node-js 0.1.13

//! Builtin op handlers (compiler-emitted `CallBuiltin` ids) plus the JS standard
//! library (`console`, `Math`, `JSON`, `Object`, array/string methods) reachable
//! from the host. Handlers pop their arguments off the VM operand stack and
//! return the result value, which the VM pushes back.

use crate::host::{self, ops, with_host, FuncVal, JsObj, ObjKind};
use fusevm::{NumOp, Value, VM};
use indexmap::IndexMap;

/// Register every node-js builtin id on a VM.
pub fn install(vm: &mut VM) {
    vm.register_builtin(ops::GETLOCAL, b_getlocal);
    vm.register_builtin(ops::SETLOCAL, b_setlocal);
    vm.register_builtin(ops::SETLOCAL_STRICT, b_setlocal_strict);
    vm.register_builtin(ops::DECLARE, b_declare);
    vm.register_builtin(ops::DECLARE_CONST, b_declare_const);
    vm.register_builtin(ops::MARK_HOLE, b_mark_hole);
    vm.register_builtin(ops::DELNAME, b_delname);
    vm.register_builtin(ops::GETATTR, b_getattr);
    vm.register_builtin(ops::SETATTR, b_setattr);
    vm.register_builtin(ops::GETITEM, b_getitem);
    vm.register_builtin(ops::SETITEM, b_setitem);
    vm.register_builtin(ops::DELITEM, b_delitem);
    vm.register_builtin(ops::MKSTR, b_mkstr);
    vm.register_builtin(ops::MKARR, b_mkarr);
    vm.register_builtin(ops::MKOBJ, b_mkobj);
    vm.register_builtin(ops::CALL, b_call);
    vm.register_builtin(ops::CALL_METHOD, b_call_method);
    vm.register_builtin(ops::CALL_VALUE, b_call_value);
    vm.register_builtin(ops::NEW, b_new);
    vm.register_builtin(ops::TRUTHY, b_truthy);
    vm.register_builtin(ops::TOSTR, b_tostr);
    vm.register_builtin(ops::MKFUNC, b_mkfunc);
    vm.register_builtin(ops::GETITER, b_getiter);
    vm.register_builtin(ops::FORITER, b_foriter);
    vm.register_builtin(ops::FORIN_KEYS, b_forin_keys);
    vm.register_builtin(ops::FORIN_ALIVE, b_forin_alive);
    vm.register_builtin(ops::HOIST_TDZ, b_hoist_tdz);
    vm.register_builtin(ops::NEW_SPREAD, b_new_spread);
    vm.register_builtin(ops::SUPER_CALL_SPREAD, b_super_call_spread);
    vm.register_builtin(ops::CONTAINS, b_contains);
    vm.register_builtin(ops::SIG_RETURN, b_sig_return);
    vm.register_builtin(ops::BINOP, b_binop);
    vm.register_builtin(ops::UNARY, b_unary);
    vm.register_builtin(ops::STRICT_EQ, b_strict_eq);
    vm.register_builtin(ops::LOOSE_EQ, b_loose_eq);
    vm.register_builtin(ops::TYPEOF, b_typeof);
    vm.register_builtin(ops::LOAD_NULL, b_load_null);
    vm.register_builtin(ops::THROW, b_throw);
    vm.register_builtin(ops::TRY, b_try);
    vm.register_builtin(ops::NULLISH, b_nullish);
    vm.register_builtin(ops::UNPACK, b_unpack);
    vm.register_builtin(ops::BUILD_ARGS, b_build_args);
    vm.register_builtin(ops::THIS, b_this);
    vm.register_builtin(ops::INSTANCEOF, b_instanceof);
    vm.register_builtin(ops::DELPROP_NAME, b_delprop_name);
    vm.register_builtin(ops::APPLY, b_apply);
    vm.register_builtin(ops::APPLY_METHOD, b_apply_method);
    vm.register_builtin(ops::OBJ_REST, b_obj_rest);
    vm.register_builtin(ops::DIV, b_div);
    vm.register_builtin(ops::POW, b_pow);
    vm.register_builtin(ops::MKCLASS, b_mkclass);
    vm.register_builtin(ops::DEF_MEMBER, b_def_member);
    vm.register_builtin(ops::DEF_FIELD, b_def_field);
    vm.register_builtin(ops::SUPER_CALL, b_super_call);
    vm.register_builtin(ops::SUPER_GET, b_super_get);
    vm.register_builtin(ops::YIELD, b_yield);
    vm.register_builtin(ops::PROPKEY, b_propkey);
    vm.register_builtin(ops::NEW_TARGET, b_new_target);
    vm.register_builtin(ops::AWAIT, b_await);
    vm.register_builtin(ops::DEF_ACCESSOR, b_def_accessor);
    vm.register_builtin(ops::DBG_LINE, b_dbg_line);
    vm.register_builtin(ops::MKBIGINT, b_mkbigint);
    vm.register_builtin(ops::MKREGEX, b_mkregex);
    vm.register_builtin(ops::TAG_TMPL, b_tag_tmpl);
    vm.register_builtin(ops::GET_ASYNC_ITER, b_get_async_iter);
    vm.register_builtin(ops::ASYNC_STEP, b_async_step);
    vm.register_builtin(ops::NUM_STEP, b_num_step);
    vm.register_builtin(ops::ITER_CLOSE, b_iter_close);
    vm.register_builtin(ops::TYPEOF_NAME, b_typeof_name);
    vm.register_builtin(ops::SIG_BREAK, b_sig_break);
    vm.register_builtin(ops::SIG_CONTINUE, b_sig_continue);
    vm.register_builtin(ops::SIG_UNWIND, b_sig_unwind);
    vm.register_builtin(ops::PUSH_SCOPE, b_push_scope);
    vm.register_builtin(ops::POP_SCOPE, b_pop_scope);
    vm.register_builtin(ops::COPY_SCOPE, b_copy_scope);
    vm.register_builtin(ops::DECLARE_VAR, b_declare_var);
    vm.register_builtin(ops::HOIST_VAR, b_hoist_var);
    vm.register_builtin(ops::NAMED_EVAL, b_named_eval);
}

/// `ITER_CLOSE`: close the iterator on the stack (a for-of `break`). A generator
/// runs its pending `finally`; a user iterator object gets its `.return()` called
/// if present; a plain materialized iterator just drops. Returns `undefined`.
/// `IteratorClose` (7.4.9): resume a generator with a forced return so its
/// pending `finally` runs, or invoke a user iterator's `.return()`. A value that
/// is neither is left alone.
pub(crate) fn close_iterator(it: &Value) -> Result<(), String> {
    if with_host(|h| h.is_generator_val(it)) {
        host::gen_return(it, Value::Undef)?;
        return Ok(());
    }
    if matches!(with_host(|h| h.get(it).cloned()), Some(JsObj::Object(_))) {
        if let Some(f) = with_host(|h| host::lookup_chain(h, it, "return")) {
            if with_host(|h| host::is_callable(h, &f)) {
                host::invoke(&f, Vec::new(), Some(it.clone()))?;
            }
        }
    }
    Ok(())
}

fn b_iter_close(vm: &mut VM, _: u8) -> Value {
    let it = vm.pop();
    // A `finally` may print or yield, but the loop is done either way; an error
    // it raises still propagates.
    match close_iterator(&it) {
        Ok(()) => Value::Undef,
        Err(e) => abort(vm, e),
    }
}

/// `NUM_STEP`: the `++`/`--` core. Pops `old` and the step `tag` (`+1`/`-1`),
/// pushes `ToNumeric(old)` (a BigInt stays a BigInt, else a Number), and returns
/// `old ± 1` in the SAME numeric type — so `x++` on a BigInt neither coerces to
/// Number nor throws the mix error.
fn b_num_step(vm: &mut VM, _: u8) -> Value {
    let old = vm.pop();
    let tag = match vm.pop() {
        Value::Int(n) => n,
        Value::Float(f) => f as i64,
        _ => 1,
    };
    if with_host(|h| h.is_bigint_val(&old)) {
        let b = with_host(|h| h.as_bigint(&old)).unwrap();
        let old_n = with_host(|h| h.new_bigint(b.clone()));
        let new = with_host(|h| h.new_bigint(b + num_bigint::BigInt::from(tag)));
        vm.push(old_n);
        new
    } else {
        let n = with_host(|h| h.to_number(&old));
        vm.push(Value::Float(n));
        Value::Float(n + tag as f64)
    }
}

/// `ASYNC_STEP`: one step of a `for await` loop — returns a Promise of the
/// `{value, done}` record (see `host::async_step`).
fn b_async_step(vm: &mut VM, _: u8) -> Value {
    let iter = vm.pop();
    let r = host::async_step(&iter);
    finish(vm, r)
}

/// `MKBIGINT`: pop the canonical decimal digit string constant, allocate the heap
/// BigInt. The lexer already validated the digits, so parsing cannot fail here.
fn b_mkbigint(vm: &mut VM, _: u8) -> Value {
    let digits = sval(&vm.pop());
    match digits.parse::<num_bigint::BigInt>() {
        Ok(b) => with_host(|h| h.new_bigint(b)),
        Err(_) => abort(vm, host::type_error("invalid BigInt literal")),
    }
}

/// `TAG_TMPL`: invoke a tagged template. The compiler emits the operands as
/// `[tag, n, m, cooked×n, raw×n, values×m]` (see `compile_tagged_template`).
/// Builds the `strings` array (carrying its `.raw` array) and calls
/// `tag(strings, ...values)`.
/// Reject a non-callable where node's scheduling entry points demand one.
///
/// Every one of them validates SYNCHRONOUSLY — `try { queueMicrotask(1) }
/// catch` catches an `ERR_INVALID_ARG_TYPE` in node. Here the value was queued
/// unchecked and the failure surfaced from the event loop instead, as an
/// uncaught `1 is not a function` that killed the process past any `try` around
/// the call.
fn require_callback(cb: &Value) -> Result<(), String> {
    if with_host(|h| host::is_callable(h, cb)) {
        return Ok(());
    }
    Err(host::invalid_arg_type(
        "callback", "argument", "function", cb,
    ))
}

fn b_tag_tmpl(vm: &mut VM, argc: u8) -> Value {
    // The chunk holding this site, read before the operands are popped and
    // before any host borrow: together with the compiler's per-site ordinal it
    // names the Parse Node whose template object 13.2.8.4 caches.
    let chunk = vm.chunk.op_hash;
    let mut all = pop_n(vm, argc as usize);
    let int_of = |v: &Value| match v {
        Value::Int(n) => *n as usize,
        Value::Float(f) => *f as usize,
        _ => 0,
    };
    let this = all.remove(0);
    let tag = all.remove(0);
    let n = int_of(&all.remove(0));
    let mcount = int_of(&all.remove(0));
    let site = int_of(&all.remove(0)) as u64;
    let cooked: Vec<Value> = all.drain(0..n.min(all.len())).collect();
    let raw: Vec<Value> = all.drain(0..n.min(all.len())).collect();
    let values: Vec<Value> = all.drain(0..mcount.min(all.len())).collect();
    // GetTemplateObject caches by Parse Node, so a site evaluated twice hands
    // back the SAME object — the whole point of the caching, since a tag that
    // memoizes on the strings array (lit-html, graphql-tag) re-parses its
    // template on every call without it.
    let key = (chunk, site);
    let strings = match with_host(|h| h.template_object(key)) {
        Some(cached) => cached,
        None => {
            // strings = cooked array; strings.raw = raw array.
            let strings = with_host(|h| h.new_array(cooked));
            let raw_arr = with_host(|h| h.new_array(raw));
            // `raw` is an own property that is neither writable, enumerable, nor
            // configurable, so it stays out of `Object.keys(strings)` while
            // `getOwnPropertyNames` still reports it.
            with_host(|h| {
                h.set_fn_prop(&strings, "raw", raw_arr.clone());
                h.set_prop_attrs(
                    &strings,
                    "raw",
                    host::PropAttrs {
                        writable: false,
                        enumerable: false,
                        configurable: false,
                    },
                );
                // Steps 12-13 run SetIntegrityLevel(frozen) on the raw array and
                // then on the template object itself. Without them a tag could
                // write through its own strings array and corrupt every later
                // evaluation of the site — which is exactly what caching makes
                // reachable, so the freeze and the cache belong together.
                h.seal_object(&raw_arr, true);
                h.seal_object(&strings, true);
                h.set_template_object(key, strings.clone());
            });
            strings
        }
    };
    let mut call_args = vec![strings];
    call_args.extend(values);
    let this = match this {
        Value::Undef => None,
        v => Some(v),
    };
    let r = host::invoke(&tag, call_args, this);
    finish(vm, r)
}

/// `GET_ASYNC_ITER`: obtain an async iterator for `for await (… of …)`. If the
/// value has a `Symbol.asyncIterator`, use it; otherwise fall back to its sync
/// iterator (each yielded value is awaited). Returns the iterator object/handle.
fn b_get_async_iter(vm: &mut VM, _: u8) -> Value {
    let src = vm.pop();
    let r = host::get_async_iterator(&src).map_err(|e| {
        // `for await` names the source AND says ASYNC: `for await (const x of
        // o)` is `o is not async iterable`. Built here rather than through
        // `name_call_site`, whose suffix table has no entry that composes.
        match host::call_site_text(vm) {
            Some(t) if e.ends_with(" is not iterable") => {
                host::type_error(&format!("{t} is not async iterable"))
            }
            _ => e,
        }
    });
    finish(vm, r)
}

/// `MKREGEX`: pop `(pattern, flags)`, translate the JS pattern to a Rust `regex`,
/// and allocate a `RegExp`. A pattern using a JS feature Rust `regex` cannot
/// express (backreference/lookaround) throws a `SyntaxError` here.
fn b_mkregex(vm: &mut VM, _: u8) -> Value {
    let flags = sval(&vm.pop());
    let pattern = sval(&vm.pop());
    match crate::regexp::build_regexp(&pattern, &flags) {
        Ok(v) => v,
        Err(e) => abort(vm, e),
    }
}

/// DAP per-statement marker (`node --dap` only; the compiler emits this before
/// each statement under `debug`). Pops the source line pushed by the preceding
/// `LoadInt` and fires the debugger line hook, which pauses at breakpoints/step
/// targets. Returns `undefined` (the compiler pops it). A no-op unless a debug
/// session is active.
fn b_dbg_line(vm: &mut VM, _: u8) -> Value {
    let line = match vm.pop() {
        Value::Int(n) => n as u32,
        _ => 0,
    };
    crate::dap::on_debug_line(line);
    Value::Undef
}

/// Install an object-literal getter/setter on an object (`kind` is `member::GET`
/// or `member::SET`). Keeps the object on the stack.
fn b_def_accessor(vm: &mut VM, _: u8) -> Value {
    let func = vm.pop();
    let kind = match vm.pop() {
        Value::Int(n) => n,
        _ => 0,
    };
    let name = sval(&vm.pop());
    let obj = vm.pop();
    with_host(|h| {
        if kind == host::member::SET {
            h.set_accessor(&obj, &name, None, Some(func));
        } else {
            h.set_accessor(&obj, &name, Some(func), None);
        }
    });
    obj
}

fn b_await(vm: &mut VM, _: u8) -> Value {
    let v = vm.pop();
    match host::await_value(v) {
        Ok(r) => r,
        Err(e) => abort(vm, e),
    }
}

// ── classes / super / generators / property keys (compiler-emitted ops) ──────

fn b_mkclass(vm: &mut VM, argc: u8) -> Value {
    // The fourth argument, when present, is the FuncDef carrying the class's
    // source span.
    let source_def = match argc {
        4 => match vm.pop() {
            Value::Int(n) => Some(n as usize),
            _ => None,
        },
        _ => None,
    };
    let ctor = vm.pop();
    let parent = vm.pop();
    let name = sval(&vm.pop());
    host::build_class(&name, parent, ctor, source_def)
}

fn b_def_member(vm: &mut VM, _: u8) -> Value {
    let func = vm.pop();
    let is_static = matches!(vm.pop(), Value::Bool(true));
    let kind = match vm.pop() {
        Value::Int(n) => n,
        _ => 0,
    };
    let name = sval(&vm.pop());
    let class_val = vm.pop();
    host::define_member(&class_val, &name, kind, is_static, func);
    class_val
}

fn b_def_field(vm: &mut VM, _: u8) -> Value {
    // `name_anon`: the initializer was an anonymous function definition, so
    // 15.7.10 NamedEvaluation names its result after the field. Syntactic —
    // decided by the compiler, not re-derived from the produced value.
    let name_anon = matches!(vm.pop(), Value::Bool(true));
    let thunk = vm.pop();
    let name = sval(&vm.pop());
    let class_val = vm.pop();
    host::define_field(&class_val, &name, thunk, name_anon);
    class_val
}

/// `super(...args)` in a derived constructor: run the parent constructor on the
/// current `this`, then this class's field initializers.
/// `SUPER_CALL_SPREAD` — `super(...xs)`, where the argument list is built at
/// run time. Shares everything below with the fixed-arity form; only where the
/// arguments come from differs.
fn b_super_call_spread(vm: &mut VM, _: u8) -> Value {
    let arr = vm.pop();
    let args = host::iter_all(&arr).unwrap_or_default();
    super_call_with(vm, args)
}

fn b_super_call(vm: &mut VM, argc: u8) -> Value {
    let args = pop_n(vm, argc as usize);
    super_call_with(vm, args)
}

fn super_call_with(vm: &mut VM, args: Vec<Value>) -> Value {
    let this = with_host(|h| h.current_this());
    let this = match this {
        Some(t) => t,
        None => return abort(vm, host::type_error("'super' keyword unexpected here")),
    };
    // The class whose constructor is running = the running method's home class.
    let (parent, fields) = with_host(|h| h.super_context());
    let (parent, fields) = match parent {
        Some(p) => (p, fields),
        None => return abort(vm, host::type_error("'super' keyword unexpected here")),
    };
    let nt = with_host(|h| h.current_new_target()).unwrap_or_else(|| this.clone());
    let this = match host::super_construct(&parent, args, &this, &nt) {
        Err(e) => return abort(vm, e),
        // The parent returned an object of its own: 15.7.15 makes THAT the
        // instance, so `this` is rebound to it for the rest of the constructor
        // and it is what `new` hands back.
        Ok(Some(replacement)) => {
            with_host(|h| h.set_current_this(replacement.clone()));
            replacement
        }
        Ok(None) => this,
    };
    if !with_host(|h| h.bind_super_this()) {
        return abort(
            vm,
            "ReferenceError: Super constructor may only be called once".to_string(),
        );
    }
    // Run this (derived) class's own instance-field initializers after super.
    for (name, thunk, name_anon) in fields {
        if let Err(e) = host::init_one_field(&this, &name, &thunk, name_anon) {
            return abort(vm, e);
        }
    }
    Value::Undef
}

/// `super.name` — a method from the parent's prototype, or a getter's result.
fn b_super_get(vm: &mut VM, _: u8) -> Value {
    let name = sval(&vm.pop());
    match with_host(|h| h.super_resolve(&name)) {
        host::SuperRef::Data(v) => v,
        host::SuperRef::Getter(getter) => {
            let this = with_host(|h| h.current_this());
            match host::invoke(&getter, Vec::new(), this) {
                Ok(v) => v,
                Err(e) => abort(vm, e),
            }
        }
    }
}

/// Close every loop iterator parked on `vm`'s stack at the op now executing,
/// innermost first. Called where a chunk is about to be halted abruptly, since
/// the code that would ordinarily close them is being jumped over.
///
/// A close runs user code (a generator's `finally`), which can itself throw; the
/// error is deliberately dropped, because it must not replace the completion
/// that caused the unwind.
fn close_parked_iters(vm: &mut VM) {
    let n = host::parked_iters(vm);
    if n == 0 {
        return;
    }
    // The completion that caused the unwind is already pending on the host.
    // Closing an iterator resumes ANOTHER generator, which settles its own
    // signal/error state, so the pending one is saved across the close and put
    // back — otherwise the outer `.return()` would be lost.
    let saved = with_host(|h| (h.signal.take(), h.error.take()));
    for _ in 0..n {
        let it = vm.pop();
        let _ = close_iterator(&it);
    }
    with_host(|h| {
        h.signal = saved.0;
        h.error = saved.1;
    });
}

fn b_yield(vm: &mut VM, _: u8) -> Value {
    let v = vm.pop();
    match host::gen_yield(v) {
        Ok(sent) => {
            // A `.return()`/`.throw()` injected on resume sets a pending Return
            // signal (or error); halt the chunk so the body unwinds through any
            // enclosing `try/finally`, exactly like a source `return`/`throw`.
            if with_host(|h| h.error.is_some() || h.signal.is_some()) {
                // Halting jumps past the loop exits, so the `for…of` / `yield*`
                // iterators parked on this chunk's stack would be abandoned
                // still-suspended. They sit directly beneath the yielded value
                // (innermost last), and the compiler recorded how many are
                // there for this exact op.
                close_parked_iters(vm);
                vm.ip = vm.chunk.ops.len();
            }
            sent
        }
        // An injected `.throw()` comes back as an error rather than a signal,
        // and abandons the parked iterators the same way. The thrown value is
        // already on the host as `exc`; `close_parked_iters` puts back whatever
        // it saves, so the close cannot swallow it.
        Err(e) => {
            close_parked_iters(vm);
            abort(vm, e)
        }
    }
}

/// `PROPKEY` — ToPropertyKey (7.1.19) for an object literal's COMPUTED key.
///
/// It called `JsHost::property_key` directly, which is the primitive-only half
/// of the conversion, so an object key never ran `ToPrimitive`:
/// `{ [{toString(){return "TS"}}]: 1 }` keyed on `"[object Object]"` while the
/// member form `a[o] = 1` — which does go through `host::to_property_key` —
/// keyed on `"TS"`. The two forms are the same abstract operation and now share
/// the same implementation.
fn b_propkey(vm: &mut VM, _: u8) -> Value {
    let v = vm.pop();
    match host::to_property_key(&v) {
        Ok(k) => with_host(|h| h.new_str(k)),
        Err(e) => abort(vm, e),
    }
}

fn b_new_target(_vm: &mut VM, _: u8) -> Value {
    with_host(|h| h.current_new_target().unwrap_or(Value::Undef))
}

/// `a / b` with JS/IEEE-754 semantics. fusevm's native `Op::Div` returns `Undef`
/// for a zero divisor (so a frontend whose `/` differs must lower to a builtin —
/// its own documented guidance), but JavaScript requires `x/0 === ±Infinity` and
/// `0/0 === NaN`, so `/` is lowered here instead.
///
/// Being a builtin rather than a native op means it does NOT reach the numeric
/// hook, so `/` was the one arithmetic operator that never ran `ToPrimitive`:
/// `({valueOf(){return 7}}) / 2` was `NaN` where every other operator gave
/// `3.5`, and `new Date(2) / 1` was `NaN` instead of `2`. It goes through the
/// hook now, so `/` coerces exactly as `*` and `-` do.
fn b_div(vm: &mut VM, _: u8) -> Value {
    let b = vm.pop();
    let a = vm.pop();
    let r = numeric_hook(NumOp::Div, &a, &b);
    finish(vm, r)
}

/// `a ** b`. Same reason `/` is a builtin: fusevm's native `Op::Pow` is IEEE-754
/// `pow`, which returns 1 for `(-1) ** Infinity` and for `1 ** NaN` where the
/// spec says NaN. Routing through the numeric hook also keeps BigInt `**` on the
/// one code path that already handles it.
fn b_pow(vm: &mut VM, _: u8) -> Value {
    let b = vm.pop();
    let a = vm.pop();
    let r = numeric_hook(NumOp::Pow, &a, &b);
    finish(vm, r)
}

/// `{ ...rest } = obj`: a new object of `obj`'s own keys minus the excluded set.
fn b_obj_rest(vm: &mut VM, _: u8) -> Value {
    let excluded = vm.pop();
    let obj = vm.pop();
    // The excluded keys are normalized exactly as a property READ normalizes
    // them, not merely stringified: a symbol key lives on the object under its
    // internal `@@sym:<id>` spelling, and `str_of` renders it `Symbol(k)`, which
    // matches no key at all — so `const { [sym]: v, ...rest } = o` left the
    // symbol-keyed property in `rest`.
    let excl: Vec<String> = with_host(|h| h.iter_vec(&excluded))
        .unwrap_or_default()
        .iter()
        .filter_map(|v| host::to_property_key(v).ok())
        .collect();
    // CopyDataProperties (ECMA-262 7.3.25) copies the own ENUMERABLE keys,
    // symbol-keyed ones included. `own_enum_key_names` is what `Object.keys`
    // uses, so an ACCESSOR is in the list — reading the property map directly
    // missed one entirely, and `const { ...r } = { get g() {…} }` produced an
    // object with no `g` and never ran the getter.
    // A PROXY answers from its traps — `ownKeys`, then a
    // `getOwnPropertyDescriptor` per key to test enumerability — which
    // `own_enum_key_names` cannot see. Rest over one produced an empty object
    // and ran no traps at all.
    if with_host(|h| h.kind_of(&obj)) == Some(ObjKind::Proxy) {
        let keys = match crate::proxy::own_keys(&obj) {
            Ok(k) => k.unwrap_or_default(),
            Err(e) => return abort(vm, e),
        };
        let mut pairs: Vec<(String, Value)> = Vec::new();
        for k in keys {
            if excl.contains(&k) {
                continue;
            }
            // The enumerability test and the READ interleave per key, as node's
            // trap log shows — testing every key first and then reading them
            // all produced the right object through the wrong trap sequence.
            match crate::proxy::own_enumerable(&obj, &k) {
                Ok(false) => continue,
                Ok(true) => {}
                Err(e) => return abort(vm, e),
            }
            match get_property(&obj, &k) {
                Ok(v) => pairs.push((k, v)),
                Err(e) => return abort(vm, e),
            }
        }
        return with_host(|h| h.new_object(pairs.into_iter().collect()));
    }
    let keys: Vec<String> = with_host(|h| {
        // `own_enum_key_names` is the STRING half — the same list `Object.keys`
        // gives, so an accessor is in it. The symbol-keyed half lives in the
        // property map under the internal `@@sym:` spelling and has to be
        // collected separately, since `Object.keys` deliberately omits it.
        let mut ks = h.own_enum_key_names(&obj);
        if let Some(JsObj::Object(m)) = h.get(&obj) {
            for k in m.keys() {
                if host::is_symbol_key(k) && h.prop_attrs(&obj, k).enumerable {
                    ks.push(k.clone());
                }
            }
        }
        ks
    })
    .into_iter()
    .filter(|k| {
        // An internal slot (`@@native`, `@@bytes`, …) or a private class field
        // is not a property; a SYMBOL key shares the `@@` prefix but is one, so
        // the two cases cannot be told apart by the prefix alone.
        !excl.contains(k)
            && (host::is_symbol_key(k) || !(k.starts_with("@@") || k.starts_with('#')))
    })
    .collect();
    // Each value is read through `[[Get]]`, OUTSIDE the host borrow: a getter is
    // user code and re-entering the VM under the borrow aborts the process.
    let mut pairs: Vec<(String, Value)> = Vec::with_capacity(keys.len());
    for k in keys {
        match get_property(&obj, &k) {
            Ok(v) => pairs.push((k, v)),
            Err(e) => return abort(vm, e),
        }
    }
    with_host(|h| {
        let props: IndexMap<String, Value> = pairs.into_iter().collect();
        h.new_object(props)
    })
}

// ── helpers ──────────────────────────────────────────────────────────────────

fn pop_n(vm: &mut VM, n: usize) -> Vec<Value> {
    let mut v = Vec::with_capacity(n);
    for _ in 0..n {
        v.push(vm.pop());
    }
    v.reverse();
    v
}

/// Read a compiler-internal name string (native `Value::Str` or heap `str`).
fn sval(v: &Value) -> String {
    if let Value::Str(s) = v {
        return (**s).clone();
    }
    with_host(|h| h.as_str(v)).unwrap_or_default()
}

/// The same string, without `sval`'s deep copy. Every identifier the compiler
/// emits is a `Value::Str` constant, so a variable read or write that went
/// through `sval` heap-allocated and memcpy'd the NAME once per access — on the
/// hot path of every loop. `Value::Str` is an `Arc<String>`, so cloning the
/// handle is a refcount bump instead.
fn sname(v: &Value) -> std::sync::Arc<String> {
    match v {
        Value::Str(s) => s.clone(),
        _ => std::sync::Arc::new(sval(v)),
    }
}

fn abort(vm: &mut VM, e: String) -> Value {
    with_host(|h| h.error = Some(e));
    vm.ip = vm.chunk.ops.len();
    Value::Undef
}

/// Halt the chunk if a call left an error or non-local signal pending.
fn finish(vm: &mut VM, r: Result<Value, String>) -> Value {
    match r {
        Ok(v) => {
            if with_host(|h| h.error.is_some() || h.signal.is_some()) {
                vm.ip = vm.chunk.ops.len();
            }
            v
        }
        Err(e) => abort(vm, e),
    }
}

// ── name handlers ─────────────────────────────────────────────────────────────

/// The value a bare global identifier resolves to, or `None` if unbound.
///
/// Shared by `b_getlocal` (the `x` form) and the `globalThis.x` property read,
/// which must agree: a name reachable one way and not the other is exactly the
/// discrepancy that left `globalThis.process` undefined while `process` worked.
pub(crate) fn global_binding(name: &str) -> Option<Value> {
    global_binding_from(name, false)
}

/// [`global_binding`] restricted to what the GLOBAL OBJECT really holds.
///
/// A `globalThis.x` read falls back to the same lazy binding a bare `x` gets,
/// which is what makes `globalThis.Math` and `globalThis.process` work — but
/// the bare-identifier lookup walks the SCOPE CHAIN, so while any function was
/// running its locals were readable off `globalThis`: `function f() { let zzq =
/// 2; return typeof globalThis.zzq }` answered for a name the global object has
/// never heard of. Only the globals map and the lazy builtins below may answer
/// here.
pub(crate) fn global_object_binding(name: &str) -> Option<Value> {
    global_binding_from(name, true)
}

fn global_binding_from(name: &str, object_only: bool) -> Option<Value> {
    let bound = with_host(|h| {
        if object_only {
            h.read_global(name)
        } else {
            h.read_name(name)
        }
    });
    if let Some(v) = bound {
        return Some(v);
    }
    // Globals bound lazily: numeric sentinels + builtin namespaces.
    match name {
        "undefined" => return Some(Value::Undef),
        "NaN" => return Some(Value::Float(f64::NAN)),
        "Infinity" => return Some(Value::Float(f64::INFINITY)),
        // One object, not a fresh one per read: `globalThis === globalThis` is
        // `true` in JS, and `globalThis.x = 1` is readable back as
        // `globalThis.x`. Both were false while each read minted a new object.
        // `global` is Node's alias for the same object.
        "globalThis" | "global" => return Some(with_host(|h| h.global_object())),
        // The WHATWG `crypto` global IS `require('crypto').webcrypto`, not the
        // node-flavoured module: `globalThis.crypto.randomUUID` exists while
        // `globalThis.crypto.createHash` does not.
        "crypto" => return Some(with_host(|h| h.alloc(JsObj::Builtin("webcrypto".into())))),
        _ => {}
    }
    if is_namespace(name) || is_known_builtin(name) {
        return Some(with_host(|h| h.alloc(JsObj::Builtin(name.to_string()))));
    }
    None
}

fn b_getlocal(vm: &mut VM, _: u8) -> Value {
    let name = sname(&vm.pop());
    // A module-top-level dead zone is tracked by NAME rather than by a parked
    // marker, so that the marker is never reachable as `globalThis.<name>`. It
    // only applies when nothing on the scope chain SHADOWS the name — a class's
    // own inner binding for its name does exactly that while its static
    // initializers run.
    if with_host(|h| h.is_tdz_global(&name) && h.read_name(&name).is_none()) {
        return abort(vm, host::tdz_error(&name));
    }
    match global_binding(&name) {
        // The binding EXISTS but has not reached its declaration yet.
        Some(v) if with_host(|h| h.is_tdz(&v)) => abort(vm, host::tdz_error(&name)),
        Some(v) => v,
        None => abort(vm, host::ref_error(&name)),
    }
}

/// `HOIST_TDZ` — declare one `let`/`const`/`class` name as uninitialized at the
/// top of the scope that declares it.
fn b_hoist_tdz(vm: &mut VM, _: u8) -> Value {
    let name = sname(&vm.pop());
    with_host(|h| h.hoist_tdz(&name));
    Value::Undef
}

/// The three global VALUE properties that are `{writable: false}` (19.1.1-19.1.3).
/// Assigning to one is a silent no-op in sloppy code and a `TypeError` in strict
/// code — and, either way, never rebinds the name.
const READONLY_GLOBALS: [&str; 3] = ["undefined", "NaN", "Infinity"];

fn readonly_global_error(name: &str) -> String {
    host::type_error(&format!(
        "Cannot assign to read only property '{name}' of object '#<Object>'"
    ))
}

fn b_setlocal(vm: &mut VM, _: u8) -> Value {
    let val = vm.pop();
    let name = sname(&vm.pop());
    // Sloppy assignment to a non-writable global is DISCARDED, not applied:
    // `undefined = 1` used to rebind the name and make every later `undefined`
    // read back as `1`.
    if READONLY_GLOBALS.contains(&name.as_str()) && !with_host(|h| h.has_name(&name)) {
        return val;
    }
    // Assigning to a binding still in its temporal dead zone throws too —
    // `{ x = 1; let x }` is a ReferenceError, not an initialization.
    if with_host(|h| match h.read_name(&name) {
        Some(v) => h.is_tdz(&v),
        None => h.is_tdz_global(&name),
    }) {
        return abort(vm, host::tdz_error(&name));
    }
    // An assignment to a `const` binding throws (8.5.2 SetMutableBinding on an
    // immutable binding). This used to succeed silently.
    if !with_host(|h| h.set_name(&name, val.clone())) {
        return abort(vm, host::type_error("Assignment to constant variable."));
    }
    val
}

/// Strict-mode `x = v` (6.2.5.6 `PutValue` with an unresolvable reference):
/// where sloppy code silently creates a global, strict code throws
/// `ReferenceError: x is not defined`.
///
/// A separate opcode rather than a runtime flag: strictness is a static property
/// of the code, so the compiler already knows which of the two an assignment is
/// and sloppy code — everything in a CommonJS module without the directive —
/// keeps the exact instruction it had.
fn b_setlocal_strict(vm: &mut VM, _: u8) -> Value {
    let val = vm.pop();
    let name = sname(&vm.pop());
    if !binding_exists(&name) {
        return abort(vm, host::ref_error(&name));
    }
    if READONLY_GLOBALS.contains(&name.as_str()) && !with_host(|h| h.has_name(&name)) {
        return abort(vm, readonly_global_error(&name));
    }
    if !with_host(|h| h.set_name(&name, val.clone())) {
        return abort(vm, host::type_error("Assignment to constant variable."));
    }
    val
}

/// Whether `name` resolves to anything — a scope binding, a global, or a lazily
/// materialised builtin namespace. `global_binding` answers the same question
/// but ALLOCATES the namespace object to do it, which an assignment then throws
/// away.
fn binding_exists(name: &str) -> bool {
    if with_host(|h| h.has_name(name)) {
        return true;
    }
    matches!(
        name,
        "undefined" | "NaN" | "Infinity" | "globalThis" | "global"
    ) || is_namespace(name)
        || is_known_builtin(name)
}

fn b_declare(vm: &mut VM, _: u8) -> Value {
    let val = vm.pop();
    let name = sname(&vm.pop());
    with_host(|h| h.declare_name(&name, val.clone()));
    val
}

/// `const x = …`: like `DECLARE`, but the binding is immutable, so a later
/// assignment to the name throws instead of overwriting it.
fn b_declare_const(vm: &mut VM, _: u8) -> Value {
    let val = vm.pop();
    let name = sname(&vm.pop());
    with_host(|h| h.declare_const_name(&name, val.clone()));
    val
}

/// `var x = …` / a hoisted `function f(){}`: bind at function scope, skipping any
/// open block scopes, so the name outlives the block it was written in.
/// `var` hoisting: create the binding as `undefined` unless it already exists.
fn b_hoist_var(vm: &mut VM, _: u8) -> Value {
    let name = sname(&vm.pop());
    with_host(|h| h.hoist_var_name(&name));
    Value::Undef
}

fn b_declare_var(vm: &mut VM, _: u8) -> Value {
    let val = vm.pop();
    let name = sname(&vm.pop());
    with_host(|h| h.declare_var_name(&name, val.clone()));
    val
}

fn b_push_scope(_: &mut VM, _: u8) -> Value {
    with_host(|h| h.push_scope());
    Value::Undef
}

fn b_pop_scope(_: &mut VM, _: u8) -> Value {
    with_host(|h| h.pop_scope());
    Value::Undef
}

fn b_copy_scope(_: &mut VM, _: u8) -> Value {
    with_host(|h| h.copy_scope());
    Value::Undef
}

fn b_delname(vm: &mut VM, _: u8) -> Value {
    let name = sval(&vm.pop());
    with_host(|h| h.del_name(&name));
    Value::Bool(true)
}

fn b_this(vm: &mut VM, _: u8) -> Value {
    if with_host(|h| h.this_state()) == host::ThisState::Pending {
        return abort(vm, host::this_before_super_error());
    }
    with_host(|h| h.current_this().unwrap_or(Value::Undef))
}

fn b_load_null(_vm: &mut VM, _: u8) -> Value {
    with_host(|h| h.null())
}

// ── attribute / item handlers ─────────────────────────────────────────────────

fn b_getattr(vm: &mut VM, _: u8) -> Value {
    let name = sval(&vm.pop());
    let recv = vm.pop();
    match get_property(&recv, &name) {
        Ok(v) => v,
        Err(e) => abort(vm, e),
    }
}

/// Read `recv.name` (also the computed-key path for string keys). Walks own
/// properties, accessors, and the prototype chain (class methods / getters).
/// Read one small piece out of `recv`'s heap cell under a short borrow.
///
/// The closure must not call back into the host (`with_host` is a `RefCell`
/// borrow and re-entering panics) — which is exactly why it hands back only the
/// value needed: the caller re-enters freely afterwards. This replaces the old
/// `h.get(recv).cloned()` habit, which deep-copied a whole `Vec`/`IndexMap`/
/// `String` just to look at it.
fn peek<R>(recv: &Value, f: impl FnOnce(&JsObj) -> Option<R>) -> Option<R> {
    with_host(|h| h.get(recv).and_then(f))
}

/// The nearest `[[Prototype]]` link of `recv` that is a Proxy, when the chain
/// reaches it without a closer link already owning `name`.
///
/// A proxy prototype answers only from the position it occupies in the chain: a
/// nearer prototype that owns the key (as a data property or an accessor) still
/// wins, exactly as `OrdinaryGet` walks one link at a time.
pub(crate) fn proxy_proto_link(recv: &Value, name: &str) -> Option<Value> {
    with_host(|h| {
        let mut cur = h.proto_of(recv);
        for _ in 0..100 {
            let p = cur?;
            match h.get(&p) {
                Some(JsObj::Proxy { .. }) => return Some(p),
                Some(JsObj::Object(props)) if props.contains_key(name) => return None,
                _ => {}
            }
            if h.own_accessor(&p, name).is_some() {
                return None;
            }
            cur = h.proto_of(&p);
        }
        None
    })
}

/// The CommonJS wrapper's parameters. They are function locals in Node, not
/// global-object properties, so `globalThis.require` is `undefined` and
/// `Object.getOwnPropertyDescriptor(globalThis, 'module')` reports no property —
/// even though the bare `require` and `module` both work.
const CJS_WRAPPER_LOCALS: &[&str] = &[
    "require",
    "module",
    "exports",
    "__filename",
    "__dirname",
    "__cjs_require",
    "__cjs_resolve",
];

/// The globals node exposes as ENUMERABLE own properties of the global object —
/// the timer family and the WHATWG additions, measured on v26.8.1. Everything
/// else (`Math`, `parseInt`, the constructors) is non-enumerable.
const ENUMERABLE_GLOBALS: &[&str] = &[
    "global",
    "clearImmediate",
    "setImmediate",
    "clearInterval",
    "clearTimeout",
    "setInterval",
    "setTimeout",
    "queueMicrotask",
    "structuredClone",
    "atob",
    "btoa",
    "performance",
    "fetch",
    "crypto",
    "navigator",
    "sessionStorage",
];

pub fn get_property(recv: &Value, name: &str) -> Result<Value, String> {
    // A `#`-prefixed key is a PRIVATE name. `[[PrivateGet]]` (7.3.31) throws
    // when the receiver carries no such private element — it does NOT read back
    // as `undefined`, which is what `C.prototype.method.call({})` used to do.
    if name.starts_with('#') && !with_host(|h| h.has_private(recv, name)) {
        return Err(private_brand_message(name, false));
    }
    get_property_recv(recv, name, recv)
}

/// The `TypeError` a failed private brand check raises. Node words it two ways:
/// a private METHOD or accessor names the class the receiver should have been an
/// instance of, while a private FIELD names the member.
pub fn private_brand_message(name: &str, writing: bool) -> String {
    if with_host(|h| h.is_private_method(name)) {
        if let Some(class) = with_host(|h| h.current_home_class_name()) {
            return host::type_error(&format!("Receiver must be an instance of class {class}"));
        }
    }
    let verb = if writing { "write" } else { "read" };
    let prep = if writing { "to" } else { "from" };
    host::type_error(&format!(
        "Cannot {verb} private member {name} {prep} an object whose class did not declare it"
    ))
}

/// `[[Get]](name, receiver)` — 10.1.8. `receiver` is the object the read STARTED
/// from and is what a getter sees as `this`; it differs from `recv` only when the
/// read was forwarded down a prototype chain, which is why `Reflect.get(t, k, r)`
/// and a Proxy `get` trap's third argument both need it. Every ordinary read
/// passes `recv` itself.
/// Re-format an error's `.stack` header on its first read, the way V8 does.
///
/// The constructor could only stamp the name it was called with, so a subclass
/// that sets `this.name` after `super()` — or any `e.name = …` / `e.message = …`
/// before the first read — left a stale header. Node re-reads both properties at
/// format time, including one inherited from the prototype (`E.prototype.name`).
///
/// It is formatted ONCE: node caches the string, so renaming AFTER a read does
/// not change what later reads return. `@@stackRaw` is the not-yet-formatted
/// marker and is dropped here; an explicit `e.stack = …` drops it too, so an
/// assignment is never clobbered by a later read.
/// The key of node's DEFAULT `Error.prepareStackTrace`. Recognised by name so
/// the ordinary stack path can skip the hook round-trip when nothing custom is
/// installed.
pub const DEFAULT_PREPARE: &str = "ErrorPrepareStackTrace";

pub fn materialize_stack(recv: &Value) {
    let Some(frames) = with_host(|h| match h.get(recv) {
        Some(JsObj::Object(p)) => p.get("@@stackRaw").cloned(),
        _ => None,
    }) else {
        return;
    };
    // A custom `Error.prepareStackTrace` replaces the string entirely (V8's
    // stack-introspection hook, which every source-map library installs). It was
    // honoured only by `Error.captureStackTrace`, so an ordinary `err.stack`
    // read bypassed it and handed back the default text.
    let prep = with_host(|h| h.builtin_static("Error", "prepareStackTrace"));
    if let Some(f) = prep.filter(|f| {
        // The default hook produces exactly what the fast path below produces,
        // so it is skipped rather than called.
        !matches!(
            with_host(|h| h.get(f).cloned()),
            Some(JsObj::Builtin(ref n)) if n == DEFAULT_PREPARE
        ) && matches!(
            with_host(|h| h.get(f).cloned()),
            Some(JsObj::Func(_)) | Some(JsObj::Builtin(_)) | Some(JsObj::BoundFunc { .. })
        )
    }) {
        // Clear the raw marker FIRST: the hook may read `.stack` itself, and a
        // second materialization would re-enter this path forever.
        with_host(|h| {
            if let Some(JsObj::Object(p)) = h.get_mut(recv) {
                p.shift_remove("@@stackRaw");
            }
        });
        let limit = with_host(|h| h.stack_trace_limit());
        if let Ok(sites) = crate::module::callsite_stack(limit) {
            if let Ok(out) = host::invoke(&f, vec![recv.clone(), sites], None) {
                with_host(|h| {
                    if let Some(JsObj::Object(p)) = h.get_mut(recv) {
                        p.insert("stack".into(), out);
                    }
                });
                return;
            }
        }
    }
    with_host(|h| {
        let frames = h.str_of(&frames);
        let name = host::lookup_chain(h, recv, "name")
            .map(|v| h.str_of(&v))
            .unwrap_or_else(|| "Error".to_string());
        let message = host::lookup_chain(h, recv, "message")
            .map(|v| h.str_of(&v))
            .unwrap_or_default();
        let header = if message.is_empty() {
            name
        } else {
            format!("{name}: {message}")
        };
        let sv = h.new_str(format!("{header}{frames}"));
        if let Some(JsObj::Object(p)) = h.get_mut(recv) {
            p.insert("stack".into(), sv);
            p.shift_remove("@@stackRaw");
        }
    });
}

pub fn get_property_recv(recv: &Value, name: &str, receiver: &Value) -> Result<Value, String> {
    // `[[Get]]` on a Proxy: the handler's `get` trap, or a forward to the
    // target. Checked before anything else so no ordinary-object shortcut can
    // read past the handler.
    if let Some(v) = crate::proxy::get(recv, name, receiver)? {
        return Ok(v);
    }
    if with_host(|h| h.is_nullish(recv)) {
        return Err(host::type_error(&format!(
            "Cannot read properties of {} (reading '{name}')",
            with_host(|h| h.str_of(recv))
        )));
    }
    if name == "stack" {
        materialize_stack(recv);
    }
    // A `DOMException`'s `name`/`message`/`code` are prototype accessors over
    // internal slots, so they resolve here rather than out of a property map.
    if let Some(v) = dom_exception_slot(recv, name) {
        return Ok(v);
    }
    // A read off `globalThis` for a name the object does not own falls back to
    // the same lazy global binding the bare identifier gets. Without it the
    // global object was an empty bag: `globalThis.process`, `.console`, `.Math`
    // and `.JSON` were all `undefined`, so `process === globalThis.process` was
    // `false` and any `globalThis.X` feature probe reported the feature missing.
    if with_host(|h| h.is_global_object(recv)) {
        let own = with_host(|h| match h.get(recv) {
            Some(JsObj::Object(p)) => p.contains_key(name),
            _ => false,
        });
        // The CommonJS wrapper's parameters are function locals in Node, not
        // global-object properties: `typeof globalThis.require` is `undefined`
        // there even though the bare `require` works.
        if !own && !CJS_WRAPPER_LOCALS.contains(&name) {
            if let Some(v) = global_object_binding(name) {
                return Ok(v);
            }
        }
    }
    // Accessor (own or inherited getter) takes precedence over the chain walk.
    // The getter runs with the RECEIVER as `this`, not the object that owns it.
    if let Some((getter, _)) = with_host(|h| host::lookup_accessor(h, recv, name)) {
        return match getter {
            Some(g) => host::invoke(&g, Vec::new(), Some(receiver.clone())),
            None => Ok(Value::Undef), // set-only property reads as undefined
        };
    }
    // `Symbol.toStringTag` read as an ordinary property. The builtins that carry
    // one expose it to a plain read, not just to `Object.prototype.toString` —
    // `new Uint8Array(1)[Symbol.toStringTag]` is `'Uint8Array'`, and a `Buffer`
    // inherits `'Uint8Array'` from the typed-array prototype it now really has.
    // Anything the receiver's own chain provides wins (a class may define its
    // own getter), so this is only the fallback.
    if name == "@@toStringTag" && with_host(|h| host::lookup_chain(h, recv, name)).is_none() {
        if let Some(tag) = with_host(|h| well_known_tag(h, recv)) {
            return Ok(with_host(|h| h.new_str(tag)));
        }
    }
    // `constructor`: a user class/function sets it on the prototype chain, and
    // that wins; otherwise every builtin instance reports its native
    // constructor (so `[].constructor`, `new Map().constructor`,
    // `Promise.resolve(1).constructor`, `(5).constructor` match Node).
    if name == "constructor" {
        if let Some(v) = with_host(|h| {
            match h.get(recv) {
                Some(JsObj::Object(p)) => p.get("constructor").cloned(),
                _ => None,
            }
            .or_else(|| host::lookup_chain(h, recv, "constructor"))
        }) {
            return Ok(v);
        }
        // An intrinsic prototype the receiver's CHAIN reaches owns a
        // `constructor` too, and it wins over the receiver's own kind:
        // `Object.create(Map.prototype).constructor` is `Map`, not `Object`.
        // Deciding from the kind alone also mis-named the receiver in every
        // message that renders one — the brand-check errors say `#<Map>`.
        if let Some(c) = chain_intrinsic_ctors(recv)
            .into_iter()
            .find(|c| is_builtin_ctor(c))
        {
            return Ok(with_host(|h| h.alloc(JsObj::Builtin(c.to_string()))));
        }
        if let Some(cn) = with_host(|h| default_ctor_name(h, recv)) {
            return Ok(with_host(|h| h.alloc(JsObj::Builtin(cn.to_string()))));
        }
    }
    // `__proto__` (Annex B B.2.2.1) is an accessor on `Object.prototype`, so it
    // answers for EVERY object that inherits from it, not only plain ones —
    // `[].__proto__` is `Array.prototype`. Only the plain-object arm handled it,
    // so an array, function or builtin instance read `undefined`. An object with
    // a null prototype inherits no such accessor and reads `undefined`, which is
    // why this is skipped there rather than answering `null`.
    if name == "__proto__"
        && !with_host(|h| h.has_null_proto(recv))
        && peek(recv, |o| match o {
            JsObj::Object(p) => Some(p.contains_key("__proto__")),
            _ => Some(false),
        }) != Some(true)
    {
        return Ok(prototype_of(recv));
    }
    // An ACCESSOR member read off the intrinsic prototype ITSELF is not a
    // method: it RUNS the getter with that prototype as `this`, and all but two
    // of `RegExp.prototype`'s then fail their brand check and throw. Every one
    // answered `undefined`, so both the value and the failure were invisible.
    // Both representations of a prototype reach here — the namespace handles
    // and the real objects (`Symbol.prototype`, `String.prototype`).
    if let Some(ctor) = intrinsic_proto_of(recv) {
        if is_proto_accessor(&ctor, name) {
            return proto_getter_call(&ctor, name, recv);
        }
    }
    let kind = with_host(|h| h.kind_of(recv));
    #[allow(unused_mut)]
    let mut out = match kind {
        Some(ObjKind::Object) => {
            let numeric = !name.is_empty() && name.bytes().all(|b| b.is_ascii_digit());
            // A view over a DETACHED buffer reports zero extent. Its own
            // `length`/`byteLength`/`byteOffset` properties still hold the old
            // numbers — the buffer does not know its views, so it cannot rewrite
            // them — and reading them straight back made a detached view still
            // look eight bytes long.
            if matches!(name, "length" | "byteLength" | "byteOffset")
                && crate::stdlib::typedarray::view_detached(recv)
            {
                match crate::stdlib::native_tag(recv).as_deref() {
                    Some("TypedArray") => return Ok(Value::Float(0.0)),
                    // A DataView THROWS where a typed array answers zero — its
                    // extent accessors are brand-checked and node reports the
                    // getter by name.
                    Some("DataView") => {
                        return Err(crate::stdlib::typedarray::detached_error(
                            "get DataView.prototype",
                            name,
                            false,
                        ))
                    }
                    _ => {}
                }
            }
            // Typed-array element read (`ta[i]`): elements live in a hidden
            // `@@elems`, not as own numeric props, so intercept integer keys.
            if numeric && crate::stdlib::native_tag(recv).as_deref() == Some("TypedArray") {
                if let Some(v) = crate::stdlib::typedarray::elem_get(recv, name) {
                    return Ok(v);
                }
            }
            // `buf[i]`: a Buffer's bytes live in a hidden `@@bytes` array, not as
            // own numeric props, so integer keys read through to it.
            if numeric
                && peek(recv, |o| match o {
                    JsObj::Object(p) => Some(p.contains_key("@@bytes")),
                    _ => None,
                })
                .unwrap_or(false)
            {
                return Ok(crate::stdlib::buffer::byte_get(recv, name));
            }
            if let Some(v) = peek(recv, |o| match o {
                JsObj::Object(p) => p.get(name).cloned(),
                _ => None,
            }) {
                v
            } else if let Some(link) = proxy_proto_link(recv, name) {
                // A Proxy sitting in the prototype chain. `OrdinaryGet` (10.1.8.1
                // step 4) forwards to the parent's `[[Get]]` with the ORIGINAL
                // receiver, so the trap sees the child as `receiver` and `this`
                // inside a trap-served getter resolves to the child, not the
                // proxy. `lookup_chain` cannot do this: it reads property maps,
                // and a proxy has none.
                return Ok(crate::proxy::get(&link, name, recv)?.expect("link is a proxy"));
            } else if let Some(v) = with_host(|h| host::lookup_chain(h, recv, name)) {
                // A method / data property inherited from the prototype chain.
                v
            } else if crate::stdlib::native_tag(recv)
                .map(|tag| crate::stdlib::instance_has_method(&tag, name))
                .unwrap_or(false)
            {
                // A native instance method read as a property (`server.listen`) →
                // a bound method, dispatched via `instance_call` when invoked.
                bound_method(recv, name)
            } else if is_object_method(name) && !with_host(|h| h.has_null_proto(recv)) {
                // `Object.create(null)` inherits nothing, so `toString`/`valueOf`
                // read as `undefined` there — which is also what makes
                // `Object.create(null) + 1` the spec `TypeError` instead of a
                // silent `"[object Object]1"`.
                bound_method(recv, name)
            } else {
                Value::Undef
            }
        }
        Some(ObjKind::Class) | Some(ObjKind::Func) | Some(ObjKind::BoundFunc) => {
            function_property(recv, name)
        }
        // A method READ off an instance (`[].slice`, `new Map().get`) is a bound
        // thunk here. It is a function value, so it answers the function
        // properties: `[].slice.name` was `undefined` where node reports
        // `slice`, and `String([].slice)` fell through to
        // `Object.prototype.toString`.
        Some(ObjKind::BoundMethod) => bound_method_property(recv, name),
        Some(ObjKind::Symbol) => match name {
            "description" => {
                match peek(recv, |o| match o {
                    JsObj::Symbol { desc, .. } => desc.clone(),
                    _ => None,
                }) {
                    Some(d) => with_host(|h| h.new_str(d)),
                    None => Value::Undef,
                }
            }
            "toString" => bound_method(recv, name),
            // Anything else a symbol answers, it inherits from
            // `Symbol.prototype`. The arm used to stop at `undefined`, so
            // `Symbol('x')[Symbol.toPrimitive]` and `Symbol('x').valueOf` read
            // as absent even though the prototype defines both — a symbol is an
            // ordinary object for the purpose of a property LOOKUP, only its
            // methods are branded.
            _ => with_host(|h| {
                h.ensure_wrapper_protos();
                h.native_proto("Symbol")
            })
            .and_then(|p| with_host(|h| host::lookup_chain(h, &p, name)))
            .unwrap_or(Value::Undef),
        },
        Some(ObjKind::BigInt) => {
            if matches!(
                name,
                "toString" | "valueOf" | "toLocaleString" | "constructor"
            ) {
                bound_method(recv, name)
            } else {
                Value::Undef
            }
        }
        Some(ObjKind::RegExp) => {
            // A RegExp holds no collection, so cloning the compiled pattern here
            // does not scale with any input size; `regexp_property` re-enters the
            // host to allocate `source`/`flags`, so it cannot run under a borrow.
            let r = peek(recv, |o| match o {
                JsObj::RegExp(r) => Some(r.clone()),
                _ => None,
            });
            match r {
                Some(r) => crate::regexp::regexp_property(&r, name).unwrap_or_else(|| {
                    // An OWN property beats the prototype method of the same
                    // name, which is ordinary resolution order. It mattered once
                    // the symbol-keyed methods existed: `re[Symbol.match] =
                    // false` disowns the regexp label (7.2.8), and the method
                    // was shadowing the assignment so the value never took.
                    if let Some(v) = with_host(|h| h.fn_prop(recv, name)) {
                        return v;
                    }
                    if crate::regexp::is_regexp_method(name) {
                        bound_method(recv, name)
                    } else {
                        Value::Undef
                    }
                }),
                None => Value::Undef,
            }
        }
        // A WeakMap/WeakSet has NO `size` (its contents are not observable), so
        // the read must be `undefined` rather than a live count.
        Some(ObjKind::Map) => {
            let (len, weak) = peek(recv, |o| match o {
                JsObj::Map { entries, weak } => Some((entries.len(), *weak)),
                _ => None,
            })
            .unwrap_or((0, false));
            match name {
                "size" if !weak => Value::Float(len as f64),
                "@@iterator" => bound_method(recv, name),
                _ if is_map_method(name) => bound_method(recv, name),
                _ => with_host(|h| h.fn_prop(recv, name)).unwrap_or(Value::Undef),
            }
        }
        Some(ObjKind::Set) => {
            let (len, weak) = peek(recv, |o| match o {
                JsObj::Set { entries, weak } => Some((entries.len(), *weak)),
                _ => None,
            })
            .unwrap_or((0, false));
            match name {
                "size" if !weak => Value::Float(len as f64),
                "@@iterator" => bound_method(recv, name),
                _ if is_set_method(name) => bound_method(recv, name),
                _ => with_host(|h| h.fn_prop(recv, name)).unwrap_or(Value::Undef),
            }
        }
        Some(ObjKind::Generator) => {
            // A generator IS its own iterator, so it answers for the matching
            // symbol — `@@asyncIterator` for an async one, `@@iterator` for a
            // sync one. Neither was advertised, so `ag()[Symbol.asyncIterator]`
            // was `undefined` even though `for await` over it worked through a
            // different path.
            let want = if with_host(|h| h.is_async_gen_val(recv)) {
                "@@asyncIterator"
            } else {
                "@@iterator"
            };
            if name == want || is_generator_method(name) || crate::stdlib::iterator::is_helper(name)
            {
                bound_method(recv, name)
            } else {
                with_host(|h| h.fn_prop(recv, name)).unwrap_or(Value::Undef)
            }
        }
        Some(ObjKind::Promise) => {
            if matches!(name, "then" | "catch" | "finally") {
                bound_method(recv, name)
            } else {
                with_host(|h| h.fn_prop(recv, name)).unwrap_or(Value::Undef)
            }
        }
        Some(ObjKind::Iter) => {
            if matches!(name, "next" | "return" | "@@iterator")
                || crate::stdlib::iterator::is_helper(name)
            {
                bound_method(recv, name)
            } else {
                with_host(|h| h.fn_prop(recv, name)).unwrap_or(Value::Undef)
            }
        }
        Some(ObjKind::Array) => {
            if name == "length" {
                let n = peek(recv, |o| match o {
                    JsObj::Array(items) => Some(items.len()),
                    _ => None,
                })
                .unwrap_or(0);
                Value::Float(n as f64)
            } else if let Ok(i) = name.parse::<usize>() {
                peek(recv, |o| match o {
                    JsObj::Array(items) => items.get(i).cloned(),
                    _ => None,
                })
                // An index PAST an `arguments` object's length is an ordinary
                // own property in the side table, since adding one must not
                // move `length`. The array read alone could not see it, so the
                // write was invisible to every later read.
                .or_else(|| with_host(|h| h.fn_prop(recv, name)))
                .unwrap_or(Value::Undef)
            } else if name == "@@iterator"
                || is_object_method(name)
                // An `arguments` object is array-BACKED here but is not an
                // Array: node's exposes no `Array.prototype` method, which is
                // exactly why the idiom is `Array.prototype.slice.call(args)`.
                // Exposing them made `arguments.map` a function.
                || (is_array_method(name) && !is_arguments(recv))
            {
                bound_method(recv, name)
            } else if let Some(v) = with_host(|h| h.fn_prop(recv, name)) {
                // Extra own props attached to an array (e.g. `RegExp.exec` result's
                // `.index`/`.input`/`.groups`).
                v
            } else {
                Value::Undef
            }
        }
        Some(ObjKind::Str) => {
            // `.length` and `s[i]` count UTF-16 code units, not code points.
            if name == "length" {
                let n = peek(recv, |o| match o {
                    JsObj::Str(s) => Some(crate::utf16::len(s)),
                    _ => None,
                })
                .unwrap_or(0);
                Value::Float(n as f64)
            } else if let Ok(i) = name.parse::<usize>() {
                match peek(recv, |o| match o {
                    JsObj::Str(s) => crate::utf16::Units::of(s).unit_str(i),
                    _ => None,
                }) {
                    Some(c) => with_host(|h| h.new_str(c)),
                    None => Value::Undef,
                }
            } else if name == "@@iterator" || is_string_method(name) {
                bound_method(recv, name)
            } else {
                Value::Undef
            }
        }
        Some(ObjKind::Builtin) => {
            let ns = peek(recv, |o| match o {
                JsObj::Builtin(ns) => Some(ns.clone()),
                _ => None,
            })
            .unwrap_or_default();
            let v = namespace_property(&ns, name);
            // `Function.prototype`'s methods READ off a builtin function. The
            // CALL forms (`Math.max.call(null, 1, 2)`) already dispatched, but
            // the read answered `undefined` — so `typeof Math.max.bind` was
            // `"undefined"`, and `String(Math.max)` found no `toString` to
            // invoke and fell back to `Object.prototype.toString`'s
            // `[object Function]` where node reports the native-code form.
            if matches!(v, Value::Undef)
                && is_function_method(name)
                && host::builtin_is_callable(&ns)
            {
                return Ok(bound_method(recv, name));
            }
            v
        }
        _ => {
            // Primitive numbers/booleans: method access -> bound method.
            if matches!(recv, Value::Float(_) | Value::Int(_)) && is_number_method(name) {
                bound_method(recv, name)
            } else {
                Value::Undef
            }
        }
    };
    // Every object INHERITS the `Object.prototype` methods, and each kind's
    // read arm above knows only its OWN. So `typeof new Map().toString`,
    // `typeof f.hasOwnProperty` and `typeof /a/.propertyIsEnumerable` all
    // answered `undefined` — for Map the CALL already worked, which is the
    // read and the dispatch disagreeing about the same method.
    //
    // Which prototype owns the name is decided by the same helper the `in`
    // operator uses, so the two cannot drift, and the result is the SHARED
    // intrinsic rather than a per-read thunk.
    // `arguments.callee` (and `.caller`) is a POISON PILL in strict code — the
    // accessor throws rather than answering, which is how a strict function
    // keeps its caller unreachable. It read back as `undefined` here, which a
    // feature probe reads as "not supported" rather than "forbidden".
    // Measured: on an ARGUMENTS object only `callee` is poisoned (`caller` is
    // simply absent and reads `undefined`); on a strict FUNCTION both `caller`
    // and `arguments` are.
    if name == "callee" && is_arguments(recv) && with_host(|h| h.current_strict()) {
        return Err(host::type_error(POISON_PILL));
    }
    if matches!(name, "caller" | "arguments")
        && matches!(
            with_host(|h| h.kind_of(recv)),
            Some(ObjKind::Func) | Some(ObjKind::Class)
        )
    {
        return poison_pill_read(recv);
    }
    // `arguments.callee` in SLOPPY code is the running function — the
    // pre-`class` self-reference idiom. It read back `undefined`.
    if name == "callee" && is_arguments(recv) {
        if let Some(f) = with_host(|h| h.fn_prop(recv, "@@callee")) {
            return Ok(f);
        }
    }
    // A method SYNTHESIZED from the receiver's kind is only reachable while the
    // receiver's intrinsic prototype is still on its chain. `Object
    // .setPrototypeOf(a, {})` must make `a.join` `undefined`; the kind arm
    // above answers from the kind alone and cannot know the link changed. Only
    // a synthesized value is dropped — the two shapes a method read produces —
    // and only when the receiver does not own the name itself.
    if matches!(
        with_host(|h| h.get(&out).cloned()),
        Some(JsObj::BoundMethod { .. })
    ) || matches!(
        with_host(|h| h.get(&out).cloned()),
        Some(JsObj::Builtin(ns)) if ns.starts_with("@proto:")
    ) {
        // The kind arms synthesize their OWN kind's methods, so that is the
        // prototype whose reachability decides. Clearing the value here lets
        // the `inherited_method_owner` fallback below re-supply the
        // `Object.prototype` form where one exists — which is why
        // `a.toString` stays a function after the link is replaced while
        // `a.join` does not.
        if !own_intrinsic_reachable(recv) && !has_own_for_shadow(recv, name) {
            out = Value::Undef;
        }
    }
    // A key the receiver does not OWN is looked up on its prototype chain. The
    // exotic arms above answer from their own storage and stop, so an array
    // given a prototype inherited nothing through a read: with
    // `Object.setPrototypeOf(a, {1: 'q'})`, `a[1]` was `undefined` at an elided
    // index and at one past the end, while `1 in a` already answered true —
    // the two views of the same question disagreeing. An accessor was found
    // (`lookup_accessor` walks), so only DATA properties went missing.
    //
    // A plain object's arm already consults the chain, and an array with no
    // explicit prototype has no links to walk, so this changes neither.
    if !name.starts_with('#') && !name.starts_with("@@") && !has_own_for_shadow(recv, name) {
        if matches!(out, Value::Undef) {
            if let Some(v) = with_host(|h| host::lookup_chain(h, recv, name)) {
                return Ok(v);
            }
        }
        // Then a monkey-patched intrinsic prototype member, which shadows the
        // synthesized one: after `Array.prototype.join = f`, `[1, 2].join` must
        // BE `f`. An explicitly-set prototype above wins over it, as the chain
        // order requires.
        if let Some(v) = inherited_builtin_static(recv, name) {
            return Ok(v);
        }
    }
    if matches!(out, Value::Undef) && !name.starts_with('#') {
        if let Some(owner) = inherited_method_owner(recv, name) {
            // An INHERITED accessor runs, it does not hand back a thunk, and
            // its brand check is about the receiver's internal slot rather than
            // its chain — `Object.create(Map.prototype).size` throws in node
            // even though `Map.prototype` is right there above it. This
            // answered `undefined`, which is the value a real Map would never
            // give and a plain object should never reach.
            if is_proto_accessor(owner, name) && !getter_in_flight(owner, name) {
                return proto_getter_call(owner, name, recv);
            }
            let key = format!("@proto:{owner}:{name}");
            if builtin_meta(&key).is_some() {
                return Ok(with_host(|h| h.alloc(JsObj::Builtin(key))));
            }
            // A DATA member of the prototype — `Array.prototype[Symbol
            // .unscopables]` is an object, not a method, so it is in neither
            // function table. Read it off the prototype itself rather than
            // answering `undefined`: an instance inherits it.
            let v = namespace_property(&format!("{owner}.prototype"), name);
            if !matches!(v, Value::Undef) {
                return Ok(v);
            }
        }
    }
    Ok(out)
}

/// The namespace name of the `require.cache` view. A `Builtin` rather than an
/// object literal because the module cache is the single source of truth: a
/// populated copy would answer reads correctly and silently ignore a `delete`,
/// which is the operation the property exists for.
pub const REQUIRE_CACHE: &str = "__cjs_cache";

/// The builtin constructor name for a value with no own/inherited `constructor`
/// property, so `x.constructor` (and thus `x.constructor.name`) matches Node for
/// arrays, plain objects, Map/Set, promises, iterators, functions, and boxed
/// primitives. `None` ⇒ leave `.constructor` as `undefined` (e.g. generators,
/// whose `.constructor.name` is `""` in Node — not worth modelling).
fn default_ctor_name(h: &host::JsHost, recv: &Value) -> Option<&'static str> {
    match h.get(recv) {
        Some(JsObj::Array(_)) => Some("Array"),
        Some(JsObj::Object(props)) => {
            // A native instance reports its own constructor, not Object — e.g.
            // `qs` does `buf.constructor.isBuffer(buf)`, so a Buffer's
            // `.constructor` must be `Buffer` (which carries `isBuffer`). Read
            // the `@@native` tag off the already-borrowed host (calling
            // `native_tag`, which re-enters `with_host`, would double-borrow).
            match props.get("@@native").map(|t| h.str_of(t)).as_deref() {
                Some("Buffer") => Some("Buffer"),
                Some("URL") => Some("URL"),
                Some("Date") => Some("Date"),
                Some("WeakRef") => Some("WeakRef"),
                Some("FinalizationRegistry") => Some("FinalizationRegistry"),
                Some("TextEncoder") => Some("TextEncoder"),
                Some("TextDecoder") => Some("TextDecoder"),
                Some("EventEmitter") => Some("EventEmitter"),
                Some("Timeout") => Some("Timeout"),
                Some("Immediate") => Some("Immediate"),
                _ => Some("Object"),
            }
        }
        Some(JsObj::Map { weak, .. }) => Some(if *weak { "WeakMap" } else { "Map" }),
        Some(JsObj::Set { weak, .. }) => Some(if *weak { "WeakSet" } else { "Set" }),
        Some(JsObj::Promise { .. }) => Some("Promise"),
        Some(JsObj::Str(_)) => Some("String"),
        Some(JsObj::Symbol { .. }) => Some("Symbol"),
        Some(JsObj::BigInt(_)) => Some("BigInt"),
        Some(JsObj::RegExp(_)) => Some("RegExp"),
        Some(JsObj::Iter { .. }) => Some("Iterator"),
        Some(JsObj::Func(f)) => {
            // A generator or async function is NOT an ordinary function: its
            // `[[Prototype]]` is `GeneratorFunction.prototype` (or the async
            // variants'), and so is its `constructor`. All three reported plain
            // `Function`, so `g.constructor.name` was `Function` where node
            // says `GeneratorFunction`.
            Some(match h.funcs.get(f.def_id) {
                Some(d) if d.is_generator && d.is_async => "AsyncGeneratorFunction",
                Some(d) if d.is_generator => "GeneratorFunction",
                Some(d) if d.is_async => "AsyncFunction",
                _ => "Function",
            })
        }
        Some(JsObj::Class(_)) | Some(JsObj::BoundFunc { .. }) => Some("Function"),
        _ => match recv {
            Value::Float(_) | Value::Int(_) => Some("Number"),
            Value::Bool(_) => Some("Boolean"),
            _ => None,
        },
    }
}

/// The builtin constructor *functions*, so `Ctor.name` is the constructor name.
/// Excludes the non-callable namespaces (`Math`, `JSON`, `console`, `Reflect`,
/// `process`), whose `.name` is `undefined` in Node.
///
/// Most are also globals, but not all: `Timeout`/`Immediate` are unexposed in
/// Node (`typeof Timeout === 'undefined'`) yet still name themselves through a
/// handle's `.constructor.name`, so they belong here and not in `GLOBALS`.
/// The builtins that expose a `Symbol.species` accessor. Each returns `this`,
/// so a subclass is its own species unless it overrides the getter.
fn has_species(name: &str) -> bool {
    matches!(
        name,
        "Array" | "Map" | "Set" | "WeakMap" | "WeakSet" | "Promise" | "RegExp" | "ArrayBuffer"
    ) || crate::stdlib::typedarray::is_ctor(name)
}

fn is_builtin_ctor(name: &str) -> bool {
    matches!(
        name,
        "Array"
            | "Object"
            | "Number"
            | "String"
            | "Boolean"
            | "Symbol"
            | "Function"
            | "Map"
            | "Set"
            | "WeakMap"
            | "WeakSet"
            | "Promise"
            | "BigInt"
            | "Iterator"
            | "RegExp"
            | "Date"
            | "ArrayBuffer"
            | "DataView"
            | "Uint8Array"
            | "Int8Array"
            | "Uint8ClampedArray"
            | "Int16Array"
            | "Uint16Array"
            | "Int32Array"
            | "Uint32Array"
            | "Float32Array"
            | "Float64Array"
            | "BigInt64Array"
            | "BigUint64Array"
            | "WeakRef"
            | "FinalizationRegistry"
            | "TextEncoder"
            | "TextDecoder"
            | "IncomingMessage"
            | "ServerResponse"
            | "EventEmitter"
            | "Buffer"
            | "URL"
            | "URLSearchParams"
            | "Timeout"
            | "Immediate"
    ) || host::ERROR_NAMES.contains(&name)
        // The stream base classes are constructors too, and `require('stream')`
        // IS `Stream`, so `require('stream').name` has to answer.
        || crate::stdlib::stream::is_class(name)
}

/// The intrinsic key of the method `<instance>.<method>` resolves to, so a bound
/// thunk can look its `name`/`length` up in the same table a
/// `<Ctor>.prototype.<method>` thunk uses. `None` when the receiver has no
/// builtin constructor to name (a native stdlib instance, whose methods are
/// node's own JS and have no specified arity).
fn bound_method_key(recv: &Value, method: &str) -> Option<String> {
    let ctor = with_host(|h| default_ctor_name(h, recv))?;
    Some(format!("@proto:{ctor}:{method}"))
}

/// `[[Get]]` on a bound method thunk. It is a function, so `name`, `length` and
/// the `Function.prototype` methods all answer; `length` only when the intrinsic
/// table knows the method, because inventing an arity is worse than the
/// `undefined` a caller can test for.
fn bound_method_property(recv: &Value, name: &str) -> Value {
    let method = peek(recv, |o| match o {
        JsObj::BoundMethod { name, .. } => Some(name.clone()),
        _ => None,
    })
    .unwrap_or_default();
    let key = peek(recv, |o| match o {
        JsObj::BoundMethod { recv, .. } => Some(recv.clone()),
        _ => None,
    })
    .and_then(|inner| bound_method_key(&inner, &method));
    let meta = key.as_deref().and_then(builtin_meta);
    match name {
        "name" => {
            let n = meta.map(|(n, _)| n.to_string()).unwrap_or(method);
            with_host(|h| h.new_str(n))
        }
        "length" => match meta {
            Some((_, len)) => Value::Float(len as f64),
            None => Value::Undef,
        },
        _ if is_function_method(name) => bound_method(recv, name),
        _ => with_host(|h| h.fn_prop(recv, name)).unwrap_or(Value::Undef),
    }
}

fn bound_method(recv: &Value, name: &str) -> Value {
    // An ECMAScript intrinsic is ONE function object shared by every instance:
    // `[1].push === Array.prototype.push` and `[1].push === [2].push` are both
    // true. Reading one off an instance used to mint a fresh thunk bound to that
    // instance, so every such comparison answered false — and a detached method
    // kept working on the receiver it was read off, where node throws because it
    // has no `this` at all.
    if let Some(key) = bound_method_key(recv, name) {
        if builtin_meta(&key).is_some() {
            return with_host(|h| h.alloc(JsObj::Builtin(key)));
        }
    }
    with_host(|h| {
        h.alloc(JsObj::BoundMethod {
            recv: recv.clone(),
            name: name.to_string(),
        })
    })
}

/// `Object.prototype` methods reachable on any object.
fn is_object_method(name: &str) -> bool {
    matches!(
        name,
        "hasOwnProperty"
            | "isPrototypeOf"
            | "propertyIsEnumerable"
            | "toString"
            | "toLocaleString"
            | "valueOf"
            | "constructor"
            | "__defineGetter__"
            | "__defineSetter__"
            | "__lookupGetter__"
            | "__lookupSetter__"
    )
}

/// The `Object.prototype` methods installed as thunks on the real
/// `Object.prototype` object, so `Object.prototype.toString.call(x)` and a class
/// prototype's inherited `hasOwnProperty` both resolve through the chain.
pub const OBJECT_PROTO_METHODS: &[&str] = &[
    "hasOwnProperty",
    "isPrototypeOf",
    "propertyIsEnumerable",
    "toString",
    "toLocaleString",
    "valueOf",
    "__defineGetter__",
    "__defineSetter__",
    "__lookupGetter__",
    "__lookupSetter__",
];

/// A typed array with elements cannot be frozen or sealed: its indices are
/// non-configurable by construction, so making them non-writable would violate
/// the invariant, and node refuses outright rather than half-applying it. An
/// EMPTY view and a `DataView` are both fine.
/// `TestIntegrityLevel` (7.3.16) — `Object.isFrozen` / `Object.isSealed`.
///
/// Over a PROXY it is a sequence of traps (`isExtensible`, `ownKeys`, then a
/// `getOwnPropertyDescriptor` per key), not a question for the host: the proxy
/// OBJECT was being inspected, so a frozen proxy answered false and the handler
/// never saw the query.
fn integrity_level(v: &Value, freeze: bool) -> Result<Value, String> {
    if with_host(|h| h.kind_of(v)) != Some(ObjKind::Proxy) {
        return Ok(Value::Bool(with_host(|h| h.is_sealed(v, freeze))));
    }
    // An EXTENSIBLE object is neither sealed nor frozen, whatever its keys say.
    if crate::proxy::is_extensible(v)?.unwrap_or(true) {
        return Ok(Value::Bool(false));
    }
    for key in crate::proxy::own_keys(v)?.unwrap_or_default() {
        let Some(d) = crate::proxy::get_own_descriptor(v, &key)? else {
            continue;
        };
        let flag = |name: &str| {
            with_host(|h| match h.get(&d) {
                Some(JsObj::Object(p)) => p.get(name).map(|x| h.truthy(x)).unwrap_or(false),
                _ => false,
            })
        };
        let is_data = with_host(
            |h| matches!(h.get(&d), Some(JsObj::Object(p)) if !p.contains_key("get") && !p.contains_key("set")),
        );
        if flag("configurable") || (freeze && is_data && flag("writable")) {
            return Ok(Value::Bool(false));
        }
    }
    Ok(Value::Bool(true))
}

/// `SetIntegrityLevel` (7.3.15) over a PROXY, which is a sequence of TRAPS —
/// `preventExtensions`, then `ownKeys`, then a `getOwnPropertyDescriptor` and a
/// `defineProperty` per key. It ran none of them: the host sealed the proxy
/// OBJECT, so the handler never saw the operation and the target was untouched.
///
/// Returns false for a non-proxy, which takes the ordinary path.
fn seal_proxy(v: &Value, freeze: bool) -> Result<bool, String> {
    if with_host(|h| h.kind_of(v)) != Some(ObjKind::Proxy) {
        return Ok(false);
    }
    if !crate::proxy::prevent_extensions(v)? {
        return Err(host::type_error("Object.freeze called on non-object"));
    }
    let keys = crate::proxy::own_keys(v)?.unwrap_or_default();
    for key in keys {
        // SEALING asks for no descriptor at all — it only strips
        // `configurable`, which is the same for a data property and an
        // accessor. FREEZING has to know which it is, because only a data
        // property has a `writable` to strip, and that is the one extra trap
        // call node makes.
        let accessor = if freeze {
            let Some(cur) = crate::proxy::get_own_descriptor(v, &key)? else {
                continue;
            };
            with_host(
                |h| matches!(h.get(&cur), Some(JsObj::Object(p)) if p.contains_key("get") || p.contains_key("set")),
            )
        } else {
            false
        };
        let desc = with_host(|h| {
            let mut m: IndexMap<String, Value> = IndexMap::new();
            m.insert("configurable".into(), Value::Bool(false));
            if freeze && !accessor {
                m.insert("writable".into(), Value::Bool(false));
            }
            h.new_object(m)
        });
        if !crate::proxy::define_property(v, &key, &desc)? {
            return Err(host::type_error(&format!(
                "'defineProperty' on proxy: trap returned falsish for property '{key}'"
            )));
        }
    }
    Ok(true)
}

fn reject_sealing_a_view(v: &Value, verb: &str) -> Result<(), String> {
    let has_elements = matches!(
        crate::stdlib::native_tag(v).as_deref(),
        Some("TypedArray") | Some("Buffer")
    ) && !crate::stdlib::typedarray::elem_values(v).is_empty();
    if has_elements {
        return Err(host::type_error(&format!(
            "Cannot {verb} array buffer views with elements"
        )));
    }
    Ok(())
}

pub fn is_object_builtin_method(name: &str) -> bool {
    matches!(
        name,
        "hasOwnProperty"
            | "isPrototypeOf"
            | "propertyIsEnumerable"
            | "toString"
            | "toLocaleString"
            | "valueOf"
            | "__defineGetter__"
            | "__defineSetter__"
            | "__lookupGetter__"
            | "__lookupSetter__"
    )
}

/// The `Symbol.toStringTag` STRING on `recv`'s chain, if any — steps 16-17 of
/// 20.1.3.6, the hook by which a class names its own brand.
///
/// A Proxy has no chain to probe: the step is an unconditional
/// `Get(O, @@toStringTag)`, so its `get` trap decides. Probing first (as an
/// ordinary receiver does, to keep the read off objects that carry no tag)
/// would always miss and brand every tagged proxy `[object Object]`.
///
/// The read runs OUTSIDE the host borrow so a getter-valued tag can be invoked.
fn to_string_tag(recv: &Value) -> Result<Option<String>, String> {
    let tagged = with_host(|h| h.kind_of(recv)) == Some(ObjKind::Proxy)
        || with_host(|h| {
            host::lookup_chain(h, recv, "@@toStringTag").is_some()
                || host::lookup_accessor(h, recv, "@@toStringTag").is_some()
        });
    if !tagged {
        return Ok(None);
    }
    let t = get_property(recv, "@@toStringTag")?;
    Ok(with_host(|h| h.as_str(&t)))
}

/// Dispatch an `Object.prototype` builtin method on an object/instance.
pub fn object_builtin_method(recv: &Value, name: &str, args: Vec<Value>) -> Result<Value, String> {
    match name {
        // Annex B B.2.2.2-B.2.2.5. Legacy, but still present in node and still
        // reached by pre-`defineProperty` libraries; all four were missing, so
        // `o.__defineGetter__` threw "is not a function".
        "__defineGetter__" | "__defineSetter__" => {
            let getter = name == "__defineGetter__";
            let f = args.get(1).cloned().unwrap_or(Value::Undef);
            if !with_host(|h| host::is_callable(h, &f)) {
                return Err(host::type_error(&format!(
                    "Object.prototype.{name}: Expecting function"
                )));
            }
            let key = host::to_property_key(&arg0(&args))?;
            let desc = with_host(|h| {
                let mut m: IndexMap<String, Value> = IndexMap::new();
                m.insert(if getter { "get" } else { "set" }.into(), f);
                m.insert("enumerable".into(), Value::Bool(true));
                m.insert("configurable".into(), Value::Bool(true));
                h.new_object(m)
            });
            apply_descriptor(recv, &key, &desc)?;
            Ok(Value::Undef)
        }
        "__lookupGetter__" | "__lookupSetter__" => {
            let want_get = name == "__lookupGetter__";
            let key = host::to_property_key(&arg0(&args))?;
            // Walks the prototype chain, unlike `getOwnPropertyDescriptor`.
            let found = with_host(|h| host::lookup_accessor(h, recv, &key));
            Ok(match found {
                Some((g, st)) => {
                    let side = if want_get { g } else { st };
                    side.unwrap_or(Value::Undef)
                }
                None => Value::Undef,
            })
        }
        "hasOwnProperty" => {
            let k = host::to_property_key(&arg0(&args))?;
            // The global object OWNS its lazily-bound builtins and every global
            // a script created; neither lives in its property map.
            if with_host(|h| h.is_global_object(recv))
                && !CJS_WRAPPER_LOCALS.contains(&k.as_str())
                && global_object_binding(&k).is_some()
            {
                return Ok(Value::Bool(true));
            }
            // A builtin namespace/prototype receiver (`Map.prototype`) reports
            // ownership via `has_property` (its methods resolve as thunks).
            if with_host(|h| h.kind_of(recv)) == Some(ObjKind::Builtin) {
                return Ok(Value::Bool(has_property(recv, &k)?));
            }
            // `HasOwnProperty` (7.3.12) is `[[GetOwnProperty]]`, so on a Proxy it
            // is the `getOwnPropertyDescriptor` trap — NOT the `has` trap and not
            // the target's property map.
            if with_host(|h| h.kind_of(recv)) == Some(ObjKind::Proxy) {
                let d = crate::proxy::get_own_descriptor(recv, &k)?.unwrap_or(Value::Undef);
                return Ok(Value::Bool(!matches!(d, Value::Undef)));
            }
            // A Buffer's / typed array's own keys are its element indices: the
            // `length`/`byteLength` slots are internal bookkeeping, and V8
            // reports `hasOwnProperty('length')` as false for a typed array.
            // Shared with the `in` operator so the two cannot drift apart.
            if let Some(hit) = crate::stdlib::typedarray::has_index(recv, &k) {
                return Ok(Value::Bool(hit));
            }
            // A function's `length`/`name`/`prototype` and a RegExp's
            // `lastIndex` are SYNTHESIZED own properties: they read back but
            // own no map entry, so this answered false where node says true.
            if synthesized_own_descriptor(recv, &k).is_some() {
                return Ok(Value::Bool(true));
            }
            if uses_side_table(recv) {
                return Ok(Value::Bool(with_host(|h| h.fn_prop(recv, &k).is_some())));
            }
            let has = with_host(|h| match h.get(recv) {
                Some(JsObj::Object(p)) => p.contains_key(&k) || h.own_accessor(recv, &k).is_some(),
                Some(JsObj::Array(items)) => {
                    k == "length"
                        || k.parse::<usize>()
                            .map(|i| i < items.len() && !h.is_hole(recv, i))
                            .unwrap_or(false)
                }
                _ => false,
            });
            Ok(Value::Bool(has))
        }
        "isPrototypeOf" => {
            let target = arg0(&args);
            // The ARGUMENT is what gets walked, so a proxy there needs its
            // `getPrototypeOf` trap for the FIRST hop: `proto_of` reads a link a
            // proxy does not hold, which reported `false` for every proxy. From
            // the second hop on the chain is ordinary objects again, walked by
            // the recorded link exactly as before.
            let mut cur = match crate::proxy::get_prototype_of(&target)? {
                Some(p) => Some(p).filter(|p| !with_host(|h| h.is_null(p))),
                None => with_host(|h| h.proto_of(&target)),
            };
            while let Some(p) = cur {
                if with_host(|h| h.strict_eq(&p, recv)) {
                    return Ok(Value::Bool(true));
                }
                cur = with_host(|h| h.proto_of(&p));
            }
            Ok(Value::Bool(false))
        }
        "propertyIsEnumerable" => {
            let k = with_host(|h| h.str_of(&arg0(&args)));
            // Own *and* enumerable — a non-enumerable own slot reads false. On a
            // Proxy that question is `[[GetOwnProperty]]`, i.e. the descriptor
            // trap, since there is no property map to enumerate.
            if with_host(|h| h.kind_of(recv)) == Some(ObjKind::Proxy) {
                let has = crate::proxy::own_enum_string_keys(recv)?.contains(&k);
                return Ok(Value::Bool(has));
            }
            let has = with_host(|h| h.own_enum_key_names(recv).contains(&k));
            Ok(Value::Bool(has))
        }
        "toString" => {
            // An instance with a custom `toString` up the chain is handled by
            // call_method before reaching here; this is the default — and the
            // default consults `Symbol.toStringTag` (20.1.3.6 steps 16-17).
            // Only the EXPLICIT `Object.prototype.toString.call(o)` did, so a
            // tagged object branded itself `[object T]` when asked one way and
            // `[object Object]` when converted the other (`String(o)`, `${o}`,
            // `o + ''`, `o.toString()`), which is the path ordinary code takes.
            if let Some(t) = to_string_tag(recv)? {
                return Ok(with_host(|h| h.new_str(format!("[object {t}]"))));
            }
            Ok(with_host(|h| {
                let s = h.str_of(recv);
                h.new_str(s)
            }))
        }
        // `Object.prototype.toLocaleString` (20.1.3.5) is defined as
        // `Invoke(this, "toString")` — no locale behavior of its own. It was
        // installed as a thunk on `Object.prototype` but had no dispatch arm, so
        // calling it threw `is not a function` on every plain object.
        "toLocaleString" => {
            let v = host::call_method(recv, "toString", Vec::new())?;
            Ok(v)
        }
        "valueOf" => Ok(recv.clone()),
        _ => Err(host::type_error(&format!("{name} is not a function"))),
    }
}

/// `Function.prototype` methods (`call`/`apply`/`bind`) plus `Symbol.prototype`/
/// generator handling done elsewhere. Returns `Ok(None)` if `name` is not one of
/// these (so the caller can try statics).
pub fn function_builtin_method(
    recv: &Value,
    name: &str,
    args: &[Value],
) -> Result<Option<Value>, String> {
    match name {
        "call" => {
            let this = args.first().cloned();
            let rest = args.get(1..).map(|s| s.to_vec()).unwrap_or_default();
            Ok(Some(host::invoke(recv, rest, this)?))
        }
        "apply" => {
            let this = args.first().cloned();
            let arr = args.get(1).cloned().unwrap_or(Value::Undef);
            // `Function.prototype.apply` takes an ARRAY-LIKE, not an iterable
            // (10.2.4.3 → CreateListFromArrayLike): `f.apply(null, arguments)`
            // and `f.apply(null, {length: 2, 0: 'x', 1: 'y'})` are the shapes
            // this is written for, and both produced an empty list. A nullish
            // second argument means no arguments at all.
            let call_args = if matches!(arr, Value::Undef) || with_host(|h| h.is_null(&arr)) {
                Vec::new()
            } else {
                create_list_from_array_like(&arr)?
            };
            Ok(Some(host::invoke(recv, call_args, this)?))
        }
        "bind" => {
            let this = args.first().cloned().unwrap_or(Value::Undef);
            let pre = args.get(1..).map(|s| s.to_vec()).unwrap_or_default();
            Ok(Some(with_host(|h| {
                h.alloc(JsObj::BoundFunc {
                    target: recv.clone(),
                    this,
                    args: pre,
                })
            })))
        }
        "toString" => Ok(Some(with_host(|h| {
            let s = h.str_of(recv);
            h.new_str(s)
        }))),
        _ => Ok(None),
    }
}

fn is_function_method(name: &str) -> bool {
    matches!(name, "call" | "apply" | "bind" | "toString")
}
fn is_map_method(name: &str) -> bool {
    matches!(
        name,
        "get" | "set" | "has" | "delete" | "clear" | "forEach" | "keys" | "values" | "entries"
    )
}
fn is_set_method(name: &str) -> bool {
    matches!(
        name,
        "add"
            | "has"
            | "delete"
            | "clear"
            | "forEach"
            | "keys"
            | "values"
            | "entries"
            | "union"
            | "intersection"
            | "difference"
            | "symmetricDifference"
            | "isSubsetOf"
            | "isSupersetOf"
            | "isDisjointFrom"
    )
}
fn is_generator_method(name: &str) -> bool {
    matches!(name, "next" | "return" | "throw")
}

/// A property read on a function/class value: own fn-props (statics, name,
/// prototype, length) plus inherited statics and `call`/`apply`/`bind`.
fn function_property(recv: &Value, name: &str) -> Value {
    // A class static, inherited down the constructor chain.
    if with_host(|h| h.kind_of(recv)) == Some(ObjKind::Class) {
        if let Some(v) = with_host(|h| h.class_static(recv, name)) {
            return v;
        }
        // A class's own `name` and `length` are its own, not the builtin
        // ancestor's: `class A extends Array {}` has `A.name === "A"` and
        // `A.length === 0`, but both were read off `Array`. Only a class that
        // WOULD fall through to an ancestor takes this path; a plain class keeps
        // the ordinary computation below.
        if matches!(name, "name" | "length")
            && with_host(|h| h.class_static(recv, name)).is_none()
            && with_host(|h| h.class_builtin_ancestor(recv))
                .is_some_and(|a| matches!(with_host(|h| h.kind_of(&a)), Some(ObjKind::Builtin)))
        {
            if let Some(v) = with_host(|h| h.fn_prop(recv, name)) {
                return v;
            }
            if name == "name" {
                let n = with_host(|h| h.callable_name(recv));
                return with_host(|h| h.new_str(n));
            }
            // The class's own constructor decides its arity; with no explicit
            // one the implicit `constructor(...args)` has length 0.
            let ctor = with_host(|h| match h.get(recv) {
                Some(JsObj::Class(c)) => c.ctor.clone(),
                _ => None,
            });
            return match ctor {
                Some(c) => get_property(&c, "length").unwrap_or(Value::Float(0.0)),
                None => Value::Float(0.0),
            };
        }
        // `Symbol.species` is an accessor returning `this`, so a subclass that
        // does not override it IS its own species. Reading it off the builtin
        // ancestor below would answer with the ancestor — `A[Symbol.species]`
        // came back as `Array`, which sent every derived result to a plain
        // array.
        if name == "@@species"
            && with_host(|h| h.class_static(recv, "@@species")).is_none()
            && with_host(|h| h.class_builtin_ancestor(recv))
                .is_some_and(|a| matches!(with_host(|h| h.kind_of(&a)), Some(ObjKind::Builtin)))
        {
            return recv.clone();
        }
        // The chain may bottom out in a BUILTIN constructor (`class D extends
        // Array {}`), whose statics `class_static` cannot see — it only walks
        // `ClassVal.parent` links between user classes. Finish the lookup with an
        // ordinary read on that ancestor so `D.from` inherits `Array.from`.
        if let Some(anc) = with_host(|h| h.class_builtin_ancestor(recv)) {
            if let Ok(v) = get_property(&anc, name) {
                if !matches!(v, Value::Undef) {
                    return v;
                }
            }
        }
    } else if let Some(v) = with_host(|h| h.fn_prop(recv, name)) {
        return v;
    }
    // A method inherited via the function's [[Prototype]] chain (set with
    // `Object.setPrototypeOf(fn, proto)` — the `router` package makes each router
    // *function* inherit `route`/`use`/`get`/… from `Router.prototype` this way).
    if let Some(v) = with_host(|h| host::lookup_chain(h, recv, name)) {
        return v;
    }
    match name {
        "name" => with_host(|h| {
            let n = h.callable_name(recv);
            h.new_str(n)
        }),
        "length" => Value::Float(with_host(|h| h.func_arity(recv)) as f64),
        "prototype" => ensure_fn_prototype(recv),
        _ if is_function_method(name) => bound_method(recv, name),
        _ => Value::Undef,
    }
}

/// The `.prototype` of a function value, auto-created on first access (as Node
/// does for every non-arrow function) with `.constructor` linking back. Arrow
/// functions have no `prototype`.
fn ensure_fn_prototype(recv: &Value) -> Value {
    if let Some(p) = with_host(|h| h.fn_prop(recv, "prototype")) {
        return p;
    }
    // Only a constructor gets one: an arrow, a method definition and an async
    // function are not constructors, and a class sets its own (10.2.5).
    if with_host(|h| h.kind_of(recv)) != Some(ObjKind::Func) {
        return Value::Undef;
    }
    if !with_host(|h| h.owns_prototype(recv)) {
        return Value::Undef;
    }
    with_host(|h| {
        let proto = h.new_object(IndexMap::new());
        if let Some(JsObj::Object(p)) = h.get_mut(&proto) {
            p.insert("constructor".to_string(), recv.clone());
        }
        h.hide_prop(&proto, "constructor");
        h.set_fn_prop(recv, "prototype", proto.clone());
        proto
    })
}

/// The numeric constants a core namespace owns, in the order node reports them
/// under `getOwnPropertyNames`. ONE table rather than a value match plus a name
/// list: the enumeration and the read have to agree, and they did not — every
/// one of these read correctly while `Object.getOwnPropertyNames(Math)` omitted
/// all eight of Math's, so a member that plainly exists was invisible to any
/// reflective copy of the namespace.
///
/// Each is `{ writable: false, enumerable: false, configurable: false }`, which
/// is what separates them from the methods alongside them.
pub fn namespace_constants(ns: &str) -> &'static [(&'static str, f64)] {
    const MATH: &[(&str, f64)] = &[
        ("E", std::f64::consts::E),
        ("LN10", std::f64::consts::LN_10),
        ("LN2", std::f64::consts::LN_2),
        ("LOG10E", std::f64::consts::LOG10_E),
        ("LOG2E", std::f64::consts::LOG2_E),
        ("PI", std::f64::consts::PI),
        ("SQRT1_2", std::f64::consts::FRAC_1_SQRT_2),
        ("SQRT2", std::f64::consts::SQRT_2),
    ];
    const NUMBER: &[(&str, f64)] = &[
        ("MAX_VALUE", f64::MAX),
        // The smallest positive value a Number can hold, which is the
        // smallest SUBNORMAL double (`5e-324`), not Rust's
        // `f64::MIN_POSITIVE` — that is the smallest *normal* double,
        // `2.2250738585072014e-308`, ~256 binary orders of magnitude too
        // large.
        // The literal, not `f64::from_bits(1)`: that is only const-callable from
        // Rust 1.83 and this crate's MSRV is 1.80. It parses to the same
        // bit pattern — the smallest positive subnormal.
        ("MIN_VALUE", 5e-324),
        ("NaN", f64::NAN),
        ("NEGATIVE_INFINITY", f64::NEG_INFINITY),
        ("POSITIVE_INFINITY", f64::INFINITY),
        ("MAX_SAFE_INTEGER", 9007199254740991.0),
        ("MIN_SAFE_INTEGER", -9007199254740991.0),
        ("EPSILON", f64::EPSILON),
    ];
    match ns {
        "Math" => MATH,
        "Number" => NUMBER,
        _ => &[],
    }
}

/// The descriptor of `<ns>.<key>`, whose attributes fall into four groups —
/// measured on node v26.8.1:
///
/// ```text
/// Math.PI, Number.MAX_SAFE_INTEGER, Number.prototype   w=false e=false c=false
/// Math.max.name, Math.max.length                       w=false e=false c=true
/// Math.floor, Array.from, Array.prototype.slice        w=true  e=false c=true
/// require('path').join                                 w=true  e=true  c=true
/// ```
///
/// So: a constant (and a constructor's `prototype`) is frozen, a function's own
/// `name`/`length` is read-only but configurable, and everything else is an
/// ordinary method — enumerable exactly when the namespace enumerates it, which
/// is what separates a core module's exports from an ECMAScript namespace's.
fn builtin_member_descriptor(ns: &str, key: &str, value: Value) -> Value {
    let frozen = namespace_constants(ns).iter().any(|(k, _)| *k == key)
        || key == "prototype"
        || (ns == "Symbol" && host::WELL_KNOWN_SYMBOLS.contains(&key));
    let own_fn_meta = matches!(key, "name" | "length") && host::builtin_is_callable(ns);
    // A key a SCRIPT assigned is an ordinary writable/enumerable/configurable
    // data property, whatever the namespace's built-in members look like — the
    // synthesized answer reported it non-enumerable, so a monkey-patched member
    // described itself as one of the intrinsics.
    let assigned = !intrinsic_proto_member(ns, key)
        && !crate::stdlib::namespace_keys(ns).iter().any(|k| k == key)
        && with_host(|h| h.builtin_static(ns, key).is_some());
    let enumerable = assigned
        || (!frozen && !own_fn_meta && crate::stdlib::namespace_keys(ns).iter().any(|k| k == key));
    with_host(|h| {
        let mut m: IndexMap<String, Value> = IndexMap::new();
        m.insert("value".into(), value);
        m.insert(
            "writable".into(),
            Value::Bool(assigned || (!frozen && !own_fn_meta)),
        );
        m.insert("enumerable".into(), Value::Bool(enumerable));
        m.insert("configurable".into(), Value::Bool(assigned || !frozen));
        h.new_object(m)
    })
}

/// Whether `<ns>.<key>` may be deleted — the `configurable` half of
/// [`builtin_member_descriptor`], split out so `delete` can ask without
/// building a descriptor object.
/// Whether `key` is one of the members the intrinsic prototype namespace `ns`
/// really defines — as opposed to a name a script added. An assignment over one
/// of these is a `[[Set]]` and leaves its attributes alone.
fn intrinsic_proto_member(ns: &str, key: &str) -> bool {
    intrinsic_proto_members(ns).is_some_and(|members| {
        members
            .iter()
            .any(|m| m.strip_prefix('+').unwrap_or(m) == key)
    })
}

fn builtin_member_configurable(ns: &str, key: &str) -> bool {
    !(namespace_constants(ns).iter().any(|(k, _)| *k == key)
        || key == "prototype"
        || (ns == "Symbol" && host::WELL_KNOWN_SYMBOLS.contains(&key)))
}

/// The value of `<ns>.<name>` when it is one of those constants.
fn namespace_constant(ns: &str, name: &str) -> Option<f64> {
    namespace_constants(ns)
        .iter()
        .find(|(k, _)| *k == name)
        .map(|(_, v)| *v)
}

/// Whether `ctor` is a WebIDL interface, whose prototype members are plain
/// assigned — and so ENUMERABLE — rather than the non-enumerable ones an
/// ECMAScript builtin defines. The generated member table records the same
/// distinction with its `+` prefix.
fn is_webidl_proto(ctor: &str) -> bool {
    intrinsic_proto_members(&format!("{ctor}.prototype"))
        .is_some_and(|ms| ms.iter().any(|m| m.starts_with('+')))
}

/// The intrinsic constructor a value's own kind implies — the prototype it
/// inherits with no explicit link.
pub(crate) fn own_ctor_name(h: &host::JsHost, v: &Value) -> Option<&'static str> {
    default_ctor_name(h, v)
}

/// Whether `ctor.prototype` defines `key` as a NON-WRITABLE data property, so
/// an object inheriting it refuses an assignment to that name.
pub(crate) fn is_proto_readonly(ctor: &str, key: &str) -> bool {
    crate::arity::PROTO_READONLY
        .binary_search_by(|(k, _)| (*k).cmp(ctor))
        .ok()
        .is_some_and(|i| crate::arity::PROTO_READONLY[i].1.contains(&key))
}

/// Whether `ctor.prototype` defines `key` as an ACCESSOR rather than a data
/// property or a method.
pub(crate) fn is_proto_accessor(ctor: &str, key: &str) -> bool {
    crate::arity::PROTO_ACCESSORS
        .binary_search_by(|(k, _)| (*k).cmp(ctor))
        .ok()
        .is_some_and(|i| crate::arity::PROTO_ACCESSORS[i].1.contains(&key))
}

/// The constructor whose `.prototype` IS `recv`, whichever of the two
/// representations it uses — a `Builtin` namespace handle or a real object.
pub(crate) fn intrinsic_proto_of(recv: &Value) -> Option<String> {
    with_host(|h| match h.get(recv) {
        Some(JsObj::Builtin(ns)) => ns.strip_suffix(".prototype").map(str::to_string),
        _ => h.intrinsic_proto_ctor(recv).map(str::to_string),
    })
}

/// The getter function of an intrinsic prototype accessor, as a first-class
/// value — what `Object.getOwnPropertyDescriptor(Map.prototype, 'size').get`
/// hands back, and the form a library uses to borrow one.
fn proto_getter(ctor: &str, key: &str) -> Value {
    with_host(|h| h.alloc(JsObj::Builtin(format!("@protoget:{ctor}:{key}"))))
}

/// Whether `recv` carries the internal slot `ctor`'s accessor demands. This is
/// a BRAND check, not a chain walk: `Object.create(Map.prototype).size` throws
/// in node even though `Map.prototype` is right there on the chain.
fn brand_matches(recv: &Value, ctor: &str) -> bool {
    if let Some(tag) = crate::stdlib::native_tag(recv) {
        if tag == ctor || (ctor == "TypedArray" && tag == "TypedArray") {
            return true;
        }
    }
    match ctor {
        "TypedArray" => crate::stdlib::native_tag(recv).as_deref() == Some("TypedArray"),
        "ArrayBuffer" => with_host(
            |h| matches!(h.get(recv), Some(JsObj::Object(p)) if p.contains_key("@@bytes")),
        ),
        _ => {
            let own = match wrapped_primitive(recv).as_ref().and_then(wrapper_ctor_of) {
                Some(c) => Some(c),
                None => with_host(|h| default_ctor_name(h, recv)),
            };
            own == Some(ctor)
        }
    }
}

thread_local! {
    /// The `(ctor, key)` prototype accessors whose tail read is in flight.
    ///
    /// A getter's last step reads the value off the receiver, and when the
    /// receiver does not STORE it that read walks the chain, finds the same
    /// accessor and runs it again: `new TextDecoder().fatal` recursed until the
    /// stack overflowed and aborted the process. An accessor already in flight
    /// answers `undefined` for its own key rather than re-entering — the value
    /// a missing internal slot has, and what node reports for one.
    static GETTERS_IN_FLIGHT: std::cell::RefCell<Vec<(String, String)>> =
        const { std::cell::RefCell::new(Vec::new()) };
}

/// Whether `ctor`'s `key` getter is already running further down the stack.
fn getter_in_flight(ctor: &str, key: &str) -> bool {
    GETTERS_IN_FLIGHT.with(|g| g.borrow().iter().any(|(c, k)| c == ctor && k == key))
}

/// Invoke an intrinsic prototype's getter against `recv` — the body behind the
/// `@protoget:` thunks.
///
/// Reading one OFF THE PROTOTYPE (`Map.prototype.size`) is the case that was
/// wrong: it answered `undefined` where node runs the getter, fails the brand
/// check and throws. `RegExp.prototype` is the documented exception — 22.2.6.10
/// and .13 return `"(?:)"` and `""` for it specifically, so the one receiver
/// that would otherwise throw for every flag reads two of them back.
pub(crate) fn proto_getter_call(ctor: &str, key: &str, recv: &Value) -> Result<Value, String> {
    let is_the_prototype = with_host(
        |h| matches!(h.get(recv), Some(JsObj::Builtin(ns)) if *ns == format!("{ctor}.prototype")),
    );
    if is_the_prototype && ctor == "RegExp" {
        // 22.2.6.x each carry the same step: when `this` IS `%RegExp.prototype%`
        // the getter returns rather than throwing. `source` and `flags` have
        // their own values there; every flag getter answers `undefined`.
        return Ok(match key {
            "source" => with_host(|h| h.new_str("(?:)".to_string())),
            "flags" => with_host(|h| h.new_str(String::new())),
            _ => Value::Undef,
        });
    }
    // `RegExp.prototype.flags` (22.2.6.5) is the one that is GENERIC: it reads
    // the individual flag properties off whatever object it is handed and
    // concatenates their letters, so a plain object answers `""` rather than
    // throwing, and one carrying `global`/`ignoreCase` answers `"gi"`.
    if ctor == "RegExp" && key == "flags" && !brand_matches(recv, ctor) {
        if !with_host(|h| is_object_like(h, recv)) {
            return Err(regexp_brand_error(key, recv));
        }
        let mut out = String::new();
        for (prop, letter) in REGEXP_FLAG_LETTERS {
            let v = get_property(recv, prop)?;
            if with_host(|h| h.truthy(&v)) {
                out.push(*letter);
            }
        }
        return Ok(with_host(|h| h.new_str(out)));
    }
    // `Function.prototype.arguments`/`caller` are POISON PILLS (10.2.4.1): both
    // the getter and the setter throw for every receiver, which is how a strict
    // function keeps its caller unreachable. They are not brand checks and do
    // not name the receiver.
    if ctor == "Function" && matches!(key, "arguments" | "caller") {
        return poison_pill_read(recv);
    }
    if !brand_matches(recv, ctor) {
        return Err(match ctor {
            "RegExp" => regexp_brand_error(key, recv),
            "Symbol" => {
                host::type_error("Symbol.prototype.description requires that 'this' be a Symbol")
            }
            _ => host::type_error(&format!(
                "Method get {ctor}.prototype.{key} called on incompatible receiver {}",
                brand_receiver_string(recv)
            )),
        });
    }
    // A native instance keeps an accessor's value in the hidden `@@<key>` slot,
    // so that the public name can be a getter on the prototype rather than an
    // own enumerable property. Read it straight: the chain walk below would
    // find this same accessor and run it again.
    if let Some(v) = with_host(|h| match h.get(recv) {
        Some(JsObj::Object(p)) => p.get(&format!("@@{key}")).cloned(),
        _ => None,
    }) {
        return Ok(v);
    }
    GETTERS_IN_FLIGHT.with(|g| g.borrow_mut().push((ctor.to_string(), key.to_string())));
    let out = get_property(recv, key);
    GETTERS_IN_FLIGHT.with(|g| {
        g.borrow_mut().pop();
    });
    out
}

/// `Function.prototype.arguments`/`caller` read against `recv`.
///
/// The pill is conditional and the condition is the RECEIVER, not the reading
/// code: a sloppy non-arrow function answers `null` (node stopped populating
/// these long ago but kept them readable), and everything else — an arrow, a
/// strict function, a non-function — throws. Keying it on the READER's
/// strictness, which is what this did, made `strictFn.arguments` answer
/// `undefined` from sloppy code and a sloppy function throw from strict code:
/// wrong in both directions.
pub(crate) fn poison_pill_read(recv: &Value) -> Result<Value, String> {
    if with_host(|h| h.fn_is_sloppy(recv)) {
        return Ok(with_host(|h| h.null()));
    }
    Err(host::type_error(POISON_PILL))
}

/// The message both halves of the `arguments`/`caller` poison pill throw.
pub(crate) const POISON_PILL: &str = "'caller', 'callee', and 'arguments' properties may not be accessed on strict mode functions or the arguments objects for calls to them";

/// How a REJECTED receiver is rendered in a brand-check message.
///
/// `no_side_effects_string` answers for most of them, but two kinds differ:
/// an intrinsic PROTOTYPE renders `#<Map>` rather than `[object Map]`, and so
/// does an `ArrayBuffer`/`DataView` instance, which this host tags natively and
/// that function therefore brands. Node draws the line at whether the value is
/// one of the ES5-era classes (`Array`, `Date`, `RegExp` are `[object X]`); the
/// two cases here are the ones that fall on the other side of it.
fn brand_receiver_string(recv: &Value) -> String {
    if let Some(ctor) = intrinsic_proto_of(recv) {
        return format!("#<{ctor}>");
    }
    match crate::stdlib::native_tag(recv).as_deref() {
        Some(tag @ ("ArrayBuffer" | "DataView")) => format!("#<{tag}>"),
        _ => no_side_effects_string(recv),
    }
}

/// The flag properties `RegExp.prototype.flags` reads, in the order 22.2.6.5
/// concatenates their letters.
const REGEXP_FLAG_LETTERS: &[(&str, char)] = &[
    ("hasIndices", 'd'),
    ("global", 'g'),
    ("ignoreCase", 'i'),
    ("multiline", 'm'),
    ("dotAll", 's'),
    ("unicode", 'u'),
    ("unicodeSets", 'v'),
    ("sticky", 'y'),
];

/// `RegExp.prototype`'s flag getters word their brand failure their own way,
/// and `flags` distinguishes a non-object receiver from a non-RegExp one
/// because 22.2.6.5 reads the individual flags off any object it is given.
fn regexp_brand_error(key: &str, recv: &Value) -> String {
    if key == "flags" && !with_host(|h| matches!(recv, Value::Obj(_)) && !h.is_null(recv)) {
        return host::type_error(&format!(
            "RegExp.prototype.flags getter called on non-object {}",
            no_side_effects_string(recv)
        ));
    }
    host::type_error(&format!(
        "RegExp.prototype.{key} getter called on non-RegExp object"
    ))
}

/// A property on a builtin namespace object (`Math.PI`, `Number.MAX_SAFE_INTEGER`,
/// `console.log`).
pub fn namespace_property(ns: &str, name: &str) -> Value {
    // `require.cache[id]` — a LIVE view of the module cache, not a copy, so a
    // read sees whatever is loaded now and `delete` (see `delete_property`)
    // actually invalidates.
    if ns == REQUIRE_CACHE {
        return crate::module::cache_get(name).unwrap_or(Value::Undef);
    }
    // A property a SCRIPT assigned onto this namespace wins over everything
    // synthesized below, including a member the namespace really has. That is
    // what monkey-patching an intrinsic is: `Array.prototype.join = f` must make
    // `[1, 2].join()` call `f`, and a polyfill's `Array.prototype.at = impl` has
    // to read back at all. Only the two `Error` hooks consulted this table, so
    // every other assignment onto a builtin — the whole polyfill idiom — was
    // stored by `set_property` and then never read: the write appeared to
    // succeed, `Object.isExtensible` said true, and the value came back
    // `undefined`.
    if let Some(v) = with_host(|h| h.builtin_static(ns, name)) {
        return v;
    }
    // The ENTRY script's `require` is this builtin rather than the per-module
    // closure, so its `cache` has to be handed out here too.
    // `require.extensions` — the legacy loader map. Deprecated but still read
    // (and sometimes written) by tooling that hooks module loading, and it was
    // absent entirely. The three keys node ships are present; installing a
    // custom loader through them is NOT honoured by this runtime's loader, so
    // the map reports what it can serve rather than pretending otherwise.
    // `util.promisify.custom` — the registered symbol a module attaches to a
    // callback function to supply its own promisified form. It was `undefined`,
    // so the lookup that decides whether to use one always missed.
    if ns == "util.promisify" && name == "custom" {
        return with_host(|h| h.symbol_for("nodejs.util.promisify.custom"));
    }
    // `process.memoryUsage.rss()` — node's fast path for the one figure that
    // does not need the whole object built.
    if ns == "process.memoryUsage" && name == "rss" {
        return with_host(|h| h.alloc(JsObj::Builtin("process.memoryUsage.rss".to_string())));
    }
    if ns == "require" && name == "extensions" {
        return with_host(|h| {
            let mut m: IndexMap<String, Value> = IndexMap::new();
            for ext in [".js", ".json", ".node"] {
                let f = h.alloc(JsObj::Builtin(format!("@@extension:{ext}")));
                m.insert(ext.to_string(), f);
            }
            h.new_object(m)
        });
    }
    // `require.resolve.paths(spec)` — the directories a lookup would search:
    // `null` for a core module, the `node_modules` chain otherwise.
    if ns == "require.resolve" && name == "paths" {
        return with_host(|h| h.alloc(JsObj::Builtin("require.resolve.paths".to_string())));
    }
    if ns == "require" && name == "cache" {
        return with_host(|h| h.alloc(JsObj::Builtin(REQUIRE_CACHE.to_string())));
    }
    // The legacy numeric codes `DOMException` carries as statics
    // (`DOMException.ABORT_ERR` is 20), named by uppercasing the error name.
    if ns == "DOMException" {
        if let Some((_, code)) = DOM_EXCEPTION_CODES
            .iter()
            .find(|(n, _)| legacy_code_name(n) == name)
        {
            return Value::Float(*code);
        }
    }
    // Numeric constants.
    if let Some(k) = namespace_constant(ns, name) {
        return Value::Float(k);
    }
    // `Ctor.name` on a builtin constructor is the constructor name (`Array.name`
    // === "Array"); non-callable namespaces (`Math`/`JSON`) fall through to
    // `undefined`.
    // `GeneratorFunction.prototype` and the two async variants are REAL objects
    // in `native_protos`, not `Builtin("X.prototype")` namespace handles — they
    // sit on the prototype chain of every generator/async function, which a
    // handle cannot do. Without this the read fell through to `undefined`.
    if name == "prototype"
        && matches!(
            ns,
            "GeneratorFunction" | "AsyncFunction" | "AsyncGeneratorFunction"
        )
    {
        return with_host(|h| {
            h.ensure_native_protos();
            h.native_proto(ns).unwrap_or(Value::Undef)
        });
    }
    // `Error.prepareStackTrace` has a DEFAULT hook in node
    // (`ErrorPrepareStackTrace`), so a library probing `if
    // (Error.prepareStackTrace)` finds one. Reading `undefined` sent that probe
    // down the wrong branch. The default renders the header plus the frames,
    // which is what the fast path in `materialize_stack` already produces — it
    // recognises this exact builtin and skips the round trip.
    if ns == "Error" && name == "prepareStackTrace" {
        return with_host(|h| h.builtin_static("Error", "prepareStackTrace")).unwrap_or_else(
            || with_host(|h| h.alloc(JsObj::Builtin(DEFAULT_PREPARE.to_string()))),
        );
    }
    // `Error.stackTraceLimit` defaults to 10 and is settable; an assignment
    // lands in the builtin-static side table, which the read below consults
    // first. Without a default the READ was `undefined`, so a library doing
    // `const old = Error.stackTraceLimit` and restoring it later installed
    // `undefined` and disabled the limit permanently.
    if ns == "Error" && name == "stackTraceLimit" {
        return with_host(|h| h.builtin_static("Error", "stackTraceLimit"))
            .unwrap_or(Value::Float(10.0));
    }
    // `Ctor[Symbol.species]` is an accessor returning `this` on every builtin
    // that has one (23.1.2.5, 27.2.4.7, …). It was absent, so the species
    // protocol had nothing to read and every derived result came back a plain
    // builtin.
    if name == "@@species" && has_species(ns) {
        return with_host(|h| h.alloc(JsObj::Builtin(ns.to_string())));
    }
    if name == "name" && is_builtin_ctor(ns) {
        return with_host(|h| h.new_str(ns.to_string()));
    }
    // A well-known symbol (`Symbol.iterator`, `Symbol.toPrimitive`, …) used as a
    // computed property/method key.
    if ns == "Symbol" && host::WELL_KNOWN_SYMBOLS.contains(&name) {
        return with_host(|h| h.well_known_symbol(name));
    }
    // Non-function constants on a stdlib namespace (`path.sep`, `os.EOL`,
    // `buffer.Buffer`, `url.URL`).
    if let Some(v) = crate::stdlib::constant(ns, name) {
        return v;
    }
    // `Ctor.prototype` on a builtin constructor (`Object.prototype`,
    // `Array.prototype`, …): a prototype namespace whose methods are callable
    // thunks (`Object.prototype.toString.call(x)` is a load-time idiom in the
    // `get-intrinsic`/`function-bind` family).
    if name == "prototype" && is_builtin_ctor(ns) {
        // Same reasoning as the native prototypes below, for the error
        // hierarchy: `new Error(...)` links its `[[Prototype]]` to the REAL
        // `error_protos` object, so `Error.prototype` has to read back that same
        // object. It resolved to a fresh `Builtin("Error.prototype")` thunk
        // instead, which is a FUNCTION — so `Object.getPrototypeOf(new
        // Error("x")) === Error.prototype` was false, and `typeof
        // Error.prototype` was `"function"` where node says `"object"`.
        if host::ERROR_NAMES.contains(&ns) {
            if let Some(p) = with_host(|h| {
                h.ensure_error_protos();
                host::error_proto_of(h, ns)
            }) {
                return p;
            }
        }
        // `Buffer`/`Uint8Array` have real prototype *objects* — a Buffer's
        // `[[Prototype]]` points at one, so `Object.getPrototypeOf(buf) ===
        // Buffer.prototype` must compare equal, which a freshly-allocated
        // `Builtin` handle never can.
        if let Some(p) = with_host(|h| {
            h.ensure_native_protos();
            h.native_proto(ns)
        }) {
            return p;
        }
        let _ = ns;
        return with_host(|h| h.alloc(JsObj::Builtin(format!("{ns}.prototype"))));
    }
    // A NATIVE stdlib constructor's `.prototype` (`StringDecoder`, `Hash`,
    // `URLSearchParams`, …). These are absent from `is_builtin_ctor`, so the arm
    // above never fired and the read produced `undefined` — which broke the ES5
    // subclassing pattern libraries still ship. `iconv-lite`'s internal codec
    // reads `StringDecoder.prototype.end` at load, and threw
    // `Cannot read properties of undefined (reading 'end')`. Built from the same
    // instance-method table a method read consults, so the two cannot disagree.
    if name == "prototype" {
        if let Some(p) = with_host(|h| h.ensure_ctor_proto(ns)) {
            return p;
        }
    }
    // A method read off a builtin prototype namespace (`Array.prototype.slice`):
    // a `@proto:<Ctor>:<method>` thunk that, when invoked (typically via
    // `.call`/`.apply`), dispatches `method` against the invoke-time `this`.
    //
    // The thunk is minted only for a name the prototype REALLY carries. Minting
    // one unconditionally made every absent name answer with a function:
    // `Array.prototype.totallyBogus` was `[Function: totallyBogus]` where node
    // says `undefined`, and so was every well-known symbol a prototype does not
    // define — `Array.prototype[Symbol.toStringTag]` came back a function
    // instead of `undefined`, which is a value `Object.prototype.toString` and
    // every `typeof`/truthiness test downstream then read wrong.
    //
    // Existence is decided by the generated intrinsic table, which is read out
    // of the reference engine, so this cannot drift from what node defines.
    // A name the prototype does not define but `Object.prototype` does is
    // INHERITED, and node hands back Object.prototype's own function object
    // (`Map.prototype.toString === Object.prototype.toString` is `true`), so it
    // resolves to the `Object` thunk rather than a per-ctor one. That is also
    // what makes `String(Map.prototype)` print `[object Map]`: `Map.prototype`
    // has no own `toString`, and the inherited one is the generic tag reader,
    // not a Map method that rejects a non-Map `this`.
    if let Some(ctor) = ns.strip_suffix(".prototype") {
        // `Array.prototype[Symbol.unscopables]` (23.1.3.38) is a DATA property,
        // not an intrinsic function, so it is not in the arity table the lookup
        // above consults. It lists the methods a `with` block must NOT bring
        // into scope — the ones added after `with` existed, so old code using a
        // variable of the same name keeps working.
        if name == "@@unscopables" && ctor == "Array" {
            return with_host(|h| {
                let mut m: IndexMap<String, Value> = IndexMap::new();
                for k in [
                    "at",
                    "copyWithin",
                    "entries",
                    "fill",
                    "find",
                    "findIndex",
                    "findLast",
                    "findLastIndex",
                    "flat",
                    "flatMap",
                    "includes",
                    "keys",
                    "toReversed",
                    "toSorted",
                    "toSpliced",
                    "values",
                ] {
                    m.insert(k.to_string(), Value::Bool(true));
                }
                let o = h.new_object(m);
                let null = h.null();
                h.set_proto(&o, null);
                o
            });
        }
        if builtin_meta(&format!("@proto:{ctor}:{name}")).is_some() {
            return with_host(|h| h.alloc(JsObj::Builtin(format!("@proto:{ctor}:{name}"))));
        }
        if ctor != "Object" && builtin_meta(&format!("@proto:Object:{name}")).is_some() {
            return with_host(|h| h.alloc(JsObj::Builtin(format!("@proto:Object:{name}"))));
        }
        // `constructor` is excluded from the table because it is not a method:
        // it is the constructor function itself, and node compares equal
        // (`Array.prototype.constructor === Array`). It used to resolve to a
        // `@proto:Array:constructor` thunk, which is a different object every
        // read and so never compared equal to anything.
        if name == "constructor" && is_builtin_ctor(ctor) {
            return with_host(|h| h.alloc(JsObj::Builtin(ctor.to_string())));
        }
        return Value::Undef;
    }
    let qualified = format!("{ns}.{name}");
    if is_known_builtin(&qualified) {
        return with_host(|h| h.alloc(JsObj::Builtin(qualified)));
    }
    // A property the user stuck on this builtin namespace (`Error.prepareStackTrace`).
    if let Some(v) = with_host(|h| h.builtin_static(ns, name)) {
        return v;
    }
    // A builtin FUNCTION's own `name` and `length` (10.3.3-4: every one has
    // both). `Math.max.name` was `undefined` — as was every `.name` a library
    // reads to identify a callback it was handed. The non-callable namespaces
    // fall through: `Math.name` and `require('fs').length` really are undefined.
    if host::builtin_is_callable(ns) {
        match name {
            "name" => {
                if let Some(n) = proto_getter_name(ns) {
                    return with_host(|h| h.new_str(n));
                }
                return with_host(|h| h.new_str(builtin_name(ns).to_string()));
            }
            // Only the intrinsics have a specified arity; a core-module
            // function's is a property of node's own JS source, so it stays
            // `undefined` rather than being invented here.
            "length" => {
                // A getter takes no argument (10.2.9 / the accessor grammar),
                // so its `length` is 0 — it is not in the intrinsic table,
                // which holds only named functions.
                if proto_getter_name(ns).is_some() {
                    return Value::Float(0.0);
                }
                if let Some((_, len)) = builtin_meta(ns) {
                    return Value::Float(len as f64);
                }
            }
            _ => {}
        }
    }
    Value::Undef
}

/// Dispatch a `@proto:<Ctor>:<method>` thunk (a method read off a builtin
/// prototype, e.g. `Object.prototype.toString`) against `recv` (its invoke-time
/// `this`). `Object.prototype.toString` yields the `[object Tag]` brand string
/// libraries type-check on; every other method routes through normal method
/// dispatch on `recv`.
/// The TypeError a `<Ctor>.prototype.<method>` thunk throws when it is invoked
/// with NO receiver — `const f = [].push; f(1)`.
///
/// Reading a method off an instance used to mint a thunk bound to that
/// instance, so a detached method silently kept working on the object it came
/// from. Now that it is the shared intrinsic, a bare call has no `this` and has
/// to say so. Node words it four ways, and which one a method gets is not
/// something that can be derived — the split was measured across every method
/// of each prototype:
///
/// ```text
/// ToObject(this)         "Cannot convert undefined or null to object"
/// RequireObjectCoercible "<Ctor>.prototype.<m> called on null or undefined"
/// brand check            "<Ctor>.prototype.<m> requires that 'this' be a <X>"
/// everything else        the generic incompatible-receiver message
/// ```
fn nullish_receiver_error(ctor: &str, method: &str, recv: &str) -> Option<String> {
    // `Array.prototype` splits: the CALLBACK-taking methods plus `concat` and
    // the two `indexOf` family members name themselves, the rest go through
    // `ToObject` and report its message.
    const ARRAY_NAMED: &[&str] = &[
        "concat",
        "every",
        "filter",
        "find",
        "findIndex",
        "findLast",
        "findLastIndex",
        "forEach",
        "indexOf",
        "map",
        "reduce",
        "reduceRight",
        "some",
    ];
    const TO_OBJECT: &str = "Cannot convert undefined or null to object";
    let named = |c: &str| format!("{c}.prototype.{method} called on null or undefined");
    let branded =
        |c: &str, want: &str| format!("{c}.prototype.{method} requires that 'this' be a {want}");
    // The generic form names the receiver, so a `null` one must not be reported
    // as `undefined`.
    let generic = |c: &str, m: &str| {
        format!("Method {c}.prototype.{m} called on incompatible receiver {recv}")
    };
    Some(match ctor {
        "Array" if ARRAY_NAMED.contains(&method) => named("Array"),
        "Array" => TO_OBJECT.to_string(),
        // `Object.prototype.toString` is the one method that ACCEPTS a nullish
        // receiver — it answers `[object Undefined]`.
        "Object" if method == "toString" => return None,
        "Object" if method == "toLocaleString" => named("Object"),
        "Object" => TO_OBJECT.to_string(),
        // Both aliases report the LEGACY name in the message, which is the one
        // place `name` and the message disagree.
        "String" if method == "trimStart" => named("String").replace("trimStart", "trimLeft"),
        "String" if method == "trimEnd" => named("String").replace("trimEnd", "trimRight"),
        "String" if matches!(method, "toString" | "valueOf") => branded("String", "String"),
        "String" => named("String"),
        "Number" => branded("Number", "Number"),
        "Boolean" => branded("Boolean", "Boolean"),
        "Symbol" => branded("Symbol", "Symbol"),
        "Function" if method == "bind" => "Bind must be called on a function".to_string(),
        "Function" if matches!(method, "call" | "apply") => format!(
            "Function.prototype.{method} was called on undefined, which is undefined and not a function"
        ),
        "Function" => branded("Function", "Function"),
        // `Promise.prototype.catch`/`finally` are written in terms of `then`, so
        // a nullish receiver fails inside them and reports that instead.
        "Promise" if method == "catch" => {
            "Cannot read properties of undefined (reading 'then')".to_string()
        }
        "Promise" if method == "finally" => {
            "Promise.prototype.finally called on non-object".to_string()
        }
        "Date" if method == "toJSON" => TO_OBJECT.to_string(),
        // The plain GETTERS and `valueOf` read `[[DateValue]]` directly and
        // report that slot check; every setter, every `to*String` and the two
        // legacy year methods go through the generic receiver check first.
        "Date"
            if method == "valueOf"
                || (method.starts_with("get") && method != "getYear") =>
        {
            "this is not a Date object.".to_string()
        }
        // An ALIAS reports the method it aliases: `toGMTString` IS `toUTCString`
        // and `Set.prototype.keys` IS `values`, one function object each.
        "Date" if method == "toGMTString" => generic("Date", "toUTCString"),
        "Set" if method == "keys" => generic("Set", "values"),
        // Everything else that is brand-checked names itself. Node reaches this
        // wording from a `[[GetOwnProperty]]`-style slot check; here the check
        // is the receiver's kind, and only the message has to agree.
        "ArrayBuffer" | "DataView" | "RegExp" | "WeakRef" | "Map" | "Set" | "WeakMap"
        | "WeakSet" | "Promise" | "Date" => generic(ctor, method),
        "URLSearchParams" => "Value of \"this\" must be of type URLSearchParams".to_string(),
        // Node's `URL` methods fail while reaching for their internal state, and
        // report the read that failed rather than the method.
        "URL" => "Cannot read properties of undefined (reading 'URL')".to_string(),
        _ => return None,
    })
}

/// Whether `<ctor>.prototype.<method>` begins with a `this<Type>Value` brand
/// check (21.1.3, 20.3.3, 22.1.3.29/.35, 21.2.3). Every `Number.prototype`
/// method does; of `String.prototype` only `toString`/`valueOf` do — the rest
/// are generic and coerce their receiver with `ToString`.
fn is_brand_checked_primitive_method(ctor: &str, method: &str) -> bool {
    match ctor {
        "Number" => matches!(
            method,
            "toString" | "toLocaleString" | "valueOf" | "toFixed" | "toExponential" | "toPrecision"
        ),
        "BigInt" => matches!(method, "toString" | "toLocaleString" | "valueOf"),
        "String" | "Boolean" => matches!(method, "toString" | "valueOf"),
        _ => false,
    }
}

/// `this<Type>Value(recv)` for `ctor` ∈ Number/String/Boolean/BigInt: the
/// primitive itself, the primitive a wrapper boxes, or — for the three
/// prototypes that are themselves wrappers (21.1.3, 22.1.3, 20.3.3) — the
/// prototype's own `+0` / `""` / `false`. `None` is the TypeError case.
fn this_primitive_value(ctor: &str, recv: &Value) -> Option<Value> {
    let expected = match ctor {
        "Number" => "number",
        "String" => "string",
        "Boolean" => "boolean",
        "BigInt" => "bigint",
        _ => return None,
    };
    let is_expected = |v: &Value| with_host(|h| h.type_of(v)) == expected;
    if is_expected(recv) {
        return Some(recv.clone());
    }
    if let Some(prim) = wrapped_primitive(recv).filter(is_expected) {
        return Some(prim);
    }
    if with_host(|h| h.intrinsic_proto_ctor(recv) == Some(ctor)) {
        return match ctor {
            "Number" => Some(Value::Float(0.0)),
            "String" => Some(with_host(|h| h.new_str(""))),
            "Boolean" => Some(Value::Bool(false)),
            _ => None,
        };
    }
    None
}

pub fn proto_method(recv: &Value, ctor_method: &str, args: Vec<Value>) -> Result<Value, String> {
    let (ctor, method) = ctor_method.split_once(':').unwrap_or(("", ctor_method));
    // A prototype ACCESSOR installed by `ensure_ctor_proto`: it reads or writes
    // the instance's hidden `@@<name>` slot, which is where the value lives now
    // that the public name is a getter rather than an own property.
    if let Some(key) = method.strip_prefix("@get@") {
        if let Some(v) = with_host(|h| match h.get(recv) {
            Some(JsObj::Object(p)) => p.get(&format!("@@{key}")).cloned(),
            _ => None,
        }) {
            return Ok(v);
        }
        // No stored slot: the value is COMPUTED, so ask the class. `KeyObject`'s
        // `symmetricKeySize` is the secret's byte length, which nothing stores.
        let tag = crate::stdlib::native_tag(recv).unwrap_or_default();
        return crate::stdlib::instance_call(&tag, recv, method, args);
    }
    if let Some(key) = method.strip_prefix("@set@") {
        let v = args.first().cloned().unwrap_or(Value::Undef);
        with_host(|h| {
            if let Some(JsObj::Object(p)) = h.get_mut(recv) {
                p.insert(format!("@@{key}"), v);
            }
        });
        crate::stdlib::instance_accessor_written(ctor, key, recv);
        return Ok(Value::Undef);
    }
    if with_host(|h| h.is_nullish(recv)) {
        let shown = if with_host(|h| h.is_null(recv)) {
            "null"
        } else {
            "undefined"
        };
        if let Some(msg) = nullish_receiver_error(ctor, method, shown) {
            return Err(format!("TypeError: {msg}"));
        }
    }
    // `Error.prototype.toString` (20.5.3.4): `name`, `message`, or `name:
    // message`, read off the chain so a subclass's `this.name = 'E'` is honored.
    if ctor == "Error" && method == "toString" {
        // A `DOMException` keeps its `name`/`message` in internal slots, so the
        // chain read below would find the class name on the prototype instead.
        if let Some(n) = dom_exception_slot(recv, "name") {
            let name = with_host(|h| h.str_of(&n));
            let msg = dom_exception_slot(recv, "message")
                .map(|m| with_host(|h| h.str_of(&m)))
                .unwrap_or_default();
            let s = if msg.is_empty() {
                name
            } else {
                format!("{name}: {msg}")
            };
            return Ok(with_host(|h| h.new_str(s)));
        }
        // `name` and `message` are read with `[[Get]]` (20.5.3.4 steps 3 and 5),
        // so a PROXY supplies them through its `get` trap. Reading the stored
        // ones first made `String(new Proxy(err, handler))` ignore the handler.
        let via_proxy = with_host(|h| h.kind_of(recv)) == Some(ObjKind::Proxy);
        let stored = (!via_proxy).then(|| with_host(|h| h.error_to_string(recv)));
        let s = match stored.flatten() {
            Some(s) => s,
            None => {
                let read = |k: &str| -> Result<Option<String>, String> {
                    Ok(host::protocol_lookup(recv, k)?.map(|v| with_host(|h| h.str_of(&v))))
                };
                let name = read("name")?.unwrap_or_else(|| "Error".into());
                let msg = read("message")?.unwrap_or_default();
                if msg.is_empty() {
                    name
                } else {
                    format!("{name}: {msg}")
                }
            }
        };
        return Ok(with_host(|h| h.new_str(s)));
    }
    // The methods that read their receiver through `thisNumberValue` /
    // `thisBooleanValue` / `thisStringValue` / `thisBigIntValue` accept only the
    // primitive, its wrapper, or the prototype object (which carries the zero
    // value) — anything else is a TypeError naming the method. Unchecked,
    // `Number.prototype.valueOf.call({})` answered `{}`, `toFixed.call({})`
    // reported "toFixed is not a function", and `Number.prototype.valueOf()`
    // recursed through the generic conversion until the stack overflowed.
    if is_brand_checked_primitive_method(ctor, method) {
        let Some(prim) = this_primitive_value(ctor, recv) else {
            return Err(format!(
                "TypeError: {ctor}.prototype.{method} requires that 'this' be a {ctor}"
            ));
        };
        return host::call_method(&prim, method, args);
    }
    // A primitive wrapper's `toString`/`valueOf`/`toLocaleString`: unwrap and
    // answer as the boxed primitive does. `Number.prototype.toString.call(5)`
    // arrives with an already-primitive receiver and needs no unwrapping.
    if matches!(ctor, "String" | "Number" | "Boolean") {
        let prim = wrapped_primitive(recv).unwrap_or_else(|| recv.clone());
        return host::call_method(&prim, method, args);
    }
    // `thisSymbolValue`/`thisBigIntValue` (20.4.3, 21.2.3) accept a WRAPPER as
    // readily as the primitive, and neither was unwrapped here. A BigInt
    // wrapper's `valueOf` therefore re-entered the generic conversion, which
    // looked `valueOf` up again and called it again: `+Object(9n)` recursed
    // until the stack overflowed and ABORTED the process, which no try/catch can
    // see. A Symbol wrapper failed the brand check below instead and reported
    // that `this` was not a Symbol, when it is one. Only a real wrapper is
    // unwrapped — `Symbol.prototype` itself boxes nothing and still has to reach
    // the brand check.
    if matches!(ctor, "Symbol" | "BigInt") {
        if let Some(prim) = wrapped_primitive(recv) {
            return host::call_method(&prim, method, args);
        }
    }
    if ctor == "Object" && method == "toString" {
        // Steps 16-17 of 20.1.3.6: a `Symbol.toStringTag` STRING on the receiver
        // (own or inherited, data property or getter) replaces the builtin brand,
        // which is how a class advertises its own (`class C { get
        // [Symbol.toStringTag]() { return 'Cee' } }` → `[object Cee]`). The read
        // runs outside the host borrow so an accessor can be invoked.
        // A Proxy has no chain to probe: 20.1.3.6 step 15 is an unconditional
        // `Get(O, @@toStringTag)`, so the `get` trap decides. Probing first (as
        // the ordinary receiver does, to keep the read off objects that have no
        // tag) would always miss and brand every tagged proxy `[object Object]`.
        if let Some(s) = to_string_tag(recv)? {
            return Ok(with_host(|h| h.new_str(format!("[object {s}]"))));
        }
        return Ok(with_host(|h| h.new_str(object_tag(h, recv))));
    }
    // These thunks now live on the real `Object.prototype` object, i.e. on the
    // receiver's own chain — routing back through `call_method` would re-resolve
    // this very thunk and recurse.
    if ctor == "Object" && is_object_builtin_method(method) {
        return object_builtin_method(recv, method, args);
    }
    // `EventEmitter.prototype.<m>` mixed onto a receiver (express's `app`): run the
    // emitter method directly against `recv` (routing back through `call_method`
    // would re-resolve the mixed-in thunk and recurse).
    if ctor == "EventEmitter" {
        return crate::stdlib::events::instance_call(recv, method, args);
    }
    // Same recursion hazard for the exotics with a real prototype object: the
    // thunk now lives ON the receiver's prototype chain, so `call_method` would
    // re-resolve this very thunk. Dispatch straight to the native instance
    // implementation when the receiver is in fact an instance of `ctor`.
    if ctor == "Buffer" && crate::stdlib::native_tag(recv).as_deref() == Some("Buffer") {
        return crate::stdlib::buffer::instance_call(recv, method, &args);
    }
    // The shared typed-array methods now live on the `%TypedArray%.prototype`
    // intermediate, so their thunks are tagged `TypedArray`; `Uint8Array` still
    // appears for anything read directly off `Uint8Array.prototype`. Both
    // dispatch the same way, and both must bypass `call_method` or the thunk
    // would re-resolve itself off the receiver's chain and recurse.
    if ctor == "Uint8Array" || ctor == "TypedArray" {
        match crate::stdlib::native_tag(recv).as_deref() {
            Some("Buffer") => return crate::stdlib::buffer::instance_call(recv, method, &args),
            Some("TypedArray") => {
                return crate::stdlib::typedarray::instance_call(recv, method, &args)
            }
            _ => {}
        }
    }
    // `Array.prototype.<m>.call(arrayLike)` — every `Array.prototype` method is
    // GENERIC over `this` (23.1.3: each starts with `ToObject(this)` and
    // `LengthOfArrayLike`), which is what makes
    // `Array.prototype.slice.call(arguments)` the idiom it is. The receiver here
    // is not an Array, so `call_method` would report the method missing.
    if ctor == "Array" && with_host(|h| h.kind_of(recv)) != Some(ObjKind::Array) {
        return array_generic(recv, method, args);
    }
    // The general form of the two special cases above: a thunk taken off a native
    // constructor's real prototype, invoked with a receiver that IS an instance of
    // that constructor. Routing back through `call_method` would re-resolve this
    // very thunk off the receiver's own chain and recurse forever, which is why
    // each such prototype needed a hand-written bypass; now they all have one.
    // A SUBCLASS counts: `SecretKeyObject` reaches `KeyObject.prototype.equals`
    // through its chain, and requiring an exact tag match sent that call back
    // into `call_method`, which re-resolved this same thunk and recursed until
    // the stack overflowed.
    if let Some(tag) = crate::stdlib::native_tag(recv) {
        let mut c = Some(tag.as_str());
        while let Some(t) = c {
            if t == ctor {
                return crate::stdlib::instance_call(&tag, recv, method, args);
            }
            c = crate::stdlib::native_parent(t);
        }
    }
    // A BRANDED method reached with a receiver that has no such internal slot.
    // Every arm above dispatches a receiver that IS an instance, so arriving
    // here with one of these constructors means the brand check failed — the
    // spec's very first step for each of them (24.2.3.x reads `[[SetData]]`,
    // 24.1.3.x `[[MapData]]`, 27.2.5.4 `[[PromiseState]]`, 23.2.3.x
    // `ValidateTypedArray`). Falling through to ordinary dispatch reported
    // `union is not a function`, which says the method does not exist rather
    // than that the receiver is the wrong kind of object.
    // `Date.prototype`'s methods split in two: the ones that read the time value
    // (`ThisTimeValue`, 21.4.4.x) report `this is not a Date object.`, and the
    // rest take the ordinary branded form. Measured on node v26.8.1:
    // `Date.prototype.getTime.call({})` is the first, `.toISOString.call({})`
    // and `.setHours.call({})` the second.
    if ctor == "Date" && crate::stdlib::native_tag(recv).as_deref() != Some("Date") {
        const THIS_TIME_VALUE: &[&str] = &[
            "getTime",
            "valueOf",
            "getYear",
            "getFullYear",
            "getMonth",
            "getDate",
            "getDay",
            "getHours",
            "getMinutes",
            "getSeconds",
            "getMilliseconds",
            "getUTCFullYear",
            "getUTCMonth",
            "getUTCDate",
            "getUTCDay",
            "getUTCHours",
            "getUTCMinutes",
            "getUTCSeconds",
            "getUTCMilliseconds",
            "getTimezoneOffset",
        ];
        if THIS_TIME_VALUE.contains(&method) {
            return Err(host::type_error("this is not a Date object."));
        }
        // `toJSON` (21.4.4.37) is deliberately generic — it converts the
        // receiver and INVOKES `toISOString` on it, so it fails on the missing
        // method rather than on a brand.
        if method != "toJSON" {
            return Err(host::type_error(&format!(
                "Method Date.prototype.{method} called on incompatible receiver {}",
                no_side_effects_string(recv)
            )));
        }
    }
    // `%TypedArray%.prototype`'s methods split the same way: `ValidateTypedArray`
    // (23.2.4.4) reports `this is not a typed array.`, while the handful that
    // check the receiver at the call boundary take the branded form. Measured
    // over all 27 shared methods on node v26.8.1; `toString` is the one that is
    // genuinely generic (it is `Array.prototype.toString`) and never brands.
    if matches!(ctor, "TypedArray" | "Uint8Array")
        && !matches!(
            crate::stdlib::native_tag(recv).as_deref(),
            Some("TypedArray") | Some("Buffer")
        )
    {
        const BRANDED: &[&str] = &[
            "slice",
            "subarray",
            "join",
            "sort",
            "at",
            "toReversed",
            "toSorted",
            "toLocaleString",
        ];
        if BRANDED.contains(&method) {
            return Err(host::type_error(&format!(
                "Method %TypedArray%.prototype.{method} called on incompatible receiver {}",
                no_side_effects_string(recv)
            )));
        }
        // The four base64/hex methods brand themselves against `Uint8Array`
        // specifically — a WRONG view is as incompatible as a plain object, and
        // the generic guard here cannot tell those apart.
        if crate::stdlib::typedarray::UINT8_PROTOTYPE_METHODS.contains(&method) {
            return Err(host::type_error(&format!(
                "Method Uint8Array.prototype.{method} called on incompatible receiver {}",
                no_side_effects_string(recv)
            )));
        }
        if method != "toString" {
            return Err(host::type_error("this is not a typed array."));
        }
    }
    // `Function.prototype.call`/`apply`/`bind` with a callable PROXY as `this`
    // (`pf.call(null, 4, 5)`, reached through the target's chain). Handing
    // that back to `call_method` read `call` off the proxy again, which
    // resolved to this same thunk, and recursed until the stack overflowed and
    // aborted the process. The three are defined on the callee alone, so they
    // run here: the proxy's `apply` trap (or its target) gets the call.
    // `toString` recursed the same way.
    if ctor == "Function"
        && matches!(method, "call" | "apply" | "bind" | "toString")
        && with_host(|h| h.kind_of(recv)) == Some(ObjKind::Proxy)
    {
        let mut rest = args.into_iter();
        let this_arg = rest.next().unwrap_or(Value::Undef);
        match method {
            "call" => return host::invoke(recv, rest.collect(), Some(this_arg)),
            "apply" => {
                let list = match rest.next() {
                    None | Some(Value::Undef) => Vec::new(),
                    Some(v) if with_host(|h| h.is_null(&v)) => Vec::new(),
                    Some(v) => create_list_from_array_like(&v)?,
                };
                return host::invoke(recv, list, Some(this_arg));
            }
            "bind" => {
                let target = recv.clone();
                let pre: Vec<Value> = rest.collect();
                return Ok(with_host(|h| {
                    h.alloc(JsObj::BoundFunc {
                        target,
                        this: this_arg,
                        args: pre,
                    })
                }));
            }
            // A proxy has no source text; V8 prints the native form for it.
            _ => return Ok(with_host(|h| h.new_str("function () { [native code] }"))),
        }
    }
    // `Symbol.prototype`'s methods are branded, and the receiver that reaches
    // them is very often NOT a symbol: `Symbol.prototype` itself is an ordinary
    // object. Without this check `Symbol.prototype.toString()` re-entered the
    // generic string conversion, which looked `toString` up again and called it
    // again — an infinite recursion that overflowed the stack and ABORTED the
    // process, which no `try`/`catch` can see. Node throws a plain TypeError.
    // The wording is Symbol's own, not the "incompatible receiver" form the
    // collections use.
    if ctor == "Symbol" && with_host(|h| h.kind_of(recv)) != Some(ObjKind::Symbol) {
        // A symbol-KEYED method is named in brackets rather than after a dot:
        // node's wording is `Symbol.prototype [ @@toPrimitive ] requires …`.
        // That is the message `String(Symbol.prototype)` produces, since the
        // conversion reaches `@@toPrimitive` before it would reach `toString`.
        let named = match method.strip_prefix("@@") {
            Some(sym) => format!("Symbol.prototype [ @@{sym} ]"),
            None => format!("Symbol.prototype.{method}"),
        };
        return Err(host::type_error(&format!(
            "{named} requires that 'this' be a Symbol"
        )));
    }
    if let Some(label) = branded_method_label(ctor, recv) {
        return Err(host::type_error(&format!(
            "Method {label}.prototype.{method} called on incompatible receiver {}",
            no_side_effects_string(recv)
        )));
    }
    host::call_method(recv, method, args)
}

/// The name a branded prototype method reports itself under when its receiver
/// fails the brand check, or `None` when `ctor`'s methods are generic over
/// `this` (every `Array.prototype` and `Object.prototype` method is) or the
/// receiver really is an instance.
///
fn branded_method_label(ctor: &str, recv: &Value) -> Option<&'static str> {
    let kind = with_host(|h| h.kind_of(recv));
    // `weak` is part of the brand: a `WeakSet` has `[[WeakSetData]]`, not
    // `[[SetData]]`, so `Set.prototype.has.call(new WeakSet())` is incompatible
    // even though both are `JsObj::Set` here.
    let weak = peek(recv, |o| match o {
        JsObj::Set { weak, .. } | JsObj::Map { weak, .. } => Some(*weak),
        _ => None,
    })
    .unwrap_or(false);
    let ok = match ctor {
        "Set" => kind == Some(ObjKind::Set) && !weak,
        "WeakSet" => kind == Some(ObjKind::Set) && weak,
        "Map" => kind == Some(ObjKind::Map) && !weak,
        "WeakMap" => kind == Some(ObjKind::Map) && weak,
        "Promise" => kind == Some(ObjKind::Promise),
        _ => return None,
    };
    if ok {
        return None;
    }
    Some(match ctor {
        "Set" => "Set",
        "WeakSet" => "WeakSet",
        "Map" => "Map",
        "WeakMap" => "WeakMap",
        _ => "Promise",
    })
}

/// V8's `Object::NoSideEffectsToString`, the rendering an engine-thrown message
/// uses for a value it must not run user code on. Measured on node v26.8.1
/// through `Map.prototype.get.call(x)`:
///
/// ```text
/// 5 / 'str' / true / null / undefined / 9n   the value's own ToString
/// Symbol('s')                                Symbol(s)
/// function f(){}                             its source text
/// new Error('e')                             Error: e
/// {} / new (class A {})                      #<Object> / #<A>
/// new Map() / Promise.resolve()              #<Map> / #<Promise>
/// [] / new Date() / /re/ / new Uint8Array()  [object Array] / [object Date] / …
/// { toString() {} } / Object.create(null)    [object Object]
/// ```
///
/// The split is one test: a receiver whose `toString` is still
/// `Object.prototype.toString` prints `#<Constructor>`, and any other receiver
/// prints what the BUILTIN brand would be — V8 never calls the user's method,
/// which is why an object with its own `toString` prints `[object Object]` and
/// not what that method returns.
fn no_side_effects_string(recv: &Value) -> String {
    if with_host(|h| host::is_primitive(h, recv)) || with_host(|h| host::is_callable(h, recv)) {
        return with_host(|h| h.str_of(recv));
    }
    if let Some(s) = with_host(|h| h.error_to_string(recv)) {
        return s;
    }
    // `native_tag` re-enters the host, so it is read BEFORE the borrow below
    // rather than inside it.
    let native = crate::stdlib::native_tag(recv).is_some();
    let brands_itself = with_host(|h| {
        // `Object.prototype.toString` reaches every object as a thunk on the
        // real prototype object, so its presence proves nothing; only a
        // toString the receiver's chain OVERRIDES it with counts.
        let overridden = host::lookup_chain(h, recv, "toString")
            .map(|f| !matches!(h.get(&f), Some(JsObj::Builtin(n)) if n == "@proto:Object:toString"))
            .unwrap_or(false);
        native
            || overridden
            || h.has_null_proto(recv)
            || !matches!(
                h.kind_of(recv),
                Some(ObjKind::Object)
                    | Some(ObjKind::Map)
                    | Some(ObjKind::Set)
                    | Some(ObjKind::Promise)
            )
    });
    if brands_itself {
        return with_host(|h| object_tag(h, recv));
    }
    let ctor = get_property(recv, "constructor")
        .ok()
        .map(|c| with_host(|h| h.callable_name(&c)))
        .filter(|n| !n.is_empty())
        .unwrap_or_else(|| "Object".to_string());
    format!("#<{ctor}>")
}

/// The value of `v[Symbol.toStringTag]` for a builtin that genuinely carries
/// one, or `None` when reading that symbol must yield `undefined`.
///
/// Every builtin brand is already computed in exactly one place (`object_tag`),
/// so this reuses it and subtracts the legacy builtins, which brand for
/// `Object.prototype.toString` but expose no `Symbol.toStringTag` property.
/// The subtracted list is measured against node v26.7.0, not assumed: `[]`,
/// `function(){}`, `{}`, `new Date()`, `/x/` and `new Error()` all read
/// `undefined`, while `Map`/`Set`/`Promise`/typed arrays/`ArrayBuffer`/
/// `DataView`/`WeakRef`/`FinalizationRegistry`/`BigInt`/`Symbol`/generators/
/// async+generator functions/`Math`/`JSON`/`Reflect`/`URL`/`URLSearchParams`/
/// `TextEncoder`/`TextDecoder` all read their brand.
pub(crate) fn well_known_tag(h: &host::JsHost, v: &Value) -> Option<String> {
    // A primitive never carries the symbol except a BigInt/Symbol wrapper, both
    // of which `object_tag` already brands.
    let tag = object_brand(h, v);
    const NO_TAG: &[&str] = &[
        "Undefined",
        "Null",
        "Boolean",
        "Number",
        "String",
        "Array",
        "Function",
        "Object",
        "Date",
        "RegExp",
        "Error",
    ];
    if NO_TAG.contains(&tag.as_str()) {
        return None;
    }
    Some(tag)
}

/// The constructor name of the nearest intrinsic prototype on `v`'s chain that
/// carries an own `Symbol.toStringTag`, if any.
fn chain_tag_ctor(h: &host::JsHost, v: &Value) -> Option<String> {
    let mut cur = h.proto_of(v);
    for _ in 0..100 {
        let p = cur?;
        if h.is_null(&p) {
            return None;
        }
        let name = match h.get(&p) {
            Some(JsObj::Builtin(ns)) => ns.strip_suffix(".prototype").map(str::to_string),
            _ => h.intrinsic_proto_ctor(&p).map(str::to_string),
        }
        // A CLASS prototype is not linked to the builtin its class extends —
        // the relationship lives on the class value — so the walk crosses over
        // there, or `Object.create(D.prototype)` for `class D extends Map`
        // finds nothing.
        .or_else(|| {
            h.class_owning_proto(&p)
                .and_then(|c| h.class_builtin_ancestor(&c))
                .map(|b| h.callable_name(&b))
                .filter(|n| !n.is_empty())
        });
        if let Some(n) = name {
            if intrinsic_proto_members(&format!("{n}.prototype"))
                .is_some_and(|ms| ms.contains(&"@@toStringTag"))
            {
                return Some(n);
            }
        }
        cur = h.proto_of(&p);
    }
    None
}

/// The `Object.prototype.toString` brand tag for `v` (`[object Array]` etc.).
/// Every builtin exotic object reports its own brand, which is how packages
/// type-test values they did not construct (`toString.call(x) ===
/// '[object Uint8Array]'`). A `Buffer` reports `Uint8Array` because in Node it
/// IS a `Uint8Array` subclass and inherits that `Symbol.toStringTag`.
pub(crate) fn object_tag(h: &host::JsHost, v: &Value) -> String {
    format!("[object {}]", object_brand(h, v))
}

/// The bare brand name behind `Object.prototype.toString` (`Array`, `Uint8Array`
/// …), without the `[object …]` wrapper. Split out so the brand and the
/// `Symbol.toStringTag` property read cannot disagree about what a value is.
/// Whether `v` is a function's `arguments` object.
///
/// Backed by an Array so indices, `length`, spread and `for-of` all work, but
/// marked so it does not pass for one: node's is an exotic, and `isArray`, the
/// brand and `util.types.isArgumentsObject` all have to tell them apart.
pub fn is_arguments(v: &Value) -> bool {
    with_host(|h| is_arguments_h(h, v))
}

/// `is_arguments` for a caller that already holds the host borrow — `object_brand`
/// runs under one, and re-entering through `with_host` aborts the process.
pub fn is_arguments_h(h: &host::JsHost, v: &Value) -> bool {
    h.fn_prop(v, "@@arguments").is_some()
}

fn object_brand(h: &host::JsHost, v: &Value) -> String {
    // A `<C>.prototype` this host built as a real object is an ORDINARY object:
    // it holds no instance slot, so only the branded few report anything but
    // `[object Object]`. Checked before the match because those prototypes are
    // plain `JsObj::Object`s and would otherwise be branded by whatever their
    // own properties happen to look like — `TypeError.prototype` has `name` and
    // `message`, which read as an Error instance.
    if let Some(ctor) = h.intrinsic_proto_ctor(v) {
        return if BRANDED_PROTOS.contains(&ctor) {
            ctor.to_string()
        } else {
            "Object".to_string()
        };
    }
    let tag: String = match v {
        Value::Undef => "Undefined".into(),
        Value::Bool(_) => "Boolean".into(),
        Value::Int(_) | Value::Float(_) => "Number".into(),
        Value::Str(_) => "String".into(),
        Value::Obj(_) => match h.get(v) {
            Some(JsObj::Null) => "Null".into(),
            Some(JsObj::Str(_)) => "String".into(),
            Some(JsObj::Array(_)) if is_arguments_h(h, v) => "Arguments".into(),
            Some(JsObj::Array(_)) => "Array".into(),
            // A lazy iterator helper brands as node does.
            Some(JsObj::Object(p))
                if p.get("@@native").map(|t| h.str_of(t)).as_deref() == Some("IteratorHelper") =>
            {
                "Iterator Helper".into()
            }
            // A `DOMException` brands by its class, not as a plain `Error`.
            Some(JsObj::Object(p)) if p.contains_key("@@domName") => "DOMException".into(),
            // 20.1.3.6 steps 5-8 brand a wrapper by its internal slot, so
            // `Object.prototype.toString.call(new Number(1))` is
            // `[object Number]` rather than `[object Object]`.
            Some(JsObj::Object(p)) if p.contains_key("@@primitive") => match p["@@primitive"] {
                Value::Bool(_) => "Boolean".into(),
                Value::Int(_) | Value::Float(_) => "Number".into(),
                _ => "String".into(),
            },
            // 20.1.3.6 step 3 brands by `IsArray`, which follows a Proxy to its
            // `[[ProxyTarget]]` — `Object.prototype.toString.call(new Proxy([],
            // {}))` is `'[object Array]'`. Everything else about a proxy brands
            // as a plain Object (a `Symbol.toStringTag` read through the `get`
            // trap is handled by the caller, before this).
            Some(JsObj::Proxy { target, .. }) => {
                let mut cur = target;
                for _ in 0..100 {
                    match h.get(cur) {
                        Some(JsObj::Proxy { target: t, .. }) => cur = t,
                        _ => break,
                    }
                }
                match h.get(cur) {
                    Some(JsObj::Array(_)) => "Array".into(),
                    _ => "Object".into(),
                }
            }
            // `function*` / `async function` / `async function*` carry their own
            // `Symbol.toStringTag` in V8 (27.3.3.2, 27.7.3.2, 27.4.3.2).
            Some(JsObj::Func(f)) => match h.funcs.get(f.def_id) {
                Some(d) if d.is_generator && d.is_async => "AsyncGeneratorFunction".into(),
                Some(d) if d.is_generator => "GeneratorFunction".into(),
                Some(d) if d.is_async => "AsyncFunction".into(),
                _ => "Function".into(),
            },
            // `Math`/`JSON`/`Reflect` are namespace OBJECTS, not callables, and
            // brand by name (21.3.1.9, 25.5.3, 28.1.14).
            Some(JsObj::Builtin(n)) if matches!(n.as_str(), "Math" | "JSON" | "Reflect") => {
                n.clone()
            }
            // A `<Ctor>.prototype` object brands as the constructor it belongs
            // to — `Object.prototype.toString.call(Set.prototype)` is
            // `[object Set]` — and a `require()`d module namespace is a plain
            // object. Neither is a function, so neither brands as one.
            Some(JsObj::Builtin(n)) if !host::builtin_is_callable(n) => {
                match n.strip_suffix(".prototype") {
                    Some(ctor) if BRANDED_PROTOS.contains(&ctor) => ctor.to_string(),
                    _ => "Object".into(),
                }
            }
            Some(JsObj::Class(_))
            | Some(JsObj::Builtin(_))
            | Some(JsObj::BoundFunc { .. })
            | Some(JsObj::BoundMethod { .. }) => "Function".into(),
            // A suspended generator object is `[object Generator]`; an async one
            // `[object AsyncGenerator]`.
            Some(JsObj::Generator { .. }) if h.is_async_gen_val(v) => "AsyncGenerator".into(),
            Some(JsObj::Generator { .. }) => "Generator".into(),
            Some(JsObj::RegExp(_)) => "RegExp".into(),
            Some(JsObj::Map { weak, .. }) => if *weak { "WeakMap" } else { "Map" }.into(),
            Some(JsObj::Set { weak, .. }) => if *weak { "WeakSet" } else { "Set" }.into(),
            Some(JsObj::Promise { .. }) => "Promise".into(),
            Some(JsObj::Symbol { .. }) => "Symbol".into(),
            Some(JsObj::BigInt(_)) => "BigInt".into(),
            // Native-tagged instances brand by their tag; a typed array brands by
            // its element kind (`@@kind`), and every Error subclass is `Error`.
            Some(JsObj::Object(p)) => match p.get("@@native").map(|t| h.str_of(t)).as_deref() {
                Some("TypedArray") => p
                    .get("@@kind")
                    .map(|k| h.str_of(k))
                    .unwrap_or_else(|| "Uint8Array".into()),
                Some("Buffer") => "Uint8Array".into(),
                // Every native class that really carries a `Symbol.toStringTag`
                // in Node brands by its own name. Verified against node v26:
                // `Object.prototype.toString.call(new WeakRef({}))` is
                // `[object WeakRef]`. The rest of the `@@native` tags
                // (`EventEmitter`, `Server`, `Hash`, `Readable`, …) are plain
                // classes with NO tag, so they stay `[object Object]` — listing
                // them here would invent a brand Node does not have.
                Some(
                    t @ ("ArrayBuffer"
                    | "DataView"
                    | "Date"
                    | "WeakRef"
                    | "FinalizationRegistry"
                    | "TextEncoder"
                    | "TextDecoder"
                    | "URL"
                    | "URLSearchParams"),
                ) => t.into(),
                _ if has_error_data(h, v) => "Error".into(),
                _ => "Object".into(),
            },
            _ => "Object".into(),
        },
        // node-js only produces the Value variants above; fusevm's shell-oriented
        // variants never arise here.
        _ => "Object".into(),
    };
    // Nothing about the value itself brands it. An ordinary object whose CHAIN
    // reaches an intrinsic prototype carrying an own `Symbol.toStringTag`
    // borrows that one: 20.1.3.6 step 15 is a `Get`, which walks.
    // `Object.prototype.toString.call(Object.create(Map.prototype))` is
    // `[object Map]` and was `[object Object]`.
    //
    // Only as a FALLBACK, and only for the prototypes that REALLY carry the
    // symbol. A typed array reaches `%TypedArray%.prototype`, whose tag is an
    // ACCESSOR returning the specific kind, so consulting the chain FIRST
    // branded every view `[object TypedArray]` instead of `[object Uint8Array]`
    // — three records caught it. `Error.prototype` carries no tag at all, so
    // inheriting from it borrows nothing.
    if tag == "Object" && !has_error_data(h, v) {
        if let Some(ctor) = chain_tag_ctor(h, v) {
            return ctor;
        }
    }
    tag
}

fn b_setattr(vm: &mut VM, _: u8) -> Value {
    let val = vm.pop();
    let name = sval(&vm.pop());
    let recv = vm.pop();
    if let Err(e) = set_property(&recv, &name, val.clone()) {
        return abort(vm, e);
    }
    val
}

/// `NAMED_EVAL` — SetFunctionName (10.2.9) for a function whose name is only
/// known at run time, i.e. one defined under a COMPUTED key: `{ [k]: () => {} }`,
/// `class C { static [k] = function(){} }`.
///
/// The compiler emits this ONLY where the grammar says NamedEvaluation applies
/// (`IsAnonymousFunctionDefinition` is a syntactic predicate, not a runtime one:
/// `{ m: someAlreadyAnonymousFn }` must NOT be renamed), so the name is set
/// unconditionally here.
///
/// A symbol key becomes `[description]` per step 2 of SetFunctionName; `kind`
/// contributes the accessor prefix, so `{ get [k](){} }` is `get <key>`.
fn b_named_eval(vm: &mut VM, _: u8) -> Value {
    let func = vm.pop();
    let kind = vm.pop().to_int();
    let key = vm.pop();
    let key = sval(&key);
    // `@@sym:<id>` / `@@iterator` — an internal symbol key. Step 2: an empty
    // description gives the empty name, not `[undefined]`.
    let base = match with_host(|h| h.symbol_of_key(&key)) {
        Some(sym) => match with_host(|h| h.get(&sym).cloned()) {
            Some(JsObj::Symbol {
                desc: Some(desc), ..
            }) => format!("[{desc}]"),
            _ => String::new(),
        },
        None => key,
    };
    let name = match kind {
        host::member::GET => format!("get {base}"),
        host::member::SET => format!("set {base}"),
        _ => base,
    };
    with_host(|h| {
        let s = h.new_str(name);
        h.set_fn_prop(&func, "name", s);
    });
    func
}

/// `[[Set]]` reachable from `crate::proxy`'s no-trap forward, which has to land
/// on the same path a plain `o.k = v` takes.
pub fn set_property_pub(recv: &Value, name: &str, val: Value) -> Result<(), String> {
    set_property(recv, name, val)
}

/// An object's OWN property as `(value, writable, configurable, is_accessor)`,
/// or `None` when it has none. Reads through a Proxy's
/// `getOwnPropertyDescriptor` trap, so it answers for any object.
pub fn own_prop_facts(obj: &Value, key: &str) -> Option<(Value, bool, bool, bool)> {
    let k = with_host(|h| h.new_str(key.to_string()));
    let d = own_descriptor_pub(obj, k).ok()?;
    if matches!(d, Value::Undef) {
        return None;
    }
    let field = |n: &str| get_property(&d, n).unwrap_or(Value::Undef);
    // Each read is hoisted out of the `with_host` borrow: `get_property` takes
    // the host itself, so reading inside the closure double-borrows.
    let value = field("value");
    let writable = field("writable");
    let configurable = field("configurable");
    let truthy = |v: &Value| with_host(|h| h.truthy(v));
    let is_accessor = with_host(|h| host::lookup_chain(h, &d, "get").is_some());
    Some((value, truthy(&writable), truthy(&configurable), is_accessor))
}

/// `OrdinarySetWithOwnDescriptor` (10.1.9.2) with a receiver distinct from the
/// object the lookup started on — what `Reflect.set(t, k, v, receiver)` and a
/// proxy `set` trap forwarding to it both need.
///
/// The distinction that matters: an accessor found on `target`'s chain RUNS,
/// with `receiver` as `this`; a data property does not write to `target` at all
/// but is CREATED on `receiver` through its `[[DefineOwnProperty]]`. Routing
/// that second case back through `[[Set]]` made a proxy receiver re-enter its
/// own `set` trap forever — the trap body `Reflect.set(t, k, v, recv)` is the
/// documented way to forward a write, so the recursion hit every faithful
/// handler.
pub fn set_with_receiver(
    target: &Value,
    key: &str,
    val: Value,
    receiver: &Value,
) -> Result<bool, String> {
    // A proxy target answers through its own trap, which re-enters here with
    // whatever receiver the handler passes on.
    if crate::proxy::parts(target).is_some() {
        return crate::proxy::set(target, key, &val, receiver);
    }
    // An accessor anywhere on the target's chain wins, and sees `receiver`.
    if let Some((_, setter)) = with_host(|h| host::lookup_accessor(h, target, key)) {
        return match setter {
            Some(s) => {
                host::invoke(&s, vec![val], Some(receiver.clone()))?;
                Ok(true)
            }
            // A getter with no setter refuses the write rather than shadowing it.
            None => Ok(false),
        };
    }
    if !with_host(|h| h.can_write_prop(target, key)) {
        return Ok(false);
    }
    // Steps 3.b-3.d: only an object can receive the property, and its OWN
    // property decides — an accessor or a read-only slot refuses, and every
    // other case defines a plain data property.
    //
    // `is_object_like`, not a shape test: a string, a symbol and a bigint are
    // PRIMITIVES that ride as `Value::Obj` handles here, so the shape check
    // passed them through to `defineProperty`, which then threw `called on
    // non-object` where 10.1.9.2 step 3.b simply reports `false`.
    if !with_host(|h| is_object_like(h, receiver)) {
        return Ok(false);
    }
    if let Some((_, writable, _, is_accessor)) = own_prop_facts(receiver, key) {
        if is_accessor || !writable {
            return Ok(false);
        }
    }
    // Steps 3.d.iii and 3.e both DEFINE, they do not assign: a setter inherited
    // by the receiver must not run, and a proxy receiver must reach its
    // `defineProperty` trap rather than its `set` trap.
    let desc = with_host(|h| {
        let mut m: IndexMap<String, Value> = IndexMap::new();
        m.insert("value".into(), val);
        m.insert("writable".into(), Value::Bool(true));
        m.insert("enumerable".into(), Value::Bool(true));
        m.insert("configurable".into(), Value::Bool(true));
        h.new_object(m)
    });
    if crate::proxy::parts(receiver).is_some() {
        return crate::proxy::define_property(receiver, key, &desc);
    }
    let k = with_host(|h| h.new_str(key.to_string()));
    define_property_pub(receiver, k, desc)?;
    Ok(true)
}

/// Whether the first argument is a PRIMITIVE — including the three that ride as
/// heap handles, which a shape test misses.
fn is_primitive_arg(args: &[Value]) -> bool {
    let v = arg0(args);
    with_host(|h| host::is_primitive(h, &v))
}

/// The `TypeError` a refused write raises in strict code, worded as V8 does.
///
/// Adding a key to a non-extensible object reports differently from assigning
/// to a read-only one, and the object is named by its brand — `#<Object>` for a
/// plain object, `[object Array]` for an array.
fn write_refused(recv: &Value, name: &str) -> String {
    let extensible = with_host(|h| h.is_extensible(recv));
    // Which of the two messages applies turns on whether the key already
    // EXISTS. Every shape that keeps its own properties in the fn-prop side
    // table answered a blanket `true` here, so adding a key to a frozen
    // function reported "read only" where node reports "not extensible".
    let has_own = with_host(|h| match h.get(recv) {
        Some(JsObj::Object(p)) => p.contains_key(name),
        Some(JsObj::Array(items)) => {
            name.parse::<usize>()
                .map(|i| i < items.len())
                .unwrap_or(false)
                || h.fn_prop(recv, name).is_some()
        }
        Some(JsObj::RegExp(_)) => name == "lastIndex" || h.fn_prop(recv, name).is_some(),
        _ => h.fn_prop(recv, name).is_some(),
    });
    if !extensible && !has_own {
        return host::type_error(&format!(
            "Cannot add property {name}, object is not extensible"
        ));
    }
    // The receiver renders the way every other brand-check message renders one
    // — `#<Object>`, `[object Array]`, `[object RegExp]`, `#<Map>`, `#<C>` for a
    // class instance, `Error: m` for an error. Only Array was special-cased, so
    // every other exotic reported `#<Object>`.
    host::type_error(&format!(
        "Cannot assign to read only property '{name}' of object '{}'",
        no_side_effects_string(recv)
    ))
}

fn set_property(recv: &Value, name: &str, val: Value) -> Result<(), String> {
    // 6.2.5.6 `PutValue` begins with `RequireObjectCoercible`: writing any
    // property of `undefined` or `null` throws, naming the key. Every such
    // write was silently discarded, so `u.x = 1` — the mirror of the single
    // most common runtime fault in JS, which the READ side already reports —
    // looked like it had succeeded.
    if with_host(|h| h.is_nullish(recv)) {
        return Err(host::type_error(&format!(
            "Cannot set properties of {} (setting '{name}')",
            with_host(|h| h.str_of(recv))
        )));
    }
    // A write to a PRIMITIVE receiver has no target — `ToObject` makes a
    // throwaway wrapper — so it is discarded in sloppy code and throws in
    // strict (10.1.9.2 / 6.2.5.6 again). The refusal was silent in both.
    // `is_primitive` rather than a shape test: a string, a symbol and a bigint
    // ride as `Value::Obj` handles in this host, so a check for a non-`Obj`
    // value caught only numbers and booleans.
    if with_host(|h| host::is_primitive(h, recv)) && with_host(|h| h.current_strict()) {
        return Err(host::type_error(&format!(
            "Cannot create property '{name}' on {} '{}'",
            with_host(|h| h.type_of(recv)),
            with_host(|h| h.str_of(recv))
        )));
    }
    // `[[PrivateSet]]` (7.3.32) refuses a receiver that carries no such private
    // element. The class's own field initializers install theirs directly
    // (`host::init_one_field`), so a declaration never reaches this check.
    if name.starts_with('#') && !with_host(|h| h.has_private(recv, name)) {
        return Err(private_brand_message(name, true));
    }
    // `[[Set]]` on a Proxy: the handler's `set` trap, or a forward to the target.
    if with_host(|h| h.kind_of(recv)) == Some(ObjKind::Proxy) {
        // A `set` trap that returns falsish REFUSED the write: silent in sloppy
        // code, a TypeError in strict, exactly as an ordinary refused write is.
        if crate::proxy::set(recv, name, &val, recv)? {
            return Ok(());
        }
        if with_host(|h| h.current_strict()) {
            return Err(host::type_error(&format!(
                "'set' on proxy: trap returned falsish for property '{name}'"
            )));
        }
        return Ok(());
    }
    // `globalThis.x = 1` creates a real global binding, so the bare `x` reads it
    // back. Writing only the own property left the two views disagreeing:
    // `globalThis.zz` was 7 while `zz` was still a `ReferenceError`.
    if with_host(|h| h.is_global_object(recv)) && !name.starts_with("@@") {
        with_host(|h| h.set_name(name, val.clone()));
    }
    // `obj.__proto__ = p` re-links the prototype — but only for the two values
    // the Annex B setter accepts, an Object or `null`. Everything else is a
    // silent no-op in Node (`o.__proto__ = 5` leaves `Object.getPrototypeOf(o)`
    // untouched and creates no own key), and a null-prototype object inherits
    // no such setter at all, so there the assignment is an ORDINARY own
    // property write. Re-linking unconditionally made `o.__proto__ = 5` set the
    // prototype to the number 5.
    if name == "__proto__" && with_host(|h| h.kind_of(recv)) == Some(ObjKind::Object) {
        if with_host(|h| h.has_null_proto(recv)) {
            // falls through to the ordinary own-property write below
        } else {
            let assignable =
                with_host(|h| h.is_null(&val) || matches!(h.kind_of(&val), Some(ObjKind::Object)));
            if assignable {
                // The `__proto__` setter runs `[[SetPrototypeOf]]`, which a
                // NON-EXTENSIBLE object refuses — and unlike an ordinary
                // refused write, the setter throws in sloppy code too. It was
                // rewriting the link of a frozen object.
                if would_cycle(recv, &val) {
                    return Err(host::type_error("Cyclic __proto__ value"));
                }
                if !with_host(|h| h.is_extensible(recv)) && !same_prototype(recv, &val) {
                    return Err(host::type_error(&format!(
                        "{} is not extensible",
                        no_side_effects_string(recv)
                    )));
                }
                with_host(|h| h.set_proto(recv, val));
            }
            return Ok(());
        }
    }
    // Every environment value is a STRING. `process.env.PORT = 8080` stores
    // "8080", so `process.env.PORT + 1` concatenates the way it does in a real
    // process; storing the number made it add instead.
    if !name.starts_with("@@")
        && with_host(
            |h| matches!(h.get(recv), Some(JsObj::Object(p)) if p.contains_key("@@envObject")),
        )
    {
        let text = with_host(|h| h.str_of(&val));
        // Write THROUGH to the real environment as well. `process.env` is not a
        // private map: node applies the change to the process, so a child
        // spawned afterwards inherits it. Keeping it only in the JS object meant
        // `process.env.NODE_ENV = 'production'` was invisible to every
        // `spawnSync`/`execSync` that followed.
        std::env::set_var(name, &text);
        let sv = with_host(|h| h.new_str(text));
        with_host(|h| {
            if let Some(JsObj::Object(p)) = h.get_mut(recv) {
                p.insert(name.to_string(), sv);
            }
        });
        return Ok(());
    }
    // Assigning `e.stack` wins permanently: drop the not-yet-formatted marker so
    // no later read re-derives a header over the top of the assigned value.
    if name == "stack" {
        with_host(|h| {
            if let Some(JsObj::Object(p)) = h.get_mut(recv) {
                p.shift_remove("@@stackRaw");
            }
        });
    }
    // An inherited/own setter accessor intercepts the write. This is checked
    // BEFORE the writable test because 10.1.9.2 branches on the descriptor
    // kind first: `writable` is a data-property attribute and means nothing on
    // an accessor, where the setter alone decides. Testing it first meant an
    // accessor defined through `Object.defineProperty` — which leaves
    // `writable` false, having no such field — silently swallowed every write
    // instead of calling its setter, so the standard clone idiom
    // `Object.create(proto, Object.getOwnPropertyDescriptors(src))` produced an
    // object whose setters did nothing. An accessor from an object literal
    // carries all-true attributes, which is why only the former broke.
    if let Some((getter, setter)) = with_host(|h| host::lookup_accessor(h, recv, name)) {
        if let Some(setter) = setter {
            let _ = host::invoke(&setter, vec![val], Some(recv.clone()));
            return Ok(());
        }
        // Only a getter: the write is refused — silent in sloppy mode, a
        // TypeError in strict code. The `return` above matters, since a
        // successful setter call must not fall into this.
        let _ = getter;
        if with_host(|h| h.current_strict()) {
            return Err(host::type_error(&format!(
                "Cannot set property {name} of #<Object> which has only a getter"
            )));
        }
        return Ok(());
    }
    // A non-writable property, or a new key on a non-extensible object, refuses
    // the write. In SLOPPY mode that is silent; in strict code it is a
    // TypeError, and the ASSIGNMENT SITE decides which — not the object. Every
    // refusal used to be silent, so `'use strict'` did not catch a write to a
    // frozen object, which is most of the reason to freeze one.
    if !with_host(|h| h.can_write_prop(recv, name)) {
        if with_host(|h| h.current_strict()) {
            return Err(write_refused(recv, name));
        }
        return Ok(());
    }
    // Writing `name`/`prototype`/statics on a function value.
    if matches!(
        with_host(|h| h.kind_of(recv)),
        Some(ObjKind::Func) | Some(ObjKind::Class)
    ) {
        with_host(|h| h.set_fn_prop(recv, name, val));
        return Ok(());
    }
    // Writing a static onto a builtin namespace/ctor (`Error.prepareStackTrace`).
    // Each bare reference is a fresh `Builtin` handle, so route to the stable
    // per-namespace side table rather than the per-index `fn_props`.
    if let Some(ns) = peek(recv, |o| match o {
        JsObj::Builtin(ns) => Some(ns.clone()),
        _ => None,
    }) {
        // `process.exitCode` is an accessor in Node, not a data property: the
        // setter validates and stores the code the process will finally exit
        // with. Landing it in the generic static table made it a write-only
        // decoration — `process.exitCode = 3` read back as 3 and the process
        // still exited 0.
        if ns == "process" && name == "exitCode" {
            return crate::stdlib::process::set_exit_code(&val);
        }
        with_host(|h| h.set_builtin_static(&ns, name, val));
        return Ok(());
    }
    // A write onto a REAL intrinsic prototype object (`Object.prototype`,
    // `String.prototype`, `TypeError.prototype`) is mirrored into the
    // per-namespace side table as well as the object's own map. Instances are
    // not linked to these objects by `proto_of` — the chain walk never reaches
    // them — so the mirror is what makes `String.prototype.pad = f` visible as
    // `"x".pad`. The own-map write below still happens, so reading the
    // prototype itself and enumerating it keep working unchanged.
    if let Some(ns) = with_host(|h| {
        h.intrinsic_proto_ctor(recv)
            .map(str::to_string)
            .or_else(|| (h.object_proto() == *recv).then(|| "Object".to_string()))
    }) {
        with_host(|h| h.set_builtin_static(&format!("{ns}.prototype"), name, val.clone()));
    }
    // `re.lastIndex = n` on a RegExp advances/resets its match cursor. The
    // writability check above already refused it on a FROZEN regexp, which it
    // could only do once `integrity_keys` learned that `lastIndex` is an own
    // property.
    if name == "lastIndex" {
        if let Some(n) = with_host(|h| match h.get(recv) {
            Some(JsObj::RegExp(_)) => Some(h.to_number(&val)),
            _ => None,
        }) {
            with_host(|h| {
                if let Some(JsObj::RegExp(r)) = h.get_mut(recv) {
                    r.last_index = if n.is_finite() && n >= 0.0 {
                        crate::utf16::U16Index::new(n as usize)
                    } else {
                        crate::utf16::U16Index::ZERO
                    };
                }
            });
            return Ok(());
        }
    }
    // An `arguments` object is an ORDINARY object with a `length` data property,
    // not an array: a write PAST the end adds an index and leaves `length`
    // alone. The array backing grew it instead, so `f(1)` followed by
    // `arguments[1] = 9` reported `arguments.length` as 2.
    if let Ok(i) = name.parse::<usize>() {
        if is_arguments(recv) && i >= array_len(recv) {
            with_host(|h| h.set_fn_prop(recv, name, val));
            return Ok(());
        }
    }
    // Typed-array element write (`ta[i] = v`): coerce + store into `@@elems`.
    if !name.is_empty() && name.bytes().all(|b| b.is_ascii_digit()) {
        let is_ta = crate::stdlib::native_tag(recv).as_deref() == Some("TypedArray");
        if is_ta && crate::stdlib::typedarray::elem_set(recv, name, &val)? {
            return Ok(());
        }
        // An index write to a view over a DETACHED buffer is DROPPED. Falling
        // through would store it as an ordinary own property, which then showed
        // up in `getOwnPropertyDescriptor` over a buffer with no bytes.
        if is_ta && crate::stdlib::typedarray::view_detached(recv) {
            return Ok(());
        }
        // `buf[i] = n` writes through to the Buffer's hidden byte array.
        if crate::stdlib::buffer::byte_set(recv, name, &val) {
            return Ok(());
        }
    }
    // Any own property on an exotic with no property map of its own. This sits
    // BELOW the exotic-specific writes above, so a RegExp's `lastIndex` still
    // moves its match cursor rather than being shadowed by a side-table entry.
    if uses_side_table(recv) {
        with_host(|h| h.set_fn_prop(recv, name, val));
        return Ok(());
    }
    // An arbitrary own prop on an array (e.g. exec-result `.index`/`.input`).
    if with_host(|h| h.kind_of(recv)) == Some(ObjKind::Array)
        && name != "length"
        && name.parse::<usize>().is_err()
    {
        with_host(|h| h.set_fn_prop(recv, name, val));
        return Ok(());
    }
    // `arr.length = n` (10.4.2.4 `ArraySetLength`) validates BEFORE it resizes,
    // and does so outside the host borrow because `ToNumber` may run a user
    // `valueOf`. An invalid length throws instead of being silently coerced to 0.
    let new_len = if name == "length" && with_host(|h| h.kind_of(recv)) == Some(ObjKind::Array) {
        let want = host::to_array_length(&val)?;
        // 10.4.2.4 steps 15-17: shrinking deletes from the END downwards and
        // STOPS at the first element that cannot be deleted, leaving the length
        // just past it. Truncating regardless discarded a non-configurable
        // element and reported a length node would not have accepted.
        let floor = with_host(|h| {
            let old = match h.get(recv) {
                Some(JsObj::Array(items)) => items.len(),
                _ => 0,
            };
            let mut stop = want;
            for i in (want..old).rev() {
                if !h.prop_attrs(recv, &i.to_string()).configurable {
                    stop = i + 1;
                    break;
                }
            }
            stop
        });
        Some(floor.max(want))
    } else {
        None
    };
    with_host(|h| match h.get_mut(recv) {
        Some(JsObj::Object(props)) => {
            // Adding a *new* array-index key must re-place it into ascending
            // integer-key order (updating an existing key keeps its position).
            let is_new = !props.contains_key(name);
            props.insert(name.to_string(), val);
            if is_new && host::array_index(name).is_some() {
                host::canonicalize_own_keys(props);
            }
        }
        Some(JsObj::Array(items)) => {
            if let Some(n) = new_len {
                // Growing `length` appends HOLES (`a=[1]; a.length=3` still has
                // just the one own key); shrinking drops any hole past the end.
                let old = items.len();
                items.resize(n, Value::Undef);
                if n > old {
                    h.mark_hole_range(recv, old..n);
                } else {
                    h.truncate_holes(recv, n);
                }
            } else if let Ok(i) = name.parse::<usize>() {
                // A write PAST the end leaves the skipped positions elided.
                let old = items.len();
                if i >= old {
                    items.resize(i + 1, Value::Undef);
                }
                items[i] = val;
                if i > old {
                    h.mark_hole_range(recv, old..i);
                }
                // …and the written index itself is no longer one. This is the
                // single site that keeps a hole record from outliving the
                // elision it describes: every array element write in the
                // language reaches it.
                h.clear_hole(recv, i);
            }
        }
        _ => {}
    });
    Ok(())
}

fn b_getitem(vm: &mut VM, _: u8) -> Value {
    let idx = vm.pop();
    let recv = vm.pop();
    let key = match host::to_property_key(&idx) {
        Ok(k) => k,
        Err(e) => return abort(vm, e),
    };
    match get_property(&recv, &key) {
        Ok(v) => v,
        Err(e) => abort(vm, e),
    }
}

fn b_setitem(vm: &mut VM, _: u8) -> Value {
    let val = vm.pop();
    let idx = vm.pop();
    let recv = vm.pop();
    let key = match host::to_property_key(&idx) {
        Ok(k) => k,
        Err(e) => return abort(vm, e),
    };
    if let Err(e) = set_property(&recv, &key, val.clone()) {
        return abort(vm, e);
    }
    val
}

/// `[[Delete]]` (10.1.10) for an already-resolved property key: the one place
/// `delete o[k]`, `delete o.k` and `Reflect.deleteProperty` all go through, so
/// the three cannot drift. Reports `false` for a non-configurable property
/// (sloppy mode ignores the failure rather than throwing) and `true` otherwise,
/// which is also what deleting an absent key reports.
pub fn delete_property(recv: &Value, key: &str) -> Result<bool, String> {
    // 13.5.1.2 step 5 runs `ToObject` on the base, which a nullish one refuses.
    // `delete u.x` reported success instead.
    if with_host(|h| h.is_nullish(recv)) {
        return Err(host::type_error(
            "Cannot convert undefined or null to object",
        ));
    }
    // `[[Delete]]` on a Proxy runs the handler's `deleteProperty` trap, which may
    // throw — the reason this reports a `Result` rather than a bare `bool`.
    if let Some(b) = crate::proxy::delete(recv, key)? {
        return Ok(b);
    }
    // `delete globalThis.x` removes a global a script created. It lives in the
    // globals map, not the object's property map, so the ordinary path reported
    // success and removed nothing — the binding stayed readable afterwards.
    if with_host(|h| h.is_global_object(recv)) && with_host(|h| h.remove_global(key)) {
        return Ok(true);
    }
    // `delete require.cache[id]` drops the module so the next `require` of that
    // file runs it again — the whole point of exposing the cache.
    if peek(recv, |o| match o {
        JsObj::Builtin(ns) => Some(ns == REQUIRE_CACHE),
        _ => None,
    }) == Some(true)
    {
        return Ok(crate::module::cache_delete(key));
    }
    // `delete process.env.X` unsets the variable in the PROCESS, not just in the
    // JS view, so a child spawned afterwards no longer sees it.
    if !key.starts_with("@@")
        && with_host(
            |h| matches!(h.get(recv), Some(JsObj::Object(p)) if p.contains_key("@@envObject")),
        )
    {
        std::env::remove_var(key);
    }
    // A member of a builtin NAMESPACE (`Math.PI`, `Number.MAX_VALUE`,
    // `Object.prototype`) is non-configurable when it is a constant or a
    // constructor's `prototype`, and `delete` of one answers false without
    // removing anything. There is no property map behind a namespace, so the
    // ordinary attribute lookup below cannot tell — it reported success for
    // every one of them.
    // A REAL intrinsic prototype object carries the write in its own map AND in
    // the side table the instance read consults, so the delete has to clear
    // both. Clearing only the map left `Object.prototype.patch` deleted as far
    // as the prototype was concerned and still inherited by every object.
    if let Some(ns) = with_host(|h| {
        h.intrinsic_proto_ctor(recv)
            .map(str::to_string)
            .or_else(|| (h.object_proto() == *recv).then(|| "Object".to_string()))
    }) {
        with_host(|h| h.remove_builtin_static(&format!("{ns}.prototype"), key));
    }
    if let Some(ns) = peek(recv, |o| match o {
        JsObj::Builtin(ns) => Some(ns.clone()),
        _ => None,
    }) {
        // A script-assigned static is an ordinary configurable property and is
        // removed from the side table the assignment landed in. Falling through
        // to the attribute check below answered true and deleted nothing, so a
        // patch survived its own `delete`.
        if with_host(|h| h.remove_builtin_static(&ns, key)) {
            return Ok(true);
        }
        if ns != REQUIRE_CACHE && !builtin_member_configurable(&ns, key) {
            return Ok(false);
        }
    }
    if !with_host(|h| h.prop_attrs(recv, key).configurable) {
        return Ok(false);
    }
    // An accessor lives in its own table, not the property map, so removing it
    // has to be explicit — otherwise `delete` reported success while the getter
    // kept answering and `in` kept reporting the key.
    if with_host(|h| h.own_accessor(recv, key).is_some()) {
        with_host(|h| h.remove_accessor(recv, key));
        return Ok(true);
    }
    with_host(|h| {
        let index = key.parse::<usize>();
        match h.get_mut(recv) {
            Some(JsObj::Object(props)) => {
                props.shift_remove(key);
                return;
            }
            Some(JsObj::Array(items)) => {
                if let Ok(i) = index {
                    if i < items.len() {
                        // `delete a[i]` punches a HOLE: the length is unchanged
                        // but the index stops being an own property.
                        items[i] = Value::Undef;
                        h.mark_hole(recv, i);
                    }
                    return;
                }
            }
            _ => {}
        }
        // A non-index key on an array (`arr.foo`, `arr[sym]`), or any own key on
        // a function/class, is an ordinary own property kept in the side table.
        h.remove_fn_prop(recv, key);
    });
    Ok(true)
}

fn b_delitem(vm: &mut VM, _: u8) -> Value {
    let strict = vm.pop();
    let idx = vm.pop();
    let recv = vm.pop();
    // `delete o[k]` keys through ToPropertyKey (7.1.19), exactly as the read and
    // the write do: `String(k)` would turn a Symbol into its `Symbol(desc)`
    // description and delete a key nothing ever wrote.
    let key = match host::to_property_key(&idx) {
        Ok(k) => k,
        Err(e) => return abort(vm, e),
    };
    match delete_property(&recv, &key) {
        Ok(false) if with_host(|h| h.truthy(&strict)) => {
            abort(vm, refused_delete_error(&recv, &key))
        }
        Ok(b) => Value::Bool(b),
        Err(e) => abort(vm, e),
    }
}

fn b_delprop_name(vm: &mut VM, _: u8) -> Value {
    let strict = vm.pop();
    let name = sval(&vm.pop());
    let recv = vm.pop();
    match delete_property(&recv, &name) {
        Ok(false) if with_host(|h| h.truthy(&strict)) => {
            abort(vm, refused_delete_error(&recv, &name))
        }
        Ok(b) => Value::Bool(b),
        Err(e) => abort(vm, e),
    }
}

/// The TypeError a STRICT `delete` of a non-configurable property raises. The
/// receiver renders the way every other brand-check message renders one.
fn refused_delete_error(recv: &Value, key: &str) -> String {
    // A PROXY names the trap that refused. Only the `delete` OPERATOR reports
    // it; `Reflect.deleteProperty` answers `false`, which is why this lives
    // here rather than in the shared `[[Delete]]`.
    if with_host(|h| h.kind_of(recv)) == Some(ObjKind::Proxy) {
        return host::type_error(&format!(
            "'deleteProperty' on proxy: trap returned falsish for property '{key}'"
        ));
    }
    // A non-callable builtin NAMESPACE renders as a plain object here — node
    // reports `#<Object>` for `Math`, not its `[object Math]` brand.
    let shown = match peek(recv, |o| match o {
        JsObj::Builtin(ns) => Some(ns.clone()),
        _ => None,
    }) {
        Some(ns) if !host::builtin_is_callable(&ns) => "#<Object>".to_string(),
        _ => no_side_effects_string(recv),
    };
    host::type_error(&format!("Cannot delete property '{key}' of {shown}"))
}

// ── constructors ──────────────────────────────────────────────────────────────

fn b_mkstr(vm: &mut VM, argc: u8) -> Value {
    let parts = pop_n(vm, argc as usize);
    let s: String = with_host(|h| parts.iter().map(|p| h.str_of(p)).collect());
    with_host(|h| h.new_str(s))
}

fn b_mkarr(vm: &mut VM, argc: u8) -> Value {
    let items = pop_n(vm, argc as usize);
    with_host(|h| h.new_array(items))
}

/// `MARK_HOLE [arr, index]`: record `arr[index]` as an ELIDED element. Emitted
/// only for an array literal that actually contains an elision, so a dense
/// literal costs nothing. Returns `undefined`; the array stays on the stack
/// underneath (the compiler `Dup`s it).
fn b_mark_hole(vm: &mut VM, _: u8) -> Value {
    let idx = vm.pop();
    let arr = vm.pop();
    let i = match idx {
        Value::Int(i) if i >= 0 => i as usize,
        _ => return Value::Undef,
    };
    with_host(|h| h.mark_hole(&arr, i));
    Value::Undef
}

fn b_mkobj(vm: &mut VM, argc: u8) -> Value {
    let flat = pop_n(vm, argc as usize);
    let mut props: IndexMap<String, Value> = IndexMap::new();
    // A literal `__proto__: x` key sets the object's prototype (not an own prop).
    let mut proto_override: Option<Value> = None;
    let mut method_keys: Vec<String> = Vec::new();
    let mut i = 0;
    while i + 2 < flat.len() || (i + 2 == flat.len() && flat.len() % 3 == 0 && i < flat.len()) {
        if i + 2 >= flat.len() {
            break;
        }
        // Tag 2: an ACCESSOR's position. An accessor lives in its own table, so
        // the literal reserves its slot here with the `@@ord:` marker key that
        // `own_enum_data_keys` resolves back — otherwise `{ get g(){}, d: 2 }`
        // enumerated `d, g`, because `DEF_ACCESSOR` runs after `MKOBJ` and its
        // marker landed at the end.
        if matches!(flat[i], Value::Int(2)) {
            let key = with_host(|h| h.str_of(&flat[i + 1]));
            props
                .entry(format!("{}{key}", host::ORD_MARKER))
                .or_insert(Value::Undef);
            i += 3;
            continue;
        }
        // Tag 3: a METHOD DEFINITION — an ordinary property whose key is also
        // recorded so the literal can become its `[[HomeObject]]` below.
        if matches!(flat[i], Value::Int(3)) {
            let key = with_host(|h| h.str_of(&flat[i + 1]));
            method_keys.push(key.clone());
            props.insert(key, flat[i + 2].clone());
            i += 3;
            continue;
        }
        let spread = matches!(flat[i], Value::Int(1));
        if spread {
            let src = flat[i + 1].clone();
            // A STRING source spreads its index properties (`{..."ab"}` is
            // `{0:'a',1:'b'}`): CopyDataProperties (7.3.25) calls ToObject, and a
            // String exotic object owns one enumerable property per UTF-16 code
            // UNIT (10.4.3). `own_enum_entries_deep` only walks heap objects, so
            // a string source contributed nothing and `{..."ab"}` was `{}`.
            // Every other primitive (number/boolean/symbol) boxes to an object
            // with no own enumerable properties, and null/undefined are ignored,
            // so those correctly stay no-ops on the path below.
            if let Some(s) = with_host(|h| h.as_str(&src)) {
                for idx in 0..crate::utf16::len(&s) {
                    if let Ok(ch) = get_property(&src, &idx.to_string()) {
                        props.insert(idx.to_string(), ch);
                    }
                }
                i += 3;
                continue;
            }
            // Object spread copies own *enumerable* properties only — never the
            // hidden `@@…` slots (copying `@@native` used to turn `{...buf}`
            // into something that still claimed to be a Buffer) and never a
            // property a descriptor marked non-enumerable.
            // A getter that throws during spread propagates as a thrown value,
            // which in the VM means aborting the frame.
            let entries = match host::own_enum_entries_deep(&src) {
                Ok(e) => e,
                Err(e) => return abort(vm, e),
            };
            for (k, v) in entries {
                props.insert(k, v);
            }
            // `CopyDataProperties` (7.3.25) copies own enumerable SYMBOL keys
            // too — only `Object.keys`/`for-in`/`JSON.stringify` skip them.
            for (k, v) in with_host(|h| h.own_symbol_entries(&src)) {
                props.insert(k, v);
            }
        } else {
            let key = with_host(|h| h.str_of(&flat[i + 1]));
            if key == "__proto__" {
                proto_override = Some(flat[i + 2].clone());
            } else {
                props.insert(key, flat[i + 2].clone());
            }
        }
        i += 3;
    }
    with_host(|h| {
        let o = h.new_object(props);
        if let Some(p) = proto_override {
            if matches!(p, Value::Obj(_)) {
                h.set_proto(&o, p);
            }
        }
        // A method DEFINED here takes the literal as its `[[HomeObject]]`, which
        // is what `super` inside it resolves through. The home object is fixed
        // at definition, so a method that merely arrives as a value
        // (`{ m: other.m }`) keeps the one it was defined with — stamping every
        // method-valued property instead rebound the original and changed what
        // IT resolved.
        for key in &method_keys {
            let m = match h.get(&o) {
                Some(JsObj::Object(p)) => p.get(key).cloned(),
                _ => None,
            };
            if let Some(m) = m {
                if let Some(JsObj::Func(f)) = h.get_mut(&m) {
                    f.home_object = Some(o.clone());
                }
            }
        }
        o
    })
}

fn b_mkfunc(vm: &mut VM, _: u8) -> Value {
    let def_id = match vm.pop() {
        Value::Int(n) => n as usize,
        Value::Float(f) => f as usize,
        _ => return abort(vm, "internal: MKFUNC id".into()),
    };
    let (is_arrow, self_name) = with_host(|h| match h.funcs.get(def_id) {
        Some(d) => (
            d.is_arrow,
            (d.self_name && !d.name.is_empty()).then(|| d.name.clone()),
        ),
        None => (false, None),
    });
    with_host(|h| {
        let mut env = h.current_env_capture();
        let this = h.current_this();
        // An arrow has no `super` of its own: it uses the enclosing METHOD's,
        // exactly as it uses the enclosing `this`. Nothing was captured, so
        // `super.m()` inside an arrow reported the method missing — in a class
        // method as well as an object literal.
        let (home_class, home_static, home_object) = if is_arrow {
            h.current_home()
        } else {
            (None, false, None)
        };
        // A named function expression closes over an extra scope holding its own
        // name, so the body can recurse through it (`function f(){ … f() … }`)
        // independently of whatever the outer binding is later set to.
        if self_name.is_some() {
            env = host::child_env(env);
        }
        let f = h.alloc(JsObj::Func(FuncVal {
            def_id,
            env: Some(env.clone()),
            this,
            is_arrow,
            home_class,
            home_static,
            home_object,
        }));
        if let Some(n) = self_name {
            env.borrow_mut().vars.insert(n, f.clone());
        }
        f
    })
}

// ── truthiness / coercion / equality ──────────────────────────────────────────

fn b_truthy(vm: &mut VM, _: u8) -> Value {
    let v = vm.pop();
    Value::Bool(with_host(|h| h.truthy(&v)))
}

fn b_nullish(vm: &mut VM, _: u8) -> Value {
    let v = vm.pop();
    Value::Bool(with_host(|h| h.is_nullish(&v)))
}

fn b_tostr(vm: &mut VM, _: u8) -> Value {
    let v = vm.pop();
    // ToString with user-`toString`/`valueOf` dispatch (template interpolation,
    // `String(x)`, object keys).
    match host::to_string_value(&v) {
        Ok(s) => s,
        Err(e) => abort(vm, e),
    }
}

fn b_typeof(vm: &mut VM, _: u8) -> Value {
    let v = vm.pop();
    with_host(|h| {
        let t = h.type_of(&v);
        h.new_str(t)
    })
}

/// `typeof <bare ident>`: read the name like `b_getlocal` but return "undefined"
/// (never a ReferenceError) when the name is unbound — JS `typeof` semantics.
fn b_typeof_name(vm: &mut VM, _: u8) -> Value {
    let name = sval(&vm.pop());
    // `typeof` does NOT excuse the temporal dead zone: it answers "undefined"
    // for an UNBOUND name, but a `let` above its declaration is bound and
    // throws. Reading the marker's type answered "function".
    if with_host(|h| h.is_tdz_global(&name) && h.read_name(&name).is_none()) {
        return abort(vm, host::tdz_error(&name));
    }
    if let Some(v) = with_host(|h| h.read_name(&name)) {
        if with_host(|h| h.is_tdz(&v)) {
            return abort(vm, host::tdz_error(&name));
        }
    }
    // Bound name (user variable) → typeof its value.
    if let Some(v) = with_host(|h| h.read_name(&name)) {
        return with_host(|h| {
            let t = h.type_of(&v);
            h.new_str(t)
        });
    }
    // Lazily-bound globals mirror `b_getlocal`: resolve to the same value it
    // would produce, then take its type (so object-namespaces like `console`/
    // `Math`/`JSON`/`process` report "object", constructors report "function").
    let t = match name.as_str() {
        "undefined" => "undefined".to_string(),
        "NaN" | "Infinity" => "number".to_string(),
        "globalThis" | "global" => "object".to_string(),
        n if is_namespace(n) || is_known_builtin(n) => {
            let v = with_host(|h| h.alloc(JsObj::Builtin(name.clone())));
            with_host(|h| h.type_of(&v)).to_string()
        }
        _ => "undefined".to_string(), // genuinely unbound → JS returns "undefined"
    };
    with_host(|h| h.new_str(t))
}

fn b_strict_eq(vm: &mut VM, _: u8) -> Value {
    let b = vm.pop();
    let a = vm.pop();
    Value::Bool(with_host(|h| h.strict_eq(&a, &b)))
}

fn b_loose_eq(vm: &mut VM, _: u8) -> Value {
    let b = vm.pop();
    let a = vm.pop();
    // Abstract Equality steps 10-11 (7.2.15): object ⇄ primitive converts the
    // object with `ToPrimitive` — a JS `valueOf`/`Symbol.toPrimitive` call, so it
    // runs before the host borrow. Object ⇄ object stays a reference check.
    let (a, b) = match with_host(|h| (host::is_primitive(h, &a), host::is_primitive(h, &b))) {
        (false, true) if coerces_against_object(&b) => match host::to_primitive(&a, "default") {
            Ok(p) => (p, b),
            Err(e) => return abort(vm, e),
        },
        (true, false) if coerces_against_object(&a) => match host::to_primitive(&b, "default") {
            Ok(p) => (a, p),
            Err(e) => return abort(vm, e),
        },
        _ => (a, b),
    };
    Value::Bool(with_host(|h| h.loose_eq(&a, &b)))
}

fn b_instanceof(vm: &mut VM, _: u8) -> Value {
    let ctor = vm.pop();
    let obj = vm.pop();
    match host::instance_of(&obj, &ctor) {
        Ok(b) => Value::Bool(b),
        Err(e) => abort(vm, e),
    }
}

// ── bitwise / unary ───────────────────────────────────────────────────────────

fn b_binop(vm: &mut VM, _: u8) -> Value {
    let b = vm.pop();
    let a = vm.pop();
    let tag = match vm.pop() {
        Value::Int(n) => n,
        _ => 0,
    };
    // Both operands are ToPrimitive-d with the number hint before ToInt32
    // (ECMA-262 13.12.1), which has to happen outside the host borrow.
    let r = host::to_primitive(&a, "number")
        .and_then(|a| host::to_primitive(&b, "number").map(|b| (a, b)))
        .and_then(|(a, b)| with_host(|h| h.bitwise(tag, &a, &b)));
    finish(vm, r)
}

fn b_unary(vm: &mut VM, _: u8) -> Value {
    let v = vm.pop();
    let tag = match vm.pop() {
        Value::Int(n) => n,
        _ => 0,
    };
    // Unary `+`/`~` on a BigInt: `+` is a hard TypeError in JS; `~x` is `-x - 1`
    // computed in arbitrary precision.
    if with_host(|h| h.is_bigint_val(&v)) {
        return match tag {
            host::unop::POS => abort(
                vm,
                host::type_error("Cannot convert a BigInt value to a number"),
            ),
            host::unop::BITNOT => {
                let b = with_host(|h| h.as_bigint(&v)).unwrap();
                let r = -(b + num_bigint::BigInt::from(1));
                with_host(|h| h.new_bigint(r))
            }
            _ => Value::Undef,
        };
    }
    // `ToNumber` outside the host borrow: an object operand's `valueOf` /
    // `Symbol.toPrimitive` is a JS call, so it cannot run under `with_host`.
    let n = match host::to_number_value(&v) {
        Ok(n) => n,
        Err(e) => return abort(vm, e),
    };
    match tag {
        host::unop::POS => Value::Float(n),
        host::unop::BITNOT => {
            let i = if n.is_finite() {
                n.trunc() as i64 as i32
            } else {
                0
            };
            Value::Float(!i as f64)
        }
        _ => Value::Undef,
    }
}

// ── membership ────────────────────────────────────────────────────────────────

fn b_contains(vm: &mut VM, _: u8) -> Value {
    let container = vm.pop();
    let key = vm.pop();
    // `x in y` requires y to be an object. V8 names both operands:
    // `Cannot use 'in' operator to search for 'a' in 5`.
    // A heap-backed PRIMITIVE — a string, a symbol, a bigint — is a
    // `Value::Obj` in this host but is not an object, so the shape test alone
    // let `'length' in 'ab'` and `'description' in Symbol('x')` answer `true`
    // where node throws. `is_primitive` is the same predicate `ToObject` and
    // `typeof` use, so the three cannot disagree about what an object is.
    if !matches!(container, Value::Obj(_)) || with_host(|h| host::is_primitive(h, &container)) {
        let (k, c) = with_host(|h| (h.property_key(&key), h.str_of(&container)));
        return abort(
            vm,
            host::type_error(&format!(
                "Cannot use 'in' operator to search for '{k}' in {c}"
            )),
        );
    }
    let k = match host::to_property_key(&key) {
        Ok(k) => k,
        Err(e) => return abort(vm, e),
    };
    match has_property(&container, &k) {
        Ok(b) => Value::Bool(b),
        Err(e) => abort(vm, e),
    }
}

// ── control ───────────────────────────────────────────────────────────────────

fn b_sig_return(vm: &mut VM, _: u8) -> Value {
    let v = vm.pop();
    with_host(|h| h.signal = Some(host::Signal::Return(v.clone())));
    vm.ip = vm.chunk.ops.len();
    v
}

/// `break [label]` whose target loop lives in an enclosing chunk (the statement is
/// inside a `try` block, which the host runs as its own chunk). Raise the signal
/// and halt this chunk; `SIG_UNWIND` after the `TRY` op re-dispatches it.
fn b_sig_break(vm: &mut VM, _: u8) -> Value {
    let label = sval(&vm.pop());
    let label = (!label.is_empty()).then_some(label);
    with_host(|h| h.signal = Some(host::Signal::Break(label)));
    vm.ip = vm.chunk.ops.len();
    Value::Undef
}

/// `continue [label]` out of a `try` block — see [`b_sig_break`].
fn b_sig_continue(vm: &mut VM, _: u8) -> Value {
    let label = sval(&vm.pop());
    let label = (!label.is_empty()).then_some(label);
    with_host(|h| h.signal = Some(host::Signal::Continue(label)));
    vm.ip = vm.chunk.ops.len();
    Value::Undef
}

/// Dispatch a pending control signal at the instruction after a `TRY`. `tag`
/// describes what the `try` is nested in (see [`host::unwind`]):
///
/// * no signal → `NONE`, execution continues normally;
/// * `Return`, or no enclosing loop in this chunk → halt the chunk so the signal
///   keeps travelling outward;
/// * `break`/`continue` targeting the enclosing loop → consume it and report
///   `BREAK`/`CONTINUE` so the compiler-emitted jump lands on the loop's exit /
///   continue target;
/// * a LABELED `break`/`continue` for some outer loop → report `BREAK` but leave
///   the signal pending, so leaving this loop re-dispatches it one level out.
fn b_sig_unwind(vm: &mut VM, _: u8) -> Value {
    let cont_tag = sval(&vm.pop());
    let brk_tag = sval(&vm.pop());
    let sig = match with_host(|h| h.signal.clone()) {
        Some(s) => s,
        None => return Value::Int(host::unwind::NONE),
    };
    // Nothing in this chunk can catch a `break`: halt so the signal keeps going.
    let propagate = |vm: &mut VM| {
        vm.ip = vm.chunk.ops.len();
        Value::Int(host::unwind::NONE)
    };
    match &sig {
        host::Signal::Return(_) => propagate(vm),
        host::Signal::Break(label) => {
            if brk_tag == host::unwind::NO_LOOP {
                return propagate(vm);
            }
            let mine = match label {
                None => true, // unlabeled: always the innermost enclosing context
                Some(l) => brk_tag == *l,
            };
            if mine {
                with_host(|h| h.signal = None);
            }
            // Not ours: still leave this context by its break exit, keeping the
            // signal pending for the next dispatch point one level out.
            Value::Int(host::unwind::BREAK)
        }
        host::Signal::Continue(label) => {
            let mine = match label {
                // Unlabeled `continue` binds to the innermost continue-catching
                // loop — which a `switch` between here and it is NOT.
                None => cont_tag != host::unwind::NO_LOOP,
                Some(l) => cont_tag == *l,
            };
            if mine {
                with_host(|h| h.signal = None);
                return Value::Int(host::unwind::CONTINUE);
            }
            if brk_tag == host::unwind::NO_LOOP {
                return propagate(vm);
            }
            // The target loop is further out: exit the innermost context here and
            // re-dispatch there.
            Value::Int(host::unwind::BREAK)
        }
    }
}

fn b_throw(vm: &mut VM, _: u8) -> Value {
    let v = vm.pop();
    let msg = with_host(|h| {
        h.exc = Some(v.clone());
        // Prefer an error object's message for the top-level report.
        error_display(h, &v)
    });
    abort(vm, msg)
}

fn error_display(h: &host::JsHost, v: &Value) -> String {
    if let Some(JsObj::Object(props)) = h.get(v) {
        let name = props
            .get("name")
            .map(|x| h.str_of(x))
            .unwrap_or_else(|| "Error".into());
        if let Some(m) = props.get("message") {
            return format!("Uncaught {name}: {}", h.str_of(m));
        }
    }
    format!("Uncaught {}", h.str_of(v))
}

fn b_try(vm: &mut VM, _: u8) -> Value {
    let id = match vm.pop() {
        Value::Int(n) => n as usize,
        _ => return abort(vm, "internal: TRY id".into()),
    };
    // Shape only. Running a `try` used to clone the whole `TryDef` — its block,
    // its handler and its finalizer bytecode — every time control entered it,
    // which for a `try` inside a loop is once per iteration.
    let (has_handler, catch_bind, has_finalizer) = match with_host(|h| h.try_shape(id)) {
        Some(t) => t,
        None => return abort(vm, "internal: unknown try id".into()),
    };
    let mut pending: Option<String> = None;
    // Each sub-block runs as its own chunk on THIS frame, so a throw part-way
    // through can leave block scopes open. Snapshot the scope and restore it
    // before the handler and after the whole statement.
    let scope = with_host(|h| h.scope_snapshot());

    with_host(|h| h.push_scope()); // the try block is its own block scope
    let body_res = host::run_chunk_keyed(host::try_key(id, 0), || {
        with_host(|h| h.try_chunk(id, 0)).expect("try block exists")
    });
    with_host(|h| h.restore_scope(scope.clone()));
    let signal_after = with_host(|h| h.signal.is_some());
    if let Err(e) = body_res {
        if signal_after {
            pending = Some(e);
        } else if has_handler {
            // Bind the thrown value (or a synthesized error) to the catch param.
            let thrown =
                with_host(|h| h.exc.clone()).unwrap_or_else(|| with_host(|h| synth_error(h, &e)));
            with_host(|h| {
                h.error = None;
                h.exc = None;
            });
            // The catch parameter is block-scoped to the handler.
            with_host(|h| h.push_scope());
            if let Some(name) = &catch_bind {
                with_host(|h| h.declare_name(name, thrown));
            }
            let hres = host::run_chunk_keyed(host::try_key(id, 1), || {
                with_host(|h| h.try_chunk(id, 1)).expect("handler exists")
            });
            with_host(|h| h.restore_scope(scope.clone()));
            if let Err(e2) = hres {
                pending = Some(e2);
            }
        } else {
            pending = Some(e);
        }
    }

    // finally always runs; a finally error/signal supersedes.
    if has_finalizer {
        let sig_before = with_host(|h| h.signal.take());
        with_host(|h| h.push_scope()); // ditto for `finally`
        let fres = host::run_chunk_keyed(host::try_key(id, 2), || {
            with_host(|h| h.try_chunk(id, 2)).expect("finalizer exists")
        });
        with_host(|h| h.restore_scope(scope.clone()));
        match fres {
            Ok(_) => {
                if with_host(|h| h.signal.is_none()) {
                    // The finalizer completed normally: the try/catch block's own
                    // abrupt completion resumes.
                    with_host(|h| h.signal = sig_before);
                } else {
                    // ECMA-262 14.15.3 TryStatement evaluation: when the finalizer's
                    // completion is abrupt (`return`/`break`/`continue` inside
                    // `finally`), that completion REPLACES the try/catch block's —
                    // including a pending throw, which is discarded, not rethrown.
                    pending = None;
                    with_host(|h| {
                        h.error = None;
                        h.exc = None;
                    });
                }
            }
            Err(e) => pending = Some(e),
        }
    }

    if let Some(e) = pending {
        return abort(vm, e);
    }
    Value::Undef
}

/// Synthesize an `Error`-shaped object from an internal error string, linked to
/// the matching builtin error prototype so `instanceof`/`.constructor` work.
pub(crate) fn synth_error(h: &mut host::JsHost, e: &str) -> Value {
    h.ensure_error_protos();
    // A `DOMException` marker: the WHATWG error NAME rides in the string, since
    // it is not one of the ECMAScript error classes below.
    if let Some(rest) = e.strip_prefix(host::DOM_MARK) {
        if let Some((name, msg)) = rest.split_once('\u{1}') {
            return dom_exception_with(h, name, msg);
        }
    }
    // A `Name [ERR_CODE]: message` head carries a Node error `code` next to the
    // error class, exactly as Node's internal errors render it in `.stack`.
    let (head, rest) = match e.split_once(": ") {
        Some((n, m)) => (n, m.to_string()),
        None => ("", e.to_string()),
    };
    let (base, code) = match head.split_once(" [") {
        Some((n, c)) if c.ends_with(']') => (n, Some(c[..c.len() - 1].to_string())),
        _ => (head, None),
    };
    let (name, mut message) = if host::ERROR_NAMES.contains(&base) {
        (base.to_string(), rest)
    } else {
        ("Error".to_string(), e.to_string())
    };
    // A `host::plain_coded_error` marker: the code rides at the head of the
    // MESSAGE rather than in the class, because Node's native-layer errors set
    // `.code` while leaving `String(err)` unbracketed (`TypeError: Invalid URL`
    // with `code === 'ERR_INVALID_URL'`). Strip it back off here — the marker is
    // internal and must never reach a user-visible `.message`.
    let mut code = code;
    // Whether `String(err)`/`err.stack` show `Name [CODE]:` — true for the
    // bracketed head, false for the marker form.
    let mut bracketed = code.is_some();
    // Extra own properties (`input`, `base`) from `host::plain_coded_error_with`.
    let mut fields: Vec<(String, String)> = Vec::new();
    if let Some(rest) = message.strip_prefix(host::CODE_MARK) {
        if let Some((c, m)) = rest.split_once('\u{1}') {
            code = Some(c.to_string());
            bracketed = false;
            let (m, fs) = host::split_error_fields(m);
            fields = fs
                .into_iter()
                .map(|(k, v)| (k.to_string(), v.to_string()))
                .collect();
            message = m.to_string();
        }
    }
    let mut props: IndexMap<String, Value> = IndexMap::new();
    let mv = h.new_str(message.clone());
    props.insert("message".into(), mv);
    if let Some(c) = &code {
        let cv = h.new_str(c.clone());
        props.insert("code".into(), cv);
        for (k, v) in fields {
            let fv = h.new_str(v);
            props.insert(k, fv);
        }
        if bracketed {
            // Marks this as a Node JS-layer error, whose `toString` brackets the
            // code. A native-layer error has the same `.code` and does not.
            props.insert("@@nodeError".into(), Value::Bool(true));
        }
    }
    let label = match (&code, bracketed) {
        (Some(c), true) => format!("{name} [{c}]"),
        _ => name.clone(),
    };
    let frames = h.stack_frames();
    let stack = if message.is_empty() {
        format!("{label}{frames}")
    } else {
        format!("{label}: {message}{frames}")
    };
    let sv = h.new_str(stack);
    props.insert("stack".into(), sv);
    // A libuv system-error message is itself the canonical encoding of the
    // error's metadata — `ENOENT: no such file or directory, open '/x'` — so a
    // filesystem/network failure recovers the enumerable `code`/`errno`/
    // `syscall`/`path` own properties that `err.code === 'ENOENT'` checks (the
    // single most common error-handling idiom in Node packages) depend on.
    for (k, v) in syscall_error_fields(&message) {
        let sv = match v {
            SysField::Str(s) => h.new_str(s),
            SysField::Num(n) => Value::Float(n),
        };
        props.insert(k.into(), sv);
    }
    let obj = h.new_object(props);
    if let Some(p) = host::error_proto_of(h, &name) {
        h.set_proto(&obj, p);
    }
    // `message`/`stack` are non-enumerable; a Node `ERR_*` error's `code` is not
    // (`Object.keys(e)` on an `ERR_INVALID_ARG_TYPE` reads `["code"]`).
    h.hide_prop(&obj, "message");
    h.hide_prop(&obj, "stack");
    obj
}

enum SysField {
    Str(String),
    Num(f64),
}

/// Decompose a libuv-shaped message (`ECODE: reason, syscall 'path'`) into the
/// own properties Node hangs off a system error. Returns empty for any message
/// that is not in that shape.
fn syscall_error_fields(message: &str) -> Vec<(&'static str, SysField)> {
    let (code, rest) = match message.split_once(": ") {
        Some((c, r))
            if c.len() >= 2
                && c.starts_with('E')
                && c.bytes()
                    .all(|b| b.is_ascii_uppercase() || b.is_ascii_digit()) =>
        {
            (c, r)
        }
        _ => return Vec::new(),
    };
    let mut out: Vec<(&'static str, SysField)> = vec![
        ("errno", SysField::Num(errno_for(code))),
        ("code", SysField::Str(code.to_string())),
    ];
    // `reason, syscall 'path'` — the path is optional (`EPIPE: …, write`).
    if let Some((_, tail)) = rest.split_once(", ") {
        let (syscall, path) = match tail.split_once(" '") {
            // A two-path message ends `'from' -> 'to'`; `err.path` is the FIRST
            // one, so the scan stops at its closing quote rather than at the
            // end of the line — which had been swallowing `' -> 'dest` into the
            // path for every `rename` and `copyFile` failure.
            Some((s, p)) => (s, p.split_once('\'').map(|(first, _)| first)),
            None => (tail, None),
        };
        out.push(("syscall", SysField::Str(syscall.to_string())));
        if let Some(p) = path {
            out.push(("path", SysField::Str(p.to_string())));
        }
    }
    out
}

/// The negative `errno` Node reports for a libuv error code on this platform.
/// Only the codes `err_str` can produce are mapped; anything else reports the
/// generic `EIO` number rather than inventing a value.
fn errno_for(code: &str) -> f64 {
    let n: i32 = match code {
        "ENOENT" => 2,
        "EACCES" => 13,
        "EEXIST" => 17,
        "ENOTDIR" => 20,
        "EISDIR" => 21,
        "EINVAL" => 22,
        "EPIPE" => 32,
        "ENOTEMPTY" => 66,
        _ => 5, // EIO
    };
    -f64::from(n)
}

// ── iteration ─────────────────────────────────────────────────────────────────

fn b_getiter(vm: &mut VM, _: u8) -> Value {
    let v = vm.pop();
    // A generator is its own iterator (resumed lazily by FORITER).
    if with_host(|h| h.is_generator_val(&v)) {
        return v;
    }
    // A Proxy's iterator comes from its traps, materialized eagerly: the
    // `lookup_chain` probe below reads the property map a proxy does not have.
    if with_host(|h| h.kind_of(&v)) == Some(ObjKind::Proxy) {
        return match crate::proxy::iterate(&v) {
            Ok(Some(items)) => with_host(|h| {
                h.alloc(JsObj::Iter {
                    items,
                    idx: 0,
                    array: None,
                })
            }),
            Ok(None) => abort(vm, "internal: kind_of said Proxy".into()),
            Err(e) => abort(vm, e),
        };
    }
    // Arrays and strings take the direct path below: they have no iterator
    // state to preserve and are the hot case, so they must not pay a property
    // lookup and a call per loop.
    let direct = matches!(
        with_host(|h| h.kind_of(&v)),
        Some(ObjKind::Array) | Some(ObjKind::Str)
    );
    // …but only while their `Symbol.iterator` is still reachable. It comes from
    // the intrinsic prototype, so replacing the link takes it away: node reports
    // `a is not iterable` for an array whose prototype is a plain object, where
    // the fast path below iterated the backing vector regardless.
    if !own_intrinsic_reachable(&v)
        && !matches!(
            get_property(&v, "@@iterator"),
            Ok(ref f) if with_host(|h| host::is_callable(h, f))
        )
    {
        let shown = with_host(|h| h.inspect(&v));
        let msg = host::type_error(&format!("{shown} is not iterable"));
        return abort(vm, host::name_call_site(vm, &shown, msg));
    }
    // Anything else with a `Symbol.iterator`: call it for the iterator object.
    //
    // Resolved as a full property READ, not a stored-property lookup. A
    // NATIVE-tagged object (`URLSearchParams`, `Headers`, `Map`, `Set`)
    // dispatches its methods through the stdlib table rather than a property
    // map, so a `lookup_chain` probe found nothing and the loop fell through to
    // materializing the value — which threw for `URLSearchParams` and
    // snapshotted for `Map`. Spreading the same object already worked, because
    // that path had been fixed and this one had not.
    if !direct {
        if let Ok(iter_fn) = get_property(&v, "@@iterator") {
            if with_host(|h| host::is_callable(h, &iter_fn)) {
                return match host::invoke(&iter_fn, Vec::new(), Some(v.clone())) {
                    Ok(it) => it,
                    Err(e) => abort(vm, e),
                };
            }
        }
    }
    // An array is iterated live, as its `values()` iterator does: a snapshot
    // missed every push during the loop, so a worklist `for (const n of q)
    // q.push(…)` stopped after the first element.
    if with_host(|h| h.kind_of(&v)) == Some(ObjKind::Array) {
        return array_iterator(&v, host::ArrayIterKind::Values);
    }
    match with_host(|h| h.iter_vec(&v)) {
        Ok(items) => with_host(|h| {
            h.alloc(JsObj::Iter {
                items,
                idx: 0,
                array: None,
            })
        }),
        // V8 names the SOURCE EXPRESSION, not the value: `for (const x of a)`
        // reports `a is not iterable`. The text was recorded for this op.
        Err(e) => {
            let shown = with_host(|h| h.inspect(&v));
            let named = host::name_call_site(vm, &shown, e);
            abort(vm, named)
        }
    }
}

fn b_forin_keys(vm: &mut VM, _: u8) -> Value {
    let v = vm.pop();
    // `for-in` over a Proxy is 14.7.5.9 `EnumerateObjectProperties`: the
    // `ownKeys` trap filtered by `[[GetOwnProperty]]`'s `enumerable`. Both traps
    // are user code, so this cannot run inside `enum_keys`'s `&mut` host borrow.
    if with_host(|h| h.kind_of(&v)) == Some(ObjKind::Proxy) {
        // `ownKeys` ONLY. The `enumerable` filter is 14.7.5.10's per-key
        // `[[GetOwnProperty]]`, which `FORIN_ALIVE` runs at the moment each key
        // is visited — so the `getOwnPropertyDescriptor` traps interleave with
        // the body the way node's do, instead of all firing up front.
        return match crate::proxy::own_keys(&v) {
            Ok(keys) => with_host(|h| {
                let out: Vec<Value> = keys
                    .unwrap_or_default()
                    .into_iter()
                    .filter(|k| !host::is_symbol_key(k))
                    .map(|k| h.new_str(k))
                    .collect();
                h.new_array(out)
            }),
            Err(e) => abort(vm, e),
        };
    }
    let mut keys = with_host(|h| h.enum_keys(&v));
    // A member patched onto the receiver's INTRINSIC prototype is enumerable
    // and inherited, so `for-in` visits it after the own keys — but the
    // intrinsic prototypes are not links `enum_keys` can walk, so its chain
    // pass never reaches them.
    if !with_host(|h| h.has_null_proto(&v)) {
        let seen: Vec<String> = keys.iter().map(|k| with_host(|h| h.str_of(k))).collect();
        for ns in intrinsic_proto_namespaces(&v) {
            for k in with_host(|h| h.builtin_static_keys(&ns)) {
                if !seen.contains(&k) && !intrinsic_proto_member(&ns, &k) {
                    keys.push(with_host(|h| h.new_str(k)));
                }
            }
        }
    }
    with_host(|h| h.new_array(keys))
}

/// The intrinsic prototype namespaces `v` inherits from, nearest first — its
/// own constructor's and then `Object`'s, the same two steps
/// `inherited_builtin_static` looks a value up in.
fn intrinsic_proto_namespaces(v: &Value) -> Vec<String> {
    let ctor = match wrapped_primitive(v).as_ref().and_then(wrapper_ctor_of) {
        Some(c) => Some(c),
        None if is_arguments(v) => Some("Object"),
        None => with_host(|h| default_ctor_name(h, v)),
    };
    let mut out: Vec<String> = ctor
        .filter(|c| *c != "Object")
        .map(|c| format!("{c}.prototype"))
        .into_iter()
        .collect();
    out.push("Object.prototype".to_string());
    out
}

/// `FORIN_ALIVE` — is `key` STILL an enumerable property of `obj`?
///
/// `for-in` takes its key list once (14.7.5.10 builds it lazily, but a snapshot
/// of the enumerable keys is observationally the same for everything except
/// this), and the body can delete a key before the loop reaches it. Node does
/// not visit a key deleted that way; without this check `delete d.z` inside the
/// loop still produced `x,y,z`.
///
/// The check is `[[GetOwnProperty]]`-shaped rather than `in`: on a Proxy it runs
/// the `getOwnPropertyDescriptor` trap, which is what node runs, and NOT the
/// `has` trap, which node never fires for `for-in`. That also puts each trap
/// call immediately before its visit, matching node's interleaving — the trap
/// log used to show every `gopd` up front because the key list was filtered
/// eagerly.
fn b_forin_alive(vm: &mut VM, _: u8) -> Value {
    let key = vm.pop();
    let obj = vm.pop();
    let name = with_host(|h| h.str_of(&key));
    if with_host(|h| h.kind_of(&obj)) == Some(ObjKind::Proxy) {
        return match crate::proxy::own_enumerable(&obj, &name) {
            Ok(b) => Value::Bool(b),
            Err(e) => abort(vm, e),
        };
    }
    // A STRING's keys are its character indices. `in` is not defined on a string
    // primitive at all, so the ordinary path below has no answer for one and
    // `for (const i in 'abc')` came back empty.
    if let Some(s) = with_host(|h| h.as_str(&obj)) {
        let len = crate::utf16::len(&s);
        return Value::Bool(name.parse::<usize>().is_ok_and(|i| i < len));
    }
    // Any other receiver: EXISTENCE only. Node re-checks that the key is still
    // there and does NOT re-check enumerability — making one non-enumerable
    // mid-loop still visits it, where re-filtering on `enumerable` dropped it.
    // (The Proxy branch above does re-check, because there the answer comes from
    // the trap node itself calls.)
    Value::Bool(has_property_ordinary(&obj, &name))
}

fn b_foriter(vm: &mut VM, _: u8) -> Value {
    let it = match vm.stack.last() {
        Some(v) => v.clone(),
        None => return abort(vm, "internal: FORITER with empty stack".into()),
    };
    // A built-in iterator: a snapshot (strings, a Proxy's items) or live over
    // an array.
    if let Some(step) = iter_step(&it) {
        return match step {
            Some(v) => {
                vm.push(v);
                Value::Bool(true)
            }
            None => Value::Bool(false),
        };
    }
    // Generator: resume one step.
    if with_host(|h| h.is_generator_val(&it)) {
        return match host::gen_resume(&it, Value::Undef) {
            Ok(host::GenStep::Yield(v)) => {
                vm.push(v);
                Value::Bool(true)
            }
            Ok(host::GenStep::Done(_)) => Value::Bool(false),
            Err(e) => abort(vm, e),
        };
    }
    // A user iterator object with a `.next()` returning `{ value, done }`.
    match host::call_method(&it, "next", Vec::new()) {
        Ok(step) => {
            let done = get_property(&step, "done")
                .map(|d| with_host(|h| h.truthy(&d)))
                .unwrap_or(true);
            if done {
                Value::Bool(false)
            } else {
                match get_property(&step, "value") {
                    Ok(v) => {
                        vm.push(v);
                        Value::Bool(true)
                    }
                    Err(e) => abort(vm, e),
                }
            }
        }
        Err(e) => abort(vm, e),
    }
}

fn b_unpack(vm: &mut VM, _: u8) -> Value {
    let star = match vm.pop() {
        Value::Int(n) => n,
        _ => -1,
    };
    let count = match vm.pop() {
        Value::Int(n) => n as usize,
        _ => 0,
    };
    let iterable = vm.pop();
    // Without a `...rest` element the pattern needs exactly `count` values and
    // must then close the iterator; draining hung on an unbounded source.
    let items = match if star < 0 {
        host::iter_take(&iterable, count)
    } else {
        host::iter_all(&iterable)
    } {
        Ok(v) => v,
        // Destructuring a non-iterable names the SOURCE EXPRESSION, the way
        // `for-of` does: `const [x] = o` reports `o is not iterable`. The text
        // was recorded for this op at compile time.
        Err(e) => {
            // Node names the source only when the pattern's right-hand side is
            // a plain IDENTIFIER — `const [x] = o` is `o is not iterable`.
            // Anything else (a member, a call, a nested pattern, a parameter)
            // reports the TYPE instead, with the property note. Measured across
            // twelve shapes rather than guessed.
            let msg = match host::call_site_text(vm) {
                Some(text) => host::type_error(&format!("{text} is not iterable")),
                None if e.ends_with(" is not iterable") => {
                    host::type_error(&not_iterable_typed(&iterable))
                }
                None => e,
            };
            return abort(vm, msg);
        }
    };
    let ordered: Vec<Value> = if star < 0 {
        (0..count)
            .map(|i| items.get(i).cloned().unwrap_or(Value::Undef))
            .collect()
    } else {
        let si = star as usize;
        let after = count.saturating_sub(si + 1);
        let rest_end = items.len().saturating_sub(after).max(si);
        let mut out: Vec<Value> = Vec::with_capacity(count);
        for i in 0..si {
            out.push(items.get(i).cloned().unwrap_or(Value::Undef));
        }
        let rest: Vec<Value> = items
            .get(si..rest_end)
            .map(|s| s.to_vec())
            .unwrap_or_default();
        out.push(with_host(|h| h.new_array(rest)));
        for j in 0..after {
            out.push(items.get(rest_end + j).cloned().unwrap_or(Value::Undef));
        }
        out
    };
    if ordered.is_empty() {
        return Value::Undef;
    }
    for it in ordered[1..].iter().rev().cloned() {
        vm.push(it);
    }
    ordered[0].clone()
}

fn b_build_args(vm: &mut VM, argc: u8) -> Value {
    let flat = pop_n(vm, argc as usize);
    let mut out = Vec::new();
    // Elided positions of an array literal (tag 2), recorded as the run-time
    // index each lands on — which only this walk knows, because a preceding
    // spread contributes an unknown number of elements. Call-argument lists,
    // the other `BUILD_ARGS` caller, cannot contain an elision, so this stays
    // empty for them.
    let mut holes: rustc_hash::FxHashSet<usize> = rustc_hash::FxHashSet::default();
    let mut i = 0;
    while i + 1 < flat.len() {
        let val = flat[i + 1].clone();
        match flat[i] {
            // Tag 1 is an ARRAY-LITERAL spread, tag 3 a CALL-ARGUMENT one. They
            // report a non-iterable differently, which is the only reason the
            // two are told apart here.
            Value::Int(1) => match host::iter_all(&val).map_err(|e| {
                let shown = with_host(|h| h.inspect(&val));
                host::name_call_site(vm, &shown, e)
            }) {
                Ok(items) => out.extend(items),
                Err(e) => return abort(vm, e),
            },
            Value::Int(3) => match host::iter_all(&val) {
                Ok(items) => out.extend(items),
                Err(e) => {
                    // A NULLISH spread names the value and what could not be
                    // read off it; anything else names the missing protocol.
                    let shown = with_host(|h| h.is_nullish(&val).then(|| h.str_of(&val)));
                    return abort(
                        vm,
                        match shown {
                            Some(s) => host::type_error(&format!(
                                "{s} is not iterable (cannot read property {s})"
                            )),
                            None if e.ends_with(" is not iterable") => host::type_error(
                                "Spread syntax requires ...iterable[Symbol.iterator] to be a function",
                            ),
                            None => e,
                        },
                    );
                }
            },
            Value::Int(2) => {
                holes.insert(out.len());
                out.push(Value::Undef);
            }
            _ => out.push(val),
        }
        i += 2;
    }
    with_host(|h| {
        let arr = h.new_array(out);
        h.install_holes(&arr, holes);
        arr
    })
}

// ── calls ──────────────────────────────────────────────────────────────────────

fn b_call(vm: &mut VM, argc: u8) -> Value {
    let mut args = pop_n(vm, argc as usize);
    let name = sval(&args.remove(0));
    let r = host::call_named(&name, args);
    // A bare name that resolved to a non-callable reports the VALUE
    // (`undefined is not a function`); node names the identifier. Resolving it
    // again to learn what the message said costs nothing off the error path.
    let r = r.map_err(|e| {
        let shown = global_binding(&name)
            .map(|v| with_host(|h| h.str_of(&v)))
            .unwrap_or_default();
        host::name_call_site(vm, &shown, e)
    });
    finish(vm, r)
}

/// `recv[0](…)` — a computed call whose key is an ARRAY INDEX rather than a
/// method name. `call_method` resolves by name and bottoms out in
/// `call_type_method`, which knows `sort`/`slice` and not `"0"`, so an element
/// that happens to be a function reported "is not a function". Read the element
/// and invoke it with `recv` as `this`, which is the receiver 13.3.6 gives it.
/// A computed call's key is a property key, so it goes through ToPropertyKey:
/// `arr[0](…)` looks up `"0"`. `sval` only unwraps an existing `Value::Str` and
/// answers "" for a number, which turned `arr[0]()` into a call to the method
/// named "" — so the key is stringified here instead.
fn call_key_of(v: &Value) -> String {
    if let Value::Str(s) = v {
        return (**s).clone();
    }
    // `ToPropertyKey`, not `ToString`. A SYMBOL key has an internal `@@name`
    // spelling that `str_of` does not produce — it renders
    // `Symbol(Symbol.iterator)` — so `obj[Symbol.iterator]()` dispatched a
    // method by that display text and reported it was not a function, for every
    // object including a plain literal with a computed symbol method. Reading
    // the same property without calling it worked, which is what hid this.
    with_host(|h| h.property_key(v))
}

fn index_element_call(recv: &Value, name: &str, args: &[Value]) -> Option<Result<Value, String>> {
    if name.is_empty() || !name.bytes().all(|b| b.is_ascii_digit()) {
        return None;
    }
    let f = get_property(recv, name).ok()?;
    with_host(|h| host::is_callable(h, &f))
        .then(|| host::invoke(&f, args.to_vec(), Some(recv.clone())))
}

fn b_call_method(vm: &mut VM, argc: u8) -> Value {
    let mut args = pop_n(vm, argc as usize);
    let recv = args.remove(0);
    let name = call_key_of(&args.remove(0));
    if let Some(r) = index_element_call(&recv, &name, &args) {
        return finish(vm, r);
    }
    let r = host::call_method(&recv, &name, args);
    // `z.f()` on a missing method is `z.f is not a function` in node, not
    // `f is not a function`: V8 names the callee as the source wrote it. The
    // text was recorded for this op at compile time.
    let r = r.map_err(|e| host::name_call_site(vm, &name, e));
    finish(vm, r)
}

fn b_call_value(vm: &mut VM, argc: u8) -> Value {
    let mut args = pop_n(vm, argc as usize);
    let callable = args.remove(0);
    let r = host::invoke(&callable, args, None);
    // The callee here is an expression, not a name, so the message it produced
    // describes the VALUE (`undefined is not a function`); node names the
    // expression. Same site table, keyed on that rendering.
    let r = r.map_err(|e| {
        let shown = with_host(|h| h.str_of(&callable));
        host::name_call_site(vm, &shown, e)
    });
    finish(vm, r)
}

/// `NEW_SPREAD` — `new C(...xs)`, where the argument list is a run-time array
/// rather than a fixed count of stack slots.
///
/// `compile_new` used to compile each argument with `compile_expr`, and a
/// spread there evaluates to the SPREAD OBJECT itself — so `new C(...[1, 2])`
/// passed the array as one argument and `new Date(...[2020, 0, 1])` built an
/// Invalid Date.
fn b_new_spread(vm: &mut VM, _: u8) -> Value {
    let args_arr = vm.pop();
    let ctor = vm.pop();
    let args = host::iter_all(&args_arr).unwrap_or_default();
    let r = host::construct(&ctor, args).map_err(|e| {
        let shown = with_host(|h| h.str_of(&ctor));
        host::name_call_site(vm, &shown, e)
    });
    finish(vm, r)
}

fn b_new(vm: &mut VM, argc: u8) -> Value {
    let mut args = pop_n(vm, argc as usize);
    let ctor = args.remove(0);
    let r = host::construct(&ctor, args);
    // `new (o.a.b.c)()` on a non-constructor names the expression, as a failed
    // call does.
    let r = r.map_err(|e| {
        let shown = with_host(|h| h.str_of(&ctor));
        host::name_call_site(vm, &shown, e)
    });
    finish(vm, r)
}

fn b_apply(vm: &mut VM, _: u8) -> Value {
    let args_arr = vm.pop();
    let callable = vm.pop();
    let args = host::iter_all(&args_arr).unwrap_or_default();
    let r = host::invoke(&callable, args, None);
    finish(vm, r)
}

fn b_apply_method(vm: &mut VM, _: u8) -> Value {
    let args_arr = vm.pop();
    let name = call_key_of(&vm.pop());
    let recv = vm.pop();
    let args = host::iter_all(&args_arr).unwrap_or_default();
    if let Some(r) = index_element_call(&recv, &name, &args) {
        return finish(vm, r);
    }
    let r = host::call_method(&recv, &name, args);
    finish(vm, r)
}

// ── numeric hook ──────────────────────────────────────────────────────────────

/// Host callback for arithmetic fusevm cannot complete natively (a non-`Int`/
/// non-`Float` operand). Supplies JavaScript `+` concatenation and coercion.
///
/// Every operand is run through `ToPrimitive` FIRST (ECMA-262 13.15.3 for `+`,
/// 13.6.3 for the other arithmetic ops, 13.10.1 for the relational ones), which
/// is what invokes a user `valueOf`/`Symbol.toPrimitive`. It has to happen here
/// rather than inside `JsHost::arith`, because calling back into JS re-enters
/// the VM and `arith` runs under the host's `RefCell` borrow.
pub fn numeric_hook(op: NumOp, a: &Value, b: &Value) -> Result<Value, String> {
    use NumOp::*;
    let (a, b) = match op {
        // `==`/`!=` only convert when the OTHER side is a primitive that can be
        // compared numerically or textually; `{} == {}` stays a reference check.
        Eq | Ne => {
            let (pa, pb) = with_host(|h| (host::is_primitive(h, a), host::is_primitive(h, b)));
            match (pa, pb) {
                (false, true) if coerces_against_object(b) => {
                    (host::to_primitive(a, "default")?, b.clone())
                }
                (true, false) if coerces_against_object(a) => {
                    (a.clone(), host::to_primitive(b, "default")?)
                }
                _ => (a.clone(), b.clone()),
            }
        }
        // `+` uses the default hint (`valueOf` first, but a string result still
        // selects concatenation); everything else uses the number hint.
        Add => (
            host::to_primitive(a, "default")?,
            host::to_primitive(b, "default")?,
        ),
        _ => (
            host::to_primitive(a, "number")?,
            host::to_primitive(b, "number")?,
        ),
    };
    reject_symbol_operand(op, &a, &b)?;
    with_host(|h| h.arith(op, &a, &b))
}

/// A symbol has no `ToNumber` and no `ToString`, so every operator except the
/// equality family rejects it (7.1.4 step 2, 7.1.17 step 2). node-js instead
/// concatenated `Symbol(desc)` into the result.
///
/// Which of the two messages V8 uses is decided by whether the operation is
/// STRING concatenation — measured on node v26.7.0, `Symbol() + ''` is
/// `Cannot convert a Symbol value to a string` while `Symbol() + 1`,
/// `Symbol() + Symbol()` and `Symbol() * 1` are all
/// `Cannot convert a Symbol value to a number`. `==`/`===` never convert
/// (`Symbol() == 1` is `false`), so they are left alone.
fn reject_symbol_operand(op: NumOp, a: &Value, b: &Value) -> Result<(), String> {
    use NumOp::*;
    if matches!(op, Eq | Ne) {
        return Ok(());
    }
    let (sym, concat) = with_host(|h| {
        let is_sym = |v: &Value| matches!(h.get(v), Some(JsObj::Symbol { .. }));
        let is_str =
            |v: &Value| matches!(v, Value::Str(_)) || matches!(h.get(v), Some(JsObj::Str(_)));
        (is_sym(a) || is_sym(b), is_str(a) || is_str(b))
    });
    if !sym {
        return Ok(());
    }
    Err(host::type_error(if matches!(op, Add) && concat {
        "Cannot convert a Symbol value to a string"
    } else {
        "Cannot convert a Symbol value to a number"
    }))
}

/// Whether a primitive `v` makes `==` against an object convert that object
/// (7.2.15 steps 10-11): numbers, strings, bigints and symbols do; `null`,
/// `undefined` and booleans are settled without a `ToPrimitive` call
/// (a boolean is coerced to a number first, and then it does).
fn coerces_against_object(v: &Value) -> bool {
    match v {
        Value::Undef => false,
        Value::Bool(_) | Value::Int(_) | Value::Float(_) | Value::Str(_) => true,
        _ => with_host(|h| !h.is_null(v)),
    }
}

// ══ standard library ═══════════════════════════════════════════════════════════

/// Namespaces reachable as bare globals.
fn is_namespace(name: &str) -> bool {
    matches!(
        name,
        "console"
            | "Math"
            | "JSON"
            | "Object"
            | "Array"
            | "Number"
            | "String"
            | "Boolean"
            | "Symbol"
            | "Reflect"
            | "Promise"
            | "process"
            | "Buffer"
            | "URL"
            | "URLSearchParams"
    )
}

const GLOBAL_FUNCS: &[&str] = &[
    "parseInt",
    "parseFloat",
    "isNaN",
    "isFinite",
    "encodeURIComponent",
    "decodeURIComponent",
    "encodeURI",
    "decodeURI",
    // Annex B legacy encoders. Still globals on every engine, and still called
    // by pre-`encodeURIComponent` library code.
    "escape",
    "unescape",
    "eval",
    "String",
    "Number",
    "Boolean",
    "Array",
    "Object",
    "Function",
    "Symbol",
    "Map",
    "Set",
    "WeakMap",
    "WeakSet",
    "Promise",
    "Error",
    "TypeError",
    "RangeError",
    "SyntaxError",
    "ReferenceError",
    "EvalError",
    "URIError",
    "AggregateError",
    "DOMException",
    "Iterator",
    "BigInt",
    "RegExp",
    "Date",
    "ArrayBuffer",
    "DataView",
    "Uint8Array",
    "Int8Array",
    "Uint8ClampedArray",
    "Int16Array",
    "Uint16Array",
    "Int32Array",
    "Uint32Array",
    "Float32Array",
    "Float64Array",
    "BigInt64Array",
    "BigUint64Array",
    "WeakRef",
    "FinalizationRegistry",
    "TextEncoder",
    "TextDecoder",
    // WHATWG Fetch globals (see `stdlib::fetch`).
    "fetch",
    "Headers",
    "Request",
    "Response",
    "Blob",
    "File",
    "FormData",
    "AbortController",
    "AbortSignal",
    "queueMicrotask",
    "setTimeout",
    "setInterval",
    "setImmediate",
    "clearTimeout",
    "clearInterval",
    "clearImmediate",
    "structuredClone",
    // Base64 helpers. They existed only as `require('buffer').btoa`, but node
    // exposes both as globals, so `btoa('abc')` was a ReferenceError.
    "btoa",
    "atob",
    "Proxy",
    "require",
    // CommonJS loader dispatch targets referenced by per-module `require`
    // closures (see `module.rs`); never written by user code.
    "__cjs_require",
    "__cjs_resolve",
    "__cjs_cache",
];

const NS_METHODS: &[&str] = &[
    "console.log",
    "console.error",
    "console.warn",
    "console.info",
    "console.debug",
    "Math.abs",
    "Math.acos",
    "Math.acosh",
    "Math.asin",
    "Math.asinh",
    "Math.atan",
    "Math.atanh",
    "Math.atan2",
    "Math.ceil",
    "Math.cbrt",
    "Math.expm1",
    "Math.clz32",
    "Math.cos",
    "Math.cosh",
    "Math.exp",
    "Math.floor",
    "Math.fround",
    "Math.hypot",
    "Math.imul",
    "Math.log",
    "Math.log1p",
    "Math.log2",
    "Math.log10",
    "Math.max",
    "Math.min",
    "Math.pow",
    "Math.random",
    "Math.round",
    "Math.sign",
    "Math.sin",
    "Math.sinh",
    "Math.sqrt",
    "Math.tan",
    "Math.tanh",
    "Math.trunc",
    "JSON.stringify",
    "JSON.parse",
    "JSON.rawJSON",
    "JSON.isRawJSON",
    "Object.keys",
    "Object.values",
    "Object.entries",
    "Object.assign",
    "Object.freeze",
    "Object.is",
    "Object.fromEntries",
    "Object.getPrototypeOf",
    "Object.setPrototypeOf",
    "Object.create",
    "Object.getOwnPropertyNames",
    "Object.getOwnPropertySymbols",
    "Object.defineProperty",
    "Object.getOwnPropertyDescriptor",
    "Object.getOwnPropertyDescriptors",
    "Object.defineProperties",
    "Object.isFrozen",
    "Object.isSealed",
    "Object.seal",
    "Object.preventExtensions",
    "Object.isExtensible",
    "Object.hasOwn",
    "Object.groupBy",
    "Array.isArray",
    "Array.from",
    "Array.fromAsync",
    "Array.of",
    "Number.isFinite",
    "Number.isInteger",
    "Number.isNaN",
    "Number.isSafeInteger",
    "Number.parseFloat",
    "Number.parseInt",
    "String.fromCharCode",
    "String.fromCodePoint",
    "String.raw",
    "Symbol.for",
    "Symbol.keyFor",
    "BigInt.asIntN",
    "BigInt.asUintN",
    "Proxy.revocable",
    "Reflect.defineProperty",
    "Reflect.deleteProperty",
    "Reflect.apply",
    "Reflect.construct",
    "Reflect.get",
    "Reflect.getOwnPropertyDescriptor",
    "Reflect.getPrototypeOf",
    "Reflect.has",
    "Reflect.isExtensible",
    "Reflect.ownKeys",
    "Reflect.preventExtensions",
    "Reflect.set",
    "Reflect.setPrototypeOf",
    "Promise.resolve",
    "Promise.reject",
    "Promise.all",
    "Promise.allSettled",
    "Promise.race",
    "Promise.any",
    "Promise.withResolvers",
    "Promise.try",
    "RegExp.escape",
    "Error.isError",
    "Map.groupBy",
    "Response.json",
    "Response.error",
    "Response.redirect",
    "AbortSignal.abort",
    "AbortSignal.timeout",
    "process.nextTick",
    "Error.captureStackTrace",
    "require.resolve",
    "require.resolve.paths",
    "process.memoryUsage.rss",
];

/// The `name` and `length` a builtin function reports, from the generated
/// intrinsic table ([`crate::arity::BUILTIN_ARITY`]). `None` for a key the table
/// does not cover — every non-function namespace (`Math`, `require('fs')`),
/// and the core-module functions, whose arity is not specified anywhere.
pub fn builtin_meta(key: &str) -> Option<(&'static str, u32)> {
    crate::arity::BUILTIN_ARITY
        .binary_search_by(|(k, _, _)| (*k).cmp(key))
        .ok()
        .map(|i| {
            let (_, name, len) = crate::arity::BUILTIN_ARITY[i];
            (name, len)
        })
}

/// The `name` a builtin function reports. The table answers for an intrinsic;
/// anything else falls back to the last segment of the key, which is what the
/// name is for every builtin this frontend synthesizes: `@proto:TypedArray:set`
/// is `set` and `fs.readFileSync` is `readFileSync`. Reporting the whole key was
/// how `[Function: @proto:TypedArray:set]` reached `console.log`.
pub fn builtin_name(key: &str) -> &str {
    if let Some((name, _)) = builtin_meta(key) {
        return name;
    }
    match key.strip_prefix("@proto:") {
        Some(rest) => rest.rsplit(':').next().unwrap_or(rest),
        // An accessor's getter is named `get <member>` (10.2.9 SetFunctionName
        // with a `get` prefix), which is what `util.inspect` prints for it and
        // what a library reads to identify one.
        None => key.rsplit('.').next().unwrap_or(key),
    }
}

/// The `name` of an intrinsic accessor's getter thunk, or `None` for anything
/// else. Kept out of `builtin_name`'s `&str` return, which cannot own the
/// `"get size"` it has to build.
pub fn proto_getter_name(key: &str) -> Option<String> {
    let (verb, rest) = match key.strip_prefix("@protoget:") {
        Some(rest) => ("get", rest),
        None => ("set", key.strip_prefix("@protoset:")?),
    };
    let (_, member) = rest.split_once(':')?;
    Some(format!("{verb} {member}"))
}

pub fn is_known_builtin(name: &str) -> bool {
    // Binary search over a sorted INDEX of the two tables rather than a scan of
    // both. This runs on every call whose callee is a builtin — `call_method`
    // asks it before dispatching `Math.max(…)` or `JSON.parse(…)` — and the
    // answer came only after a full scan of `GLOBAL_FUNCS` (77) plus a scan of
    // `NS_METHODS` up to the entry — 106 string comparisons for `Math.max`, 120
    // for `Object.keys` — because those tables are ordered for ENUMERATION (V8's
    // own order for `Math`/`Number`/`Reflect`), not for lookup. Eight probes
    // now. The index is built once per process and derived FROM those tables, so
    // it cannot drift from them.
    //
    // That is an operation count, not a measured time, and NO wall-clock win is
    // claimed. Re-measured in isolation (this hunk alone applied to the previous
    // commit, interleaved against it, minimums over ten rounds each): the A/B
    // ratio came out 0.753, 1.072, 0.994 and 0.744 across four repeats, while
    // the A/A control — the SAME binary under both labels — came out 1.084,
    // 1.072, 0.787 and 1.093. The A/B spread lies inside the A/A spread, so on
    // this machine the change is not distinguishable from noise. It is kept for
    // the comparison count and because it cannot drift from the tables it is
    // derived from, not because anything got faster.
    static SORTED: std::sync::OnceLock<Vec<&'static str>> = std::sync::OnceLock::new();
    let sorted = SORTED.get_or_init(|| {
        let mut v: Vec<&'static str> = GLOBAL_FUNCS
            .iter()
            .chain(NS_METHODS.iter())
            .copied()
            .collect();
        v.sort_unstable();
        v
    });
    sorted.binary_search(&name).is_ok() || is_namespace(name) || crate::stdlib::is_method(name)
}

// ── dynamic functions (runtime source → callable) ────────────────────────────

/// Build a callable from a complete function-expression source text — the ONE
/// dynamic-function generator on this frontend.
///
/// `src` is the exact source V8 synthesizes for the construct, WITHOUT the
/// wrapping parentheses needed to parse it as an expression: those are added
/// here, and `src` itself is retained so `Function.prototype.toString` reports
/// what V8 reports. The two callers synthesize different text and both shapes
/// are observable — see `stdlib::vm::compile_function` for the measured diff.
///
/// The body runs in the MODULE scope, never the constructing function's scope
/// (20.2.1.1.1 step 26 instantiates a dynamic function's body against the
/// *global* environment). That also makes a `var` inside the body a function
/// local: measured on node v26.7.0, `new Function('a','var zz = 5; return zz + a')`
/// returns 6 and leaves `globalThis.zz` `undefined`.
pub fn dynamic_function(src: &str) -> Result<Value, String> {
    let f = crate::eval_in_global_scope(&format!("({src})"))?;
    with_host(|h| {
        let s = h.new_str(src.to_string());
        h.set_fn_prop(&f, "@@source", s);
    });
    Ok(f)
}

/// `new Function(p1, …, pN, body)` / `Function(p1, …, pN, body)`.
///
/// Argument convention (20.2.1.1.1): the LAST argument is the body and the rest
/// are parameter-list fragments joined with `,` — so a fragment may itself hold
/// several parameters (`new Function('a,b', 'c', …)` takes three). With no
/// arguments at all, both the parameter list and the body are empty.
///
/// Measured on node v26.7.0:
///
/// ```text
/// new Function('a','b','return a+b').toString() === 'function anonymous(a,b\n) {\nreturn a+b\n}'
/// new Function().toString()                     === 'function anonymous(\n) {\n\n}'
/// new Function('a,b','c','return [a,b,c]').length === 3
/// new Function('a','b','return a+b').name       === 'anonymous'
/// ```
pub fn function_ctor(args: &[Value]) -> Result<Value, String> {
    let parts: Vec<String> = args.iter().map(|a| with_host(|h| h.str_of(a))).collect();
    let (params, body) = match parts.split_last() {
        Some((body, params)) => (params.join(","), body.clone()),
        None => (String::new(), String::new()),
    };
    dynamic_function(&format!("function anonymous({params}\n) {{\n{body}\n}}"))
}

/// `eval(src)`. `direct` selects the scope the source runs in: a DIRECT eval —
/// the literal `eval(...)` call form — evaluates in the CALLER's scope, every
/// other route to the same function value is an INDIRECT eval and evaluates in
/// the global scope (ECMA-262 19.2.1.1 `PerformEval`). The two are told apart in
/// `host::call_named`, which `ops::CALL` reaches and `ops::CALL_VALUE`/`APPLY`
/// do not.
///
/// A non-string argument is returned unchanged (19.2.1.1 step 2).
pub fn eval_source(arg: Option<&Value>, direct: bool) -> Result<Value, String> {
    let v = arg.cloned().unwrap_or(Value::Undef);
    let is_string =
        matches!(v, Value::Str(_)) || with_host(|h| matches!(h.get(&v), Some(JsObj::Str(_))));
    if !is_string {
        return Ok(v);
    }
    let src = with_host(|h| h.str_of(&v));
    // A DIRECT eval inherits the caller's strictness (19.2.1.1 step 10), which
    // decides both the early errors the COMPILE raises and the variable
    // environment below. An INDIRECT one is global-scope sloppy code.
    let caller_strict = direct && with_host(|h| h.current_strict());
    let chunk = crate::load_merged(crate::compile_completion_strict(&src, caller_strict)?);
    if !direct {
        return host::run_chunk_in_global_scope(chunk);
    }
    // A STRICT direct eval gets its OWN variable environment (19.2.1.1 step 12),
    // so its `var`s and function declarations die with it. Only a SLOPPY one
    // shares the caller's, which is the form that can inject a binding — and
    // sharing it unconditionally meant `eval('var x=1')` inside strict code
    // left `x` behind.
    let strict = caller_strict
        || src.trim_start().starts_with("'use strict'")
        || src.trim_start().starts_with("\"use strict\"");
    if !strict {
        // 19.2.1.1 steps 12-13: a SLOPPY direct eval shares the caller's
        // VARIABLE environment — which is what lets `eval('var x=1')` inject a
        // binding — but gets a fresh LEXICAL one of its own. A `let`, `const`
        // or `class` declared inside therefore dies with the eval; every one of
        // them was landing in the caller's scope, so `eval('let a=1')` left `a`
        // behind and `let a=1; eval('let a=2')` overwrote it.
        //
        // `push_scope` is exactly that split: `var` and a hoisted function
        // declaration bind to `base_env`, which this does not touch.
        with_host(|h| h.push_scope());
        let out = host::run_chunk_on(chunk);
        with_host(|h| h.pop_scope());
        return out;
    }
    let prev = with_host(|h| h.push_var_scope());
    let out = host::run_chunk_on(chunk);
    with_host(|h| h.pop_var_scope(prev));
    out
}

/// Call a resolved builtin function (global or `namespace.method`).
pub fn call_builtin_function(name: &str, args: Vec<Value>) -> Result<Value, String> {
    // `require(spec)`: the ENTRY script's top-level require — core module first,
    // else the CommonJS loader resolving from the entry file's directory.
    if name == "require" {
        let spec = with_host(|h| h.str_of(&arg0(&args)));
        return crate::module::require(&spec, &crate::module::entry_dir());
    }
    // `__cjs_require(spec, fromDir)`: a per-module `require` closure's dispatch
    // into the loader, resolving `spec` against the module's own directory.
    if name == "__cjs_require" {
        let spec = with_host(|h| h.str_of(&arg0(&args)));
        let from = with_host(|h| h.str_of(args.get(1).unwrap_or(&Value::Undef)));
        return crate::module::require(&spec, std::path::Path::new(&from));
    }
    if name == "process.memoryUsage.rss" {
        return Ok(crate::stdlib::process::memory_usage_rss());
    }
    if name == "require.resolve.paths" {
        let spec = with_host(|h| h.str_of(&arg0(&args)));
        // A core module is not looked up on disk at all.
        if crate::stdlib::is_core(&spec) {
            return Ok(with_host(|h| h.null()));
        }
        let dirs = crate::module::resolve_paths(&spec, &crate::module::entry_dir());
        return Ok(with_host(|h| {
            let items: Vec<Value> = dirs.into_iter().map(|d| h.new_str(d)).collect();
            h.new_array(items)
        }));
    }
    // A `require.extensions` entry. This runtime's loader does not dispatch
    // through the map, so calling one is the loader's own behaviour for that
    // extension rather than a hook point.
    if let Some(ext) = name.strip_prefix("@@extension:") {
        let _ = ext;
        return Ok(Value::Undef);
    }
    // `require.resolve(spec)` at the ENTRY level: resolve from the entry dir.
    if name == "require.resolve" {
        let spec = with_host(|h| h.str_of(&arg0(&args)));
        if crate::stdlib::is_core(&spec) {
            return Ok(with_host(|h| h.new_str(spec)));
        }
        return match crate::module::resolve(&spec, &crate::module::entry_dir()) {
            Some(p) => Ok(with_host(|h| h.new_str(p.to_string_lossy().to_string()))),
            None => Err(crate::host::plain_coded_error(
                "Error",
                "MODULE_NOT_FOUND",
                &format!("Cannot find module '{spec}'"),
            )),
        };
    }
    // `__cjs_resolve(spec, fromDir)`: `require.resolve` — the resolved absolute
    // path (core modules resolve to the bare specifier, as in Node).
    if name == "__cjs_resolve" {
        let spec = with_host(|h| h.str_of(&arg0(&args)));
        let from = with_host(|h| h.str_of(args.get(1).unwrap_or(&Value::Undef)));
        if crate::stdlib::is_core(&spec) {
            return Ok(with_host(|h| h.new_str(spec)));
        }
        return match crate::module::resolve(&spec, std::path::Path::new(&from)) {
            Some(p) => Ok(with_host(|h| h.new_str(p.to_string_lossy().to_string()))),
            None => Err(crate::host::plain_coded_error(
                "Error",
                "MODULE_NOT_FOUND",
                &format!("Cannot find module '{spec}'"),
            )),
        };
    }
    // `Error.captureStackTrace(target[, ctor])`: V8's stack capture. Sets
    // `target.stack`; when a custom `Error.prepareStackTrace` is installed (the
    // stack-introspection pattern used by `depd`), it is called with a synthetic
    // CallSite array and its result becomes `.stack`, else `.stack` is a string.
    if name == "Error.captureStackTrace" {
        let target = arg0(&args);
        let prep = with_host(|h| h.builtin_static("Error", "prepareStackTrace"));
        let stack = match prep {
            Some(f)
                if matches!(
                    with_host(|h| h.get(&f).cloned()),
                    Some(JsObj::Func(_)) | Some(JsObj::Builtin(_)) | Some(JsObj::BoundFunc { .. })
                ) =>
            {
                let sites = crate::module::callsite_stack(10)?;
                host::invoke(&f, vec![target.clone(), sites], None)?
            }
            _ => with_host(|h| h.new_str("")),
        };
        let _ = set_property(&target, "stack", stack);
        return Ok(Value::Undef);
    }
    // Native stdlib module methods (path/os/fs/util/assert/crypto/buffer/url).
    if let Some(r) = crate::stdlib::call(name, &args) {
        return r;
    }
    match name {
        // Node's DEFAULT `Error.prepareStackTrace`: the `Name: message` header
        // followed by one `    at <site>` line per call site. Reachable because
        // the read hands the hook out, and a library may call it directly to
        // render a stack it captured.
        DEFAULT_PREPARE => {
            let err = arg0(&args);
            let header = with_host(|h| {
                let name = host::lookup_chain(h, &err, "name")
                    .map(|v| h.str_of(&v))
                    .unwrap_or_else(|| "Error".to_string());
                let msg = host::lookup_chain(h, &err, "message")
                    .map(|v| h.str_of(&v))
                    .unwrap_or_default();
                if msg.is_empty() {
                    name
                } else {
                    format!("{name}: {msg}")
                }
            });
            let sites = args.get(1).cloned().unwrap_or(Value::Undef);
            let lines = with_host(|h| match h.get(&sites) {
                Some(JsObj::Array(items)) => items.clone(),
                _ => Vec::new(),
            });
            let mut out = header;
            for s in lines {
                let rendered = host::to_string_value(&s)
                    .map(|v| with_host(|h| h.str_of(&v)))
                    .unwrap_or_default();
                out.push_str("\n    at ");
                out.push_str(&rendered);
            }
            Ok(with_host(|h| h.new_str(out)))
        }
        "console.log" | "console.info" | "console.debug" => {
            print_line(&args, false)?;
            Ok(Value::Undef)
        }
        "console.error" | "console.warn" => {
            print_line(&args, true)?;
            Ok(Value::Undef)
        }
        "parseInt" | "Number.parseInt" => Ok(Value::Float(parse_int(&args)?)),
        "parseFloat" | "Number.parseFloat" => Ok(Value::Float(parse_float(&args)?)),
        // `isNaN`/`isFinite` are `ToNumber(x)` too (19.2.3/4).
        "isNaN" => Ok(Value::Bool(to_number_arg(&args, 0)?.is_nan())),
        "isFinite" => Ok(Value::Bool(to_number_arg(&args, 0)?.is_finite())),
        "encodeURIComponent" => uri_encode(&arg_to_string(&args, 0)?, false),
        "encodeURI" => uri_encode(&arg_to_string(&args, 0)?, true),
        "decodeURIComponent" => uri_decode(&arg_to_string(&args, 0)?, false),
        "decodeURI" => uri_decode(&arg_to_string(&args, 0)?, true),
        "escape" => legacy_escape(&with_host(|h| h.str_of(&arg0(&args)))),
        "unescape" => legacy_unescape(&with_host(|h| h.str_of(&arg0(&args)))),
        // Reaching `eval` through this table means the eval FUNCTION VALUE was
        // called — `(0, eval)(src)`, `const e = eval; e(src)`, `[eval][0](src)`.
        // Those are INDIRECT evals and run in the global scope. A literal
        // `eval(src)` is intercepted earlier, in `host::call_named`.
        "eval" => eval_source(args.first(), false),
        // `new Function(...)` and `Function(...)` are the same operation
        // (20.2.1.1 `CreateDynamicFunction` is reached from both [[Call]] and
        // [[Construct]]), so both route to the one generator.
        "Function" => function_ctor(&args),
        // `Buffer(arg[, encodingOrOffset[, length]])` — the deprecated call form
        // (DEP0005). Node still supports it and still routes it to the same place
        // `new Buffer` goes, which is why `safe-buffer`'s legacy `SafeBuffer`
        // wrapper is just `return Buffer(arg, encodingOrOffset, length)`. Measured
        // on node v26.7.0: `Buffer('abc').toString() === 'abc'`,
        // `Buffer([1,2]).toString('hex') === '0102'`, `Buffer(3).length === 3`.
        // Node emits DEP0005 once, on stderr, through the same one-shot machinery
        // `url.parse`'s DEP0169 uses, so this does too rather than staying silent
        // where Node warns.
        "Buffer" => {
            crate::stdlib::process::emit_deprecation_warning(
                "DEP0005",
                "Buffer() is deprecated due to security and usability issues. \
                 Please use the Buffer.alloc(), Buffer.allocUnsafe(), or \
                 Buffer.from() methods instead.",
            );
            crate::stdlib::construct("Buffer", &args)
                .unwrap_or_else(|| Err(host::type_error("Buffer is not a function")))
        }
        "Number.isInteger" => Ok(Value::Bool(is_integer(arg0(&args)))),
        "Number.isSafeInteger" => Ok(Value::Bool(is_safe_integer(arg0(&args)))),
        "Number.isNaN" => Ok(Value::Bool(
            matches!(arg0(&args), Value::Float(f) if f.is_nan()),
        )),
        "Number.isFinite" => Ok(Value::Bool(
            matches!(arg0(&args), Value::Float(f) if f.is_finite())
                || matches!(arg0(&args), Value::Int(_)),
        )),
        "String" => {
            if args.is_empty() {
                Ok(with_host(|h| h.new_str("")))
            } else {
                // A symbol argument stringifies to `Symbol(desc)` (explicit String()
                // is allowed); everything else via ToString method dispatch.
                host::string_ctor_value(&args[0])
            }
        }
        // `Number(v)` is NOT plain ToNumber: 21.1.1.1 step 2 converts the object
        // first and then explicitly ACCEPTS a BigInt, returning its mathematical
        // value as a Number. Only `Number` does — `+v` and `Math.abs(v)` reject
        // one — which is why this cannot just call `to_number_value`.
        "Number" => Ok(Value::Float(if args.is_empty() {
            0.0
        } else {
            let prim = host::to_primitive(&args[0], "number")?;
            match with_host(|h| h.as_bigint(&prim)) {
                Some(b) => host::bigint_to_f64(&b),
                None => host::to_number_value(&prim)?,
            }
        })),
        "BigInt" => bigint_ctor(&arg0(&args)),
        "RegExp" => regexp_ctor(&args),
        "BigInt.asIntN" | "BigInt.asUintN" => bigint_as_n(name.ends_with("asUintN"), &args),
        "Boolean" => Ok(Value::Bool(with_host(|h| h.truthy(&arg0(&args))))),
        // Each argument is truncated to a uint16 and taken as one code UNIT, so
        // `String.fromCharCode(0x1D4B3)` is U+D4B3, NOT the astral U+1D4B3, and
        // a surrogate PAIR of arguments composes into one character.
        "String.fromCharCode" => Ok(with_host(|h| {
            let units: Vec<u16> = args
                .iter()
                .map(|a| crate::utf16::to_uint16(h.to_number(a)))
                .collect();
            let s = crate::utf16::to_string_lossy(&units);
            h.new_str(s)
        })),
        // `fromCodePoint` takes whole code POINTS and rejects anything that is
        // not one — including a lone surrogate, which `fromCharCode` accepts.
        "String.fromCodePoint" => {
            let mut s = String::new();
            for a in &args {
                let n = with_host(|h| h.to_number(a));
                let cp = if n.is_finite() && n.trunc() == n && (0.0..=0x10FFFF as f64).contains(&n)
                {
                    char::from_u32(n as u32)
                } else {
                    None
                };
                match cp {
                    Some(c) => s.push(c),
                    None => {
                        return Err(format!(
                            "RangeError: Invalid code point {}",
                            with_host(|h| h.str_of(a))
                        ))
                    }
                }
            }
            Ok(new_s(s))
        }
        "String.raw" => string_raw(&args),
        // `Array(5)` === `new Array(5)` (length-5 empty), but `Array.of(5)` is `[5]`.
        "Array" => construct_builtin("Array", args),
        "Array.of" => construct_array_like(host::current_static_this(), args),
        // 23.1.2.2 `IsArray` follows a Proxy to its `[[ProxyTarget]]` rather than
        // consulting any trap, so `Array.isArray(new Proxy([], {}))` is `true`.
        "Array.isArray" => {
            let v = arg0(&args);
            let subject = crate::proxy::ultimate_target(&v).unwrap_or(v);
            Ok(Value::Bool(
                matches!(
                    with_host(|h| h.get(&subject).cloned()),
                    Some(JsObj::Array(_))
                ) && !is_arguments(&subject),
            ))
        }
        "Array.from" => array_from(args),
        "Array.fromAsync" => array_from_async(args),
        "Object" => Ok(object_call(args)),
        "Object.keys" => object_keys(args, 0),
        "Object.values" => object_keys(args, 1),
        "Object.entries" => object_keys(args, 2),
        "Object.assign" => object_assign(args),
        "Object.freeze" => {
            let v = arg0(&args);
            reject_sealing_a_view(&v, "freeze")?;
            if seal_proxy(&v, true)? {
                return Ok(v);
            }
            with_host(|h| h.seal_object(&v, true));
            Ok(v)
        }
        "Object.seal" => {
            let v = arg0(&args);
            reject_sealing_a_view(&v, "seal")?;
            if seal_proxy(&v, false)? {
                return Ok(v);
            }
            with_host(|h| h.seal_object(&v, false));
            Ok(v)
        }
        "Object.preventExtensions" => {
            let v = arg0(&args);
            if crate::proxy::prevent_extensions(&v)? {
                return Ok(v);
            }
            with_host(|h| h.prevent_extensions(&v));
            Ok(v)
        }
        // A PRIMITIVE has no integrity to speak of and 7.3.15/16 answer for it
        // without coercion: it is not extensible, and vacuously frozen and
        // sealed. Reporting it extensible and unfrozen was the opposite of
        // every one of the three.
        "Object.isFrozen" if is_primitive_arg(&args) => Ok(Value::Bool(true)),
        "Object.isSealed" if is_primitive_arg(&args) => Ok(Value::Bool(true)),
        "Object.isExtensible" if is_primitive_arg(&args) => Ok(Value::Bool(false)),
        "Object.isFrozen" => integrity_level(&arg0(&args), true),
        "Object.isSealed" => integrity_level(&arg0(&args), false),
        "Object.isExtensible" => {
            let v = arg0(&args);
            match crate::proxy::is_extensible(&v)? {
                Some(b) => Ok(Value::Bool(b)),
                None => Ok(Value::Bool(with_host(|h| h.is_extensible(&v)))),
            }
        }
        // Object.is — SameValue: like `===` but NaN is equal to NaN and +0 is
        // distinct from -0.
        "Object.is" => {
            let a = arg0(&args);
            let b = args.get(1).cloned().unwrap_or(Value::Undef);
            let num = |v: &Value| match v {
                Value::Int(n) => Some(*n as f64),
                Value::Float(f) => Some(*f),
                _ => None,
            };
            let r = match (num(&a), num(&b)) {
                (Some(x), Some(y)) => {
                    if x.is_nan() && y.is_nan() {
                        true
                    } else if x == 0.0 && y == 0.0 {
                        x.is_sign_negative() == y.is_sign_negative()
                    } else {
                        x == y
                    }
                }
                _ => with_host(|h| h.strict_eq(&a, &b)),
            };
            Ok(Value::Bool(r))
        }
        "Object.fromEntries" => object_from_entries(args),
        // `[[GetPrototypeOf]]`: a Proxy answers from its trap (which may throw),
        // so the proxy form cannot share `prototype_of`'s infallible signature.
        // `Object.getPrototypeOf` coerces a primitive to its wrapper and
        // answers; `Reflect.getPrototypeOf` requires an object (28.1.8).
        "Object.getPrototypeOf" | "Reflect.getPrototypeOf" => {
            if name == "Reflect.getPrototypeOf" {
                reflect_require_object(&arg0(&args), "getPrototypeOf")?;
            }
            let v = arg0(&args);
            match crate::proxy::get_prototype_of(&v)? {
                Some(p) => Ok(p),
                None => Ok(prototype_of(&v)),
            }
        }
        "Object.setPrototypeOf" => {
            let obj = arg0(&args);
            let proto = args.get(1).cloned().unwrap_or(Value::Undef);
            if with_host(|h| h.kind_of(&obj)) == Some(ObjKind::Proxy) {
                reject_bad_prototype(&proto)?;
                crate::proxy::set_prototype_of(&obj, &proto)?;
                return Ok(obj);
            }
            // 20.1.2.23: `RequireObjectCoercible` on the target, then the
            // prototype type check, then — only for an actual object target —
            // the extensibility check. A PRIMITIVE target is returned untouched
            // (`Object.setPrototypeOf(1, {})` is `1`), which is why the
            // extensibility test cannot come first.
            if with_host(|h| matches!(obj, Value::Undef) || h.is_null(&obj)) {
                return Err(host::type_error(
                    "Object.setPrototypeOf called on null or undefined",
                ));
            }
            reject_bad_prototype(&proto)?;
            if with_host(|h| is_object_like(h, &obj)) {
                // Setting the SAME prototype is a no-op and stays legal even on a
                // frozen object: node v26.7.0 accepts
                // `Object.setPrototypeOf(Object.freeze({}), Object.prototype)`.
                // `prototype_of`, not `proto_of`: an object with no EXPLICIT
                // link still has `Object.prototype`, and comparing against the
                // absent link would call that a change.
                if would_cycle(&obj, &proto) {
                    return Err(host::type_error("Cyclic __proto__ value"));
                }
                if !same_prototype(&obj, &proto) && !with_host(|h| h.is_extensible(&obj)) {
                    // The receiver is named by its brand, as every other
                    // refusal names it — a NULL-PROTOTYPE object is
                    // `[object Object]`, not `#<Object>`, because it has no
                    // constructor to name.
                    return Err(host::type_error(&format!(
                        "{} is not extensible",
                        no_side_effects_string(&obj)
                    )));
                }
                with_host(|h| h.set_proto(&obj, proto));
            }
            Ok(obj)
        }
        "Object.create" => object_create(args),
        "Object.getOwnPropertyNames" => object_keys(args, 3),
        "Object.getOwnPropertySymbols" => {
            let v = arg0(&args);
            require_object_coercible(&v)?;
            let syms = proxy_or_own_symbol_keys(&v)?;
            Ok(with_host(|h| h.new_array(syms)))
        }
        // `Object.hasOwn(obj, key)` — the static form of `hasOwnProperty`.
        "Object.hasOwn" => {
            let obj = arg0(&args);
            let key = args.get(1).cloned().unwrap_or(Value::Undef);
            object_builtin_method(&obj, "hasOwnProperty", vec![key])
        }
        "Object.defineProperty" => object_define_property(args),
        "Object.getOwnPropertyDescriptor" => object_get_own_descriptor(args),
        "Object.getOwnPropertyDescriptors" => object_get_own_descriptors(args),
        "Object.defineProperties" => object_define_properties(args),
        // `Object.groupBy(items, cb)` (ES2024): group into a null-prototype object
        // keyed by `ToPropertyKey(cb(item, i))`, each value an array of members.
        "Object.groupBy" => object_group_by(args),
        "Symbol" => Ok(with_host(|h| {
            let desc = args
                .first()
                .filter(|a| !matches!(a, Value::Undef))
                .map(|a| h.str_of(a));
            h.new_symbol(desc)
        })),
        "Symbol.for" => Ok(with_host(|h| {
            let key = h.str_of(&arg0(&args));
            h.symbol_for(&key)
        })),
        // `Symbol.keyFor(sym)` (20.4.2.6) is a REGISTRY lookup, not a
        // description read: it answers only for symbols `Symbol.for` created.
        // Returning the description made every symbol look registered —
        // `Symbol.keyFor(Symbol("k"))` was `"k"` where node says `undefined`.
        "Symbol.keyFor" => Ok(with_host(|h| h.symbol_registry_key(&arg0(&args)))),
        "Map" | "WeakMap" | "Set" | "WeakSet" | "Promise" => construct_builtin(name, args),
        // `Proxy` has no `[[Call]]` slot: it is constructor-only (28.2.1).
        "Proxy" => Err(host::type_error("Constructor Proxy requires 'new'")),
        "Proxy.revocable" => crate::proxy::revocable(&args),
        // `Reflect.ownKeys` reports EVERY own key, non-enumerable included —
        // the same set as `getOwnPropertyNames` (node-js has no symbol-keyed
        // own properties, so there is no second half to append).
        // `Reflect.ownKeys` is `OwnPropertyKeys` (7.3.23): every own key,
        // non-enumerable included, strings first and then the SYMBOLS.
        "Reflect.ownKeys" => {
            let v = arg0(&args);
            reflect_require_object(&v, "ownKeys")?;
            let names = object_keys(args, 3)?;
            let syms = proxy_or_own_symbol_keys(&v)?;
            if syms.is_empty() {
                return Ok(names);
            }
            let mut all = with_host(|h| h.iter_vec(&names)).unwrap_or_default();
            all.extend(syms);
            Ok(with_host(|h| h.new_array(all)))
        }
        "Reflect.getOwnPropertyDescriptor" => object_get_own_descriptor(args),
        // `Reflect.defineProperty` REPORTS success as a boolean where
        // `Object.defineProperty` throws (28.1.3). It was propagating the
        // throw, so the whole point of the reflective form was lost.
        "Reflect.defineProperty" => {
            reflect_require_object(&arg0(&args), "defineProperty")?;
            Ok(Value::Bool(object_define_property(args).is_ok()))
        }
        "Reflect.deleteProperty" => {
            let obj = arg0(&args);
            reflect_require_object(&obj, "deleteProperty")?;
            let k = host::to_property_key(&args.get(1).cloned().unwrap_or(Value::Undef))?;
            Ok(Value::Bool(delete_property(&obj, &k)?))
        }
        "Reflect.setPrototypeOf" => {
            let obj = arg0(&args);
            let p = args.get(1).cloned().unwrap_or(Value::Undef);
            if with_host(|h| h.kind_of(&obj)) == Some(ObjKind::Proxy) {
                crate::proxy::set_prototype_of(&obj, &p)?;
                return Ok(Value::Bool(true));
            }
            // 10.1.2.1: a NON-EXTENSIBLE object refuses a prototype change —
            // unless the new one is what it already has, which is a no-op. It
            // reported success and rewrote the link.
            // `Reflect` reports a refusal rather than throwing, for a cycle as
            // for a non-extensible receiver.
            if would_cycle(&obj, &p) {
                return Ok(Value::Bool(false));
            }
            if !with_host(|h| h.is_extensible(&obj)) {
                return Ok(Value::Bool(same_prototype(&obj, &p)));
            }
            with_host(|h| h.set_proto(&obj, p));
            Ok(Value::Bool(true))
        }
        "Reflect.isExtensible" => {
            let v = arg0(&args);
            match crate::proxy::is_extensible(&v)? {
                Some(b) => Ok(Value::Bool(b)),
                None => Ok(Value::Bool(with_host(|h| h.is_extensible(&v)))),
            }
        }
        "Reflect.preventExtensions" => {
            let v = arg0(&args);
            if crate::proxy::prevent_extensions(&v)? {
                return Ok(Value::Bool(true));
            }
            with_host(|h| h.prevent_extensions(&v));
            Ok(Value::Bool(true))
        }
        // `Reflect.apply(target, thisArg, argsList)` / `Reflect.construct(t, a)`.
        "Reflect.apply" => {
            let f = arg0(&args);
            let this = args.get(1).cloned();
            let list = create_list_from_array_like(&args.get(2).cloned().unwrap_or(Value::Undef))?;
            host::invoke(&f, list, this.filter(|t| !with_host(|h| h.is_nullish(t))))
        }
        // `Reflect.construct(target, args, newTarget)` — the optional third
        // argument decides which constructor's `prototype` the instance gets
        // (28.1.2). It was ignored, so the result always inherited from
        // `target` and `instanceof newTarget` was false.
        "Reflect.construct" => {
            let f = arg0(&args);
            let list = create_list_from_array_like(&args.get(1).cloned().unwrap_or(Value::Undef))?;
            let new_target = args.get(2).cloned().unwrap_or_else(|| f.clone());
            host::construct_nt(&f, list, new_target)
        }
        "Reflect.has" => {
            let obj = arg0(&args);
            reflect_require_object(&obj, "has")?;
            let k = host::to_property_key(&args.get(1).cloned().unwrap_or(Value::Undef))?;
            Ok(Value::Bool(has_property(&obj, &k)?))
        }
        // `Reflect.get(target, key, receiver)` — the optional third argument is
        // what a getter sees as `this` (28.1.6). Defaults to the target.
        "Reflect.get" => {
            let obj = arg0(&args);
            reflect_require_object(&obj, "get")?;
            let k = host::to_property_key(&args.get(1).cloned().unwrap_or(Value::Undef))?;
            let receiver = args.get(2).cloned().unwrap_or_else(|| obj.clone());
            get_property_recv(&obj, &k, &receiver)
        }
        // `Reflect.set(target, key, value, receiver)` — the optional fourth
        // argument is what a setter sees as `this`, and where a DATA property
        // lands (28.1.13). It was ignored: the setter ran against the target
        // and the property was written there.
        "Reflect.set" => {
            let obj = arg0(&args);
            reflect_require_object(&obj, "set")?;
            let k = host::to_property_key(&args.get(1).cloned().unwrap_or(Value::Undef))?;
            let v = args.get(2).cloned().unwrap_or(Value::Undef);
            let receiver = args.get(3).cloned().unwrap_or_else(|| obj.clone());
            Ok(Value::Bool(set_with_receiver(&obj, &k, v, &receiver)?))
        }
        "JSON.stringify" => json_stringify(args),
        "JSON.parse" => json_parse(args),
        "JSON.rawJSON" => json_raw(args),
        "JSON.isRawJSON" => json_is_raw(args),
        "structuredClone" => structured_clone(args),
        // The deferred drain a `Readable.from` schedules; the suffix is the
        // stream's heap index.
        _ if name.starts_with("@@transformCb:") => {
            let idx: u32 = name["@@transformCb:".len()..].parse().unwrap_or(0);
            crate::stdlib::stream::transform_callback(&Value::Obj(idx), &args)?;
            Ok(Value::Undef)
        }
        _ if name.starts_with("@@streamFlush:") => {
            let idx: u32 = name["@@streamFlush:".len()..].parse().unwrap_or(0);
            crate::stdlib::stream::flush_from(&Value::Obj(idx))?;
            Ok(Value::Undef)
        }
        // Same implementation the `buffer` module exposes; only the binding was
        // missing.
        "btoa" | "atob" => crate::stdlib::buffer::module_call(name, &args)
            .unwrap_or_else(|| Err(host::type_error(&format!("{name} is not a function")))),
        "fetch" => crate::stdlib::fetch::fetch(&args),
        // An `AbortSignal.timeout` deadline reached its macrotask: the thunk's
        // suffix is the signal's heap index.
        _ if name.starts_with("@@aborttimeout:") => {
            let idx: u32 = name["@@aborttimeout:".len()..].parse().unwrap_or(0);
            crate::stdlib::fetch::fire_timeout_abort(idx)
        }
        // The `callback` handed to a `new Writable({ write(chunk, enc, cb) })`
        // implementation. Nothing here waits on backpressure, so it only has to
        // BE callable — an implementation that ends with `cb()`, which the
        // stream contract requires, would otherwise throw.
        "@@streamWriteCallback" => Ok(Value::Undef),
        "queueMicrotask" | "process.nextTick" => {
            let cb = arg0(&args);
            require_callback(&cb)?;
            let rest = args.get(1..).map(|s| s.to_vec()).unwrap_or_default();
            enqueue_microtask(name == "process.nextTick", cb, rest);
            Ok(Value::Undef)
        }
        "setTimeout" | "setInterval" | "setImmediate" => {
            require_callback(&arg0(&args))?;
            Ok(schedule_timer(name, args))
        }
        "clearTimeout" | "clearInterval" | "clearImmediate" => {
            clear_timer(&arg0(&args));
            Ok(Value::Undef)
        }
        "Promise.resolve" => promise_resolve(arg0(&args)),
        "Promise.reject" => promise_reject(arg0(&args)),
        "Promise.all" => promise_all(args, AllMode::All),
        "Promise.allSettled" => promise_all(args, AllMode::AllSettled),
        "Promise.race" => promise_race(args, false),
        "Promise.any" => promise_race(args, true),
        // `Promise.withResolvers()` (ES2024): a new pending promise plus its own
        // resolve/reject functions, returned as `{ promise, resolve, reject }`.
        "Promise.withResolvers" => promise_with_resolvers(),
        "Promise.try" => promise_try(args),
        "RegExp.escape" => regexp_escape(args),
        "Error.isError" => error_is_error(args),
        // `Map.groupBy(items, cb)` (ES2024): group into a `Map` keyed by the raw
        // `cb(item, i)` result (SameValueZero), each value an array of members.
        "Map.groupBy" => map_group_by(args),
        n if host::ERROR_NAMES.contains(&n) => make_error_checked(name, &args),
        _ if name.starts_with("Math.") => math_fn(&name[5..], &args),
        // Internal continuations (Promise resolve/reject fns, `.finally` wrappers).
        // The executor a species-constructed promise is built with: it does
        // nothing, because the caller settles the result through its id.
        "@@pnoop" => Ok(Value::Undef),
        _ if name.starts_with("@@presolve:") => {
            let id: u32 = name[11..].parse().unwrap_or(0);
            host::resolve_promise_val(id, arg0(&args));
            Ok(Value::Undef)
        }
        _ if name.starts_with("@@preject:") => {
            let id: u32 = name[10..].parse().unwrap_or(0);
            host::reject_promise_val(id, arg0(&args));
            Ok(Value::Undef)
        }
        // The revoker `Proxy.revocable` hands back, keyed by the proxy's heap
        // index so calling it twice is the spec's no-op rather than a re-tear.
        _ if name.starts_with("@@prevoke:") => {
            let i: u32 = name[10..].parse().unwrap_or(0);
            Ok(crate::proxy::revoke(i))
        }
        _ if name.starts_with("@@finpass:") => {
            // finally(cb) on fulfill: run cb, await whatever it returned, then
            // pass the original value through.
            let i: u32 = name["@@finpass:".len()..].parse().unwrap_or(0);
            let result = host::invoke(&Value::Obj(i), Vec::new(), None)?;
            Ok(finally_chain(result, arg0(&args), false))
        }
        _ if name.starts_with("@@finthrow:") => {
            // finally(cb) on reject: same, then re-throw the original reason.
            let i: u32 = name["@@finthrow:".len()..].parse().unwrap_or(0);
            let result = host::invoke(&Value::Obj(i), Vec::new(), None)?;
            Ok(finally_chain(result, arg0(&args), true))
        }
        // The two thunks `finally_chain` hangs off that awaited promise. Each
        // carries the value it must reinstate in a one-slot cell, since a
        // builtin is identified only by its name and cannot close over one.
        _ if name.starts_with("@@finret:") => {
            let i: u32 = name["@@finret:".len()..].parse().unwrap_or(0);
            get_property(&Value::Obj(i), "0")
        }
        _ if name.starts_with("@@finrethrow:") => {
            let i: u32 = name["@@finrethrow:".len()..].parse().unwrap_or(0);
            let reason = get_property(&Value::Obj(i), "0")?;
            with_host(|h| h.exc = Some(reason.clone()));
            Err(with_host(|h| error_string(h, &reason)))
        }
        _ => Err(host::type_error(&format!("{name} is not a function"))),
    }
}

/// `BigInt(x)`: convert a boolean/number/string/bigint to a BigInt. A
/// non-integer number is a `RangeError`; an unparseable string a `SyntaxError`
/// (matching Node's messages).
/// V8 names the offending value: `BigInt(undefined)` is `Cannot convert
/// undefined to a BigInt`, `BigInt({})` is `Cannot convert [object Object] to a
/// BigInt`. The old text said "value" literally, for every input.
fn bigint_convert_error(v: &Value) -> String {
    let shown = with_host(|h| h.str_of(v));
    host::type_error(&format!("Cannot convert {shown} to a BigInt"))
}

/// `ToBigInt(v)` — 7.1.13. The conversion every BigInt-typed SINK performs: a
/// 64-bit typed array's element write, `DataView.prototype.setBigInt64`, and
/// BigInt arithmetic's operand check.
///
/// It is NOT `BigInt(v)`: a Number is a `TypeError` here (`BigInt(1)` is `1n`,
/// but `new BigInt64Array(1)[0] = 1` throws), which is the whole point of the
/// separate abstract op. Everything else follows `ToPrimitive(v, number)` then
/// the type table — booleans convert (`true` → `1n`), strings parse with a
/// `SyntaxError` on failure, and `undefined`/`null`/symbols throw.
///
/// Measured on node v26.8.1, receiver `new BigInt64Array(1)`:
///
/// ```text
/// a[0] = true            → 1n
/// a[0] = '12'            → 12n
/// a[0] = []              → 0n        (ToPrimitive → "" → 0n)
/// a[0] = ['3']           → 3n
/// a[0] = 1               → TypeError: Cannot convert 1 to a BigInt
/// a[0] = new Number(3)   → TypeError: Cannot convert 3 to a BigInt
/// a[0] = 'a'             → SyntaxError: Cannot convert a to a BigInt
/// a[0] = {}              → SyntaxError: Cannot convert [object Object] to a BigInt
/// ```
pub fn to_bigint(v: &Value) -> Result<num_bigint::BigInt, String> {
    let prim = host::to_primitive(v, "number")?;
    if let Some(b) = with_host(|h| match h.get(&prim) {
        Some(JsObj::BigInt(b)) => Some(b.clone()),
        _ => None,
    }) {
        return Ok(b);
    }
    match &prim {
        Value::Bool(b) => Ok(num_bigint::BigInt::from(*b as i64)),
        Value::Str(s) => host::parse_bigint_str(s)
            .ok_or_else(|| format!("SyntaxError: Cannot convert {s} to a BigInt")),
        _ if with_host(|h| matches!(h.get(&prim), Some(JsObj::Str(_)))) => {
            let s = with_host(|h| h.str_of(&prim));
            host::parse_bigint_str(&s)
                .ok_or_else(|| format!("SyntaxError: Cannot convert {s} to a BigInt"))
        }
        _ => Err(bigint_convert_error(&prim)),
    }
}

fn bigint_ctor(v: &Value) -> Result<Value, String> {
    use num_bigint::BigInt;
    let big = match v {
        Value::Bool(b) => BigInt::from(*b as i64),
        Value::Int(n) => BigInt::from(*n),
        Value::Float(f) => {
            if !f.is_finite() || f.fract() != 0.0 {
                let disp = with_host(|h| h.str_of(v));
                return Err(format!(
                    "RangeError: The number {disp} cannot be converted to a BigInt because it is not an integer"
                ));
            }
            // The decimal EXPANSION, not `fmt_number`: `Number.prototype
            // .toString` switches to exponential notation at 1e21, and
            // `BigInt::parse_bytes` cannot read `"1e+21"` — so `BigInt(1e21)`
            // threw `Cannot convert value to a BigInt` where node returns
            // `1000000000000000000000n`. `{:.0}` prints an integral f64's exact
            // value, which is also what node reports for a magnitude past the
            // exactly-representable range (`BigInt(1e30)` is
            // `1000000000000000019884624838656n` in both).
            match BigInt::parse_bytes(format!("{f:.0}").as_bytes(), 10) {
                Some(b) => b,
                None => return Err(bigint_convert_error(v)),
            }
        }
        Value::Str(s) => match host::parse_bigint_str(s) {
            Some(b) => b,
            None => return Err(format!("SyntaxError: Cannot convert {s} to a BigInt")),
        },
        Value::Obj(_) => match with_host(|h| h.get(v).cloned()) {
            Some(JsObj::BigInt(b)) => b,
            Some(JsObj::Str(s)) => match host::parse_bigint_str(&s) {
                Some(b) => b,
                None => return Err(format!("SyntaxError: Cannot convert {s} to a BigInt")),
            },
            _ => return Err(bigint_convert_error(v)),
        },
        _ => return Err(bigint_convert_error(v)),
    };
    Ok(with_host(|h| h.new_bigint(big)))
}

/// `new RegExp(source[, flags])` / `RegExp(...)`. A first `RegExp` argument copies
/// its source (and flags, unless new ones are given).
fn regexp_ctor(args: &[Value]) -> Result<Value, String> {
    let (source, existing_flags) = match with_host(|h| h.get(&arg0(args)).cloned()) {
        Some(JsObj::RegExp(r)) => (r.source.clone(), Some(r.flags.clone())),
        _ => {
            let a0 = arg0(args);
            // 22.2.4.1 step 9 is `ToString(pattern)`, which a SYMBOL refuses —
            // `new RegExp(sym)` was compiling the text `Symbol(d)` into a
            // pattern instead of throwing.
            let src = if matches!(a0, Value::Undef) {
                String::new()
            } else {
                arg_to_string(args, 0)?
            };
            (src, None)
        }
    };
    let flags = match args.get(1) {
        Some(v) if !matches!(v, Value::Undef) => arg_to_string(args, 1)?,
        _ => existing_flags.unwrap_or_default(),
    };
    // An empty source compiles as the JS canonical `(?:)`.
    let src = if source.is_empty() {
        "(?:)".to_string()
    } else {
        source
    };
    crate::regexp::build_regexp(&src, &flags)
}

/// `BigInt.asIntN(bits, x)` / `BigInt.asUintN(bits, x)`: wrap `x` to a `bits`-wide
/// two's-complement (signed) or unsigned integer.
fn bigint_as_n(unsigned: bool, args: &[Value]) -> Result<Value, String> {
    use num_bigint::BigInt;
    use num_traits::Signed;
    let bits = with_host(|h| h.to_number(&arg0(args))) as i64;
    if bits < 0 {
        return Err("RangeError: Invalid value: not (convertible to) a safe integer".into());
    }
    let x = match with_host(|h| h.as_bigint(&args.get(1).cloned().unwrap_or(Value::Undef))) {
        Some(b) => b,
        None => return Err(host::type_error("Cannot convert to a BigInt")),
    };
    let bits = bits as u32;
    if bits == 0 {
        return Ok(with_host(|h| h.new_bigint(BigInt::from(0))));
    }
    let modulus = BigInt::from(1) << bits; // 2^bits
                                           // Reduce into [0, 2^bits); for the signed form fold the top half negative.
    let mut r = &x % &modulus;
    if r.is_negative() {
        r += &modulus;
    }
    if !unsigned {
        let half = BigInt::from(1) << (bits - 1);
        if r >= half {
            r -= &modulus;
        }
    }
    Ok(with_host(|h| h.new_bigint(r)))
}

/// `String.raw(callSite, ...subs)`: concatenate the raw quasis (`callSite.raw`)
/// interleaved with the substitutions.
fn string_raw(args: &[Value]) -> Result<Value, String> {
    let call_site = arg0(args);
    let raw = get_property(&call_site, "raw")?;
    let raws = with_host(|h| h.iter_vec(&raw)).unwrap_or_default();
    let mut out = String::new();
    for (i, r) in raws.iter().enumerate() {
        out.push_str(&with_host(|h| h.str_of(r)));
        if i + 1 < raws.len() {
            if let Some(sub) = args.get(i + 1) {
                out.push_str(&with_host(|h| h.str_of(sub)));
            }
        }
    }
    Ok(with_host(|h| h.new_str(out)))
}

/// `Object(x)`: box/pass-through — for our model, non-object args just return a
/// fresh object; objects pass through.
/// Whether `v`'s own properties live in the fn-prop SIDE TABLE rather than in a
/// property map. A `Map`/`Set`/`Promise`/`RegExp`/generator/symbol/bigint is an
/// ordinary object that also has internal slots, so it can carry own properties
/// like anything else — but its heap variant holds only those slots, so a write
/// had nowhere to go and vanished: `m.x = 5` left `m.x` undefined.
pub fn uses_side_table(v: &Value) -> bool {
    matches!(
        with_host(|h| h.kind_of(v)),
        Some(
            ObjKind::Map
                | ObjKind::Set
                | ObjKind::Promise
                | ObjKind::RegExp
                | ObjKind::Generator
                | ObjKind::Symbol
                | ObjKind::BigInt
                | ObjKind::Iter
        )
    )
}

fn object_call(args: Vec<Value>) -> Value {
    let a = arg0(&args);
    // `Object(v)` is `ToObject(v)` (20.1.1.1): a primitive comes back BOXED,
    // not replaced by an empty object. `Object(1).valueOf()` was `undefined`.
    if matches!(a, Value::Undef) || with_host(|h| h.is_null(&a)) {
        return with_host(|h| h.new_object(IndexMap::new()));
    }
    to_object(&a)
}

/// The name of the wrapper a primitive boxes into, or `None` when the value is
/// already an object.
fn wrapper_ctor_of(v: &Value) -> Option<&'static str> {
    match v {
        Value::Int(_) | Value::Float(_) => Some("Number"),
        Value::Bool(_) => Some("Boolean"),
        Value::Obj(_) => match with_host(|h| h.get(v).cloned()) {
            Some(JsObj::Str(_)) => Some("String"),
            Some(JsObj::Symbol { .. }) => Some("Symbol"),
            Some(JsObj::BigInt(_)) => Some("BigInt"),
            _ => None,
        },
        _ => None,
    }
}

/// The primitive a wrapper object boxes (`new String("a")` → `"a"`), or `None`
/// for every other value. The slot is a hidden `@@primitive` own property —
/// the same `@@` marker convention the engine already uses for internal state,
/// so it stays out of `Object.keys` and `JSON.stringify` on its own.
pub fn wrapped_primitive(v: &Value) -> Option<Value> {
    with_host(|h| match h.get(v) {
        Some(JsObj::Object(p)) => p.get("@@primitive").cloned(),
        _ => None,
    })
}

/// `ToObject(v)` (7.1.18) for a primitive: the wrapper object with the matching
/// prototype and a `[[StringData]]`/`[[NumberData]]`/`[[BooleanData]]` slot.
///
/// A String wrapper also owns its index properties and `length`, which is what
/// makes `w[0]`, `w.length` and `Object.keys(w)` answer; all of them are
/// non-writable and non-configurable, as the exotic `String` object's are.
pub fn to_object(v: &Value) -> Value {
    let Some(ctor) = wrapper_ctor_of(v) else {
        return v.clone();
    };
    with_host(|h| h.ensure_wrapper_protos());
    let chars: Vec<String> = if ctor == "String" {
        with_host(|h| h.str_of(v))
            .chars()
            .map(|c| c.to_string())
            .collect()
    } else {
        Vec::new()
    };
    with_host(|h| {
        let mut m: IndexMap<String, Value> = IndexMap::new();
        for (i, c) in chars.iter().enumerate() {
            let s = h.new_str(c.clone());
            m.insert(i.to_string(), s);
        }
        let w = h.new_object(m);
        if ctor == "String" {
            for i in 0..chars.len() {
                h.set_prop_attrs(
                    &w,
                    &i.to_string(),
                    host::PropAttrs {
                        writable: false,
                        enumerable: true,
                        configurable: false,
                    },
                );
            }
            let len = Value::Float(chars.len() as f64);
            if let Some(JsObj::Object(p)) = h.get_mut(&w) {
                p.insert("length".into(), len);
            }
            h.set_prop_attrs(
                &w,
                "length",
                host::PropAttrs {
                    writable: false,
                    enumerable: false,
                    configurable: false,
                },
            );
        }
        if let Some(JsObj::Object(p)) = h.get_mut(&w) {
            p.insert("@@primitive".into(), v.clone());
        }
        if let Some(proto) = h.native_proto(ctor) {
            h.set_proto(&w, proto);
        }
        w
    })
}

/// Construct via `new` for the builtin constructors.
pub fn construct_builtin(name: &str, args: Vec<Value>) -> Result<Value, String> {
    // Native stdlib constructors (`new URL(...)`, `new EventEmitter()`, `new Buffer(...)`).
    if let Some(r) = crate::stdlib::construct(name, &args) {
        return r;
    }
    match name {
        "Array" => {
            // `new Array(n)` -> length-n array; `new Array(a, b)` -> [a, b].
            // A single NUMBER argument is a length and is validated as one
            // (23.1.1.1 step 6), so `new Array(-1)` / `new Array(1.5)` /
            // `new Array(2**32)` are all `RangeError: Invalid array length` on
            // node v26.7.0; only a non-number single argument is an element.
            if args.len() == 1 {
                if let Value::Float(_) | Value::Int(_) = args[0] {
                    let n = host::to_array_length(&args[0])?;
                    // Every element of `new Array(n)` is a HOLE, not a stored
                    // `undefined`: `Object.keys(Array(3))` is `[]`.
                    return Ok(with_host(|h| {
                        let a = h.new_array(vec![Value::Undef; n]);
                        h.mark_hole_range(&a, 0..n);
                        a
                    }));
                }
            }
            Ok(with_host(|h| h.new_array(args)))
        }
        "Object" => Ok(object_call(args)),
        // `new String(v)` / `new Number(v)` / `new Boolean(v)` — the wrapper
        // form. These were not constructors at all, so every one threw.
        "String" => Ok(to_object(&host::to_string_value(
            &args
                .first()
                .cloned()
                .unwrap_or_else(|| with_host(|h| h.new_str(String::new()))),
        )?)),
        "Number" => Ok(to_object(&Value::Float(match args.first() {
            Some(a) => host::to_number_value(a)?,
            None => 0.0,
        }))),
        "Boolean" => Ok(to_object(&Value::Bool(with_host(|h| {
            h.truthy(&arg0(&args))
        })))),
        "Map" | "WeakMap" => {
            let weak = name == "WeakMap";
            let m = with_host(|h| {
                h.alloc(JsObj::Map {
                    entries: indexmap::IndexMap::new(),
                    weak,
                })
            });
            if let Some(init) = args
                .first()
                .filter(|a| !matches!(a, Value::Undef) && !with_host(|h| h.is_null(a)))
            {
                // Stepped, not drained: an entry that is not a pair has to
                // stop the construction at that element and CLOSE the iterator
                // (24.1.1.2 step 8). Materializing first meant a bad entry in an
                // infinite source was never reached and the constructor HUNG.
                host::iter_for_each(init, |p, _| {
                    // 24.1.1.2 step 8.d: each entry must be an OBJECT. A string
                    // is iterable, so without this check `new Map(["ab"])`
                    // happily stored `'a' => 'b'` instead of throwing — and over
                    // an infinite source it never stopped.
                    if !with_host(|h| is_object_like(h, &p)) {
                        let shown = with_host(|h| h.str_of(&p));
                        return Err(host::type_error(&format!(
                            "Iterator value {shown} is not an entry object"
                        )));
                    }
                    // The entry is read by INDEX with `[[Get]]` (step 8.e), not
                    // iterated: an object with a `Symbol.iterator` but no `0`/`1`
                    // gives `undefined => undefined`, and an array-LIKE entry
                    // works. Iterating it instead accepted a string as a pair
                    // and rejected the array-like.
                    let k = get_property(&p, "0")?;
                    let v = get_property(&p, "1")?;
                    map_method(&m, "set", vec![k, v])?;
                    Ok(())
                })?;
            }
            Ok(m)
        }
        "Set" | "WeakSet" => {
            let weak = name == "WeakSet";
            let s = with_host(|h| {
                h.alloc(JsObj::Set {
                    entries: indexmap::IndexMap::new(),
                    weak,
                })
            });
            if let Some(init) = args
                .first()
                .filter(|a| !matches!(a, Value::Undef) && !with_host(|h| h.is_null(a)))
            {
                host::iter_for_each(init, |v, _| {
                    set_method(&s, "add", vec![v])?;
                    Ok(())
                })?;
            }
            Ok(s)
        }
        "Promise" => new_promise(arg0(&args)),
        "Proxy" => crate::proxy::create(&args),
        // `new Function(p…, body)` — the same `CreateDynamicFunction` the plain
        // call form runs (20.2.1.1). `depd`'s `wrapfunction` builds its
        // deprecation wrapper this way, so `require('body-parser')` — and with it
        // `require('express')` — dies at load without it.
        "Function" => function_ctor(&args),
        "RegExp" => regexp_ctor(&args),
        "BigInt" => Err(host::type_error("BigInt is not a constructor")),
        "Error" => make_error_checked(name, &args),
        // `new DOMException(message, name)` — the name is an ARGUMENT, and the
        // legacy numeric `code` follows from it.
        "DOMException" => Ok(dom_exception(&args)),
        n if host::ERROR_NAMES.contains(&n) => make_error_checked(name, &args),
        _ => Err(host::type_error(&format!("{name} is not a constructor"))),
    }
}

/// The legacy numeric `DOMException.code` a WHATWG error name maps to. A name
/// outside the table — including the default `"Error"` — reports 0.
pub const DOM_EXCEPTION_CODES: &[(&str, f64)] = &[
    ("IndexSizeError", 1.0),
    ("DOMStringSizeError", 2.0),
    ("HierarchyRequestError", 3.0),
    ("WrongDocumentError", 4.0),
    ("InvalidCharacterError", 5.0),
    ("NoDataAllowedError", 6.0),
    ("NoModificationAllowedError", 7.0),
    ("NotFoundError", 8.0),
    ("NotSupportedError", 9.0),
    ("InUseAttributeError", 10.0),
    ("InvalidStateError", 11.0),
    ("SyntaxError", 12.0),
    ("InvalidModificationError", 13.0),
    ("NamespaceError", 14.0),
    ("InvalidAccessError", 15.0),
    ("ValidationError", 16.0),
    ("TypeMismatchError", 17.0),
    ("SecurityError", 18.0),
    ("NetworkError", 19.0),
    ("AbortError", 20.0),
    ("URLMismatchError", 21.0),
    ("QuotaExceededError", 22.0),
    ("TimeoutError", 23.0),
    ("InvalidNodeTypeError", 24.0),
    ("DataCloneError", 25.0),
];

/// The static name a `DOMException` code is exposed under: the error name minus
/// its `Error` suffix, upper-snake-cased, plus `_ERR` — `AbortError` becomes
/// `ABORT_ERR`, `IndexSizeError` becomes `INDEX_SIZE_ERR`.
fn legacy_code_name(error_name: &str) -> String {
    let stem = error_name.strip_suffix("Error").unwrap_or(error_name);
    let mut out = String::new();
    for (i, c) in stem.chars().enumerate() {
        if c.is_ascii_uppercase() && i > 0 {
            out.push('_');
        }
        out.push(c.to_ascii_uppercase());
    }
    out.push_str("_ERR");
    out
}

/// `new DOMException(message, name)`.
///
/// The class node's `AbortSignal.reason` rejects with. Its `name` is the second
/// ARGUMENT (defaulting to `"Error"`), not the class name, and its `code` is the
/// legacy number that name maps to.
pub fn dom_exception(args: &[Value]) -> Value {
    let message = match args.first() {
        None | Some(Value::Undef) => String::new(),
        Some(v) => with_host(|h| h.str_of(v)),
    };
    let name = match args.get(1) {
        None | Some(Value::Undef) => "Error".to_string(),
        Some(v) => with_host(|h| h.str_of(v)),
    };
    with_host(|h| dom_exception_with(h, &name, &message))
}

/// `dom_exception` for a caller that already holds the host borrow.
pub(crate) fn dom_exception_with(h: &mut host::JsHost, name: &str, message: &str) -> Value {
    let name = name.to_string();
    let message = message.to_string();
    let code = DOM_EXCEPTION_CODES
        .iter()
        .find(|(n, _)| *n == name)
        .map(|(_, c)| *c)
        .unwrap_or(0.0);
    let head = if message.is_empty() {
        name.clone()
    } else {
        format!("{name}: {message}")
    };
    let e = synth_error(h, &head);
    {
        let nv = h.new_str(name);
        let mv = h.new_str(message);
        let sv = h.new_str(head);
        if let Some(JsObj::Object(p)) = h.get_mut(&e) {
            // `name`, `message` and `code` are PROTOTYPE accessors over internal
            // slots in node, so `stack` is the instance's only own property.
            // Storing them as own keys would show up in
            // `Object.getOwnPropertyNames`, which reports just `['stack']`.
            p.shift_remove("message");
            p.insert("@@domName".into(), nv);
            p.insert("@@domMessage".into(), mv);
            p.insert("@@domCode".into(), Value::Float(code));
            p.insert("stack".into(), sv);
        }
        h.ensure_error_protos();
        if let Some(proto) = host::error_proto_of(h, "DOMException") {
            h.set_proto(&e, proto);
        }
    }
    e
}

/// A `DOMException`'s `name`/`message`/`code`, which live in internal slots
/// rather than as own properties. `None` for anything else.
pub fn dom_exception_slot(recv: &Value, name: &str) -> Option<Value> {
    let slot = match name {
        "name" => "@@domName",
        "message" => "@@domMessage",
        "code" => "@@domCode",
        _ => return None,
    };
    with_host(|h| match h.get(recv) {
        Some(JsObj::Object(p)) if p.contains_key("@@domName") => p.get(slot).cloned(),
        _ => None,
    })
}

/// Build an `Error` object carrying `msg`, for stdlib callers that need to
/// throw a value with extra own properties on it.
pub(crate) fn make_error_pub(name: &str, msg: &str) -> Value {
    let m = with_host(|h| h.new_str(msg.to_string()));
    make_error_inner(name, &[m])
}

/// [`make_error`] with the message's `ToString` allowed to FAIL. A symbol
/// refuses it (20.5.1.1 step 3), so `new Error(sym)` is a TypeError where this
/// rendered `Symbol(desc)` into `.message`.
fn make_error_checked(name: &str, args: &[Value]) -> Result<Value, String> {
    if let Some(m) = args.first().filter(|m| !matches!(m, Value::Undef)) {
        // AggregateError's message is its SECOND argument.
        let idx = usize::from(name == "AggregateError");
        if idx == 0 {
            host::to_string_value(m)?;
        } else if let Some(m2) = args.get(idx).filter(|m| !matches!(m, Value::Undef)) {
            host::to_string_value(m2)?;
        }
    }
    Ok(make_error_inner(name, args))
}

fn make_error_inner(name: &str, args: &[Value]) -> Value {
    // `new AggregateError(errors, message)` takes the causes FIRST; every other
    // error constructor takes the message first.
    let agg = name == "AggregateError";
    let (errors, args) = if agg {
        (
            Some(args.first().cloned().unwrap_or(Value::Undef)),
            args.get(1..).unwrap_or(&[]),
        )
    } else {
        (None, args)
    };
    with_host(|h| {
        h.ensure_error_protos();
        let mut props: IndexMap<String, Value> = IndexMap::new();
        let msg = args
            .first()
            .filter(|a| !matches!(a, Value::Undef))
            .map(|a| h.str_of(a));
        if let Some(m) = &msg {
            let mv = h.new_str(m.clone());
            props.insert("message".into(), mv);
        }
        // `.stack` is engine-specific; a simple `Name: message` header line
        // suffices for parity (the fuzzer never prints raw stacks).
        //
        // V8 formats that header LAZILY, on the first read, from whatever `name`
        // and `message` the error carries at that moment — which is why the
        // near-universal
        //
        //     class MyErr extends Error { constructor(m) { super(m); this.name = 'MyErr'; } }
        //
        // reports `MyErr: boom` and not the `Error: boom` this built eagerly,
        // inside `super()`, before the subclass had renamed anything. `@@stackRaw`
        // carries the frames so the read can redo it; see `materialize_stack`.
        let frames = h.stack_frames();
        let stack = match &msg {
            Some(m) if !m.is_empty() => format!("{name}: {m}{frames}"),
            _ => format!("{name}{frames}"),
        };
        let sv = h.new_str(stack);
        props.insert("stack".into(), sv);
        let raw = h.new_str(frames);
        props.insert("@@stackRaw".into(), raw);
        if let Some(errs) = errors {
            // Materialize the iterable into the own `errors` array property.
            let items = h.iter_vec(&errs).unwrap_or_default();
            let arr = h.new_array(items);
            props.insert("errors".into(), arr);
        }
        // `new Error(msg, { cause })` (ES2022): installed only when the options
        // bag actually has a `cause` key, so `new Error(m, {})` leaves none.
        let opts = args.get(1);
        if let Some(cause) = opts.and_then(|o| match h.get(o) {
            Some(JsObj::Object(p)) => p.get("cause").cloned(),
            _ => None,
        }) {
            props.insert("cause".into(), cause);
        }
        let e = h.new_object(props);
        if let Some(p) = host::error_proto_of(h, name) {
            h.set_proto(&e, p);
        }
        // Every own slot an error constructor installs is non-enumerable in V8,
        // which is why `Object.keys(err)` is `[]` and `JSON.stringify(err)` is
        // `{}` — properties a *script* later assigns stay enumerable.
        for k in ["message", "stack", "errors", "cause", "@@stackRaw"] {
            h.hide_prop(&e, k);
        }
        e
    })
}

fn print_line(args: &[Value], stderr: bool) -> Result<(), String> {
    // Node's console.log(...args) === util.format(...args): printf-style
    // substitution when the first arg is a format string, else inspect-and-join.
    // A directive can THROW (`console.log('%j', 1n)`), and node lets that reach
    // the caller instead of printing a line — so nothing is written on failure.
    let line: String = crate::stdlib::util::format(args)?;
    with_host(|h| h.write_out(&format!("{line}\n"), stderr));
    Ok(())
}

fn arg0(args: &[Value]) -> Value {
    args.first().cloned().unwrap_or(Value::Undef)
}
/// `ToString(arg)` for a builtin's argument — fallible, because a SYMBOL
/// refuses the conversion (7.1.17). Every site that reached for `str_of`
/// instead rendered `Symbol(desc)` into its result and reported nothing.
fn arg_to_string(args: &[Value], i: usize) -> Result<String, String> {
    let v = args.get(i).cloned().unwrap_or(Value::Undef);
    let sv = host::to_string_value(&v)?;
    Ok(with_host(|h| h.str_of(&sv)))
}

fn arg_num(args: &[Value], i: usize) -> f64 {
    with_host(|h| h.to_number(&args.get(i).cloned().unwrap_or(Value::Undef)))
}

fn is_integer(v: Value) -> bool {
    match v {
        Value::Int(_) => true,
        Value::Float(f) => f.is_finite() && f.fract() == 0.0,
        _ => false,
    }
}
fn is_safe_integer(v: Value) -> bool {
    match v {
        Value::Float(f) => f.is_finite() && f.fract() == 0.0 && f.abs() <= 9007199254740991.0,
        Value::Int(_) => true,
        _ => false,
    }
}

/// `encodeURI`/`encodeURIComponent`: percent-encode `s`'s UTF-8 bytes, leaving
/// the unreserved set unescaped. `encodeURI` additionally preserves the reserved
/// URI characters (`;,/?:@&=+$#`) that delimit a URI's structure.
fn uri_encode(s: &str, uri: bool) -> Result<Value, String> {
    // Always-unescaped (`encodeURIComponent`'s unreserved set), per the spec.
    const UNRESERVED: &[u8] =
        b"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-_.!~*'()";
    // Reserved characters `encodeURI` leaves intact on top of the unreserved set.
    const RESERVED: &[u8] = b";,/?:@&=+$#";
    let mut out = String::with_capacity(s.len());
    for &b in s.as_bytes() {
        if UNRESERVED.contains(&b) || (uri && RESERVED.contains(&b)) {
            out.push(b as char);
        } else {
            out.push('%');
            out.push(
                char::from_digit((b >> 4) as u32, 16)
                    .unwrap()
                    .to_ascii_uppercase(),
            );
            out.push(
                char::from_digit((b & 0xf) as u32, 16)
                    .unwrap()
                    .to_ascii_uppercase(),
            );
        }
    }
    Ok(with_host(|h| h.new_str(out)))
}

/// `decodeURI`/`decodeURIComponent`: reverse `%XX` escapes back to UTF-8 text.
/// For `decodeURI`, escapes of the reserved delimiters are left as-is (the spec's
/// asymmetry with `encodeURI`). Throws `URIError` on a malformed escape.
fn uri_decode(s: &str, uri: bool) -> Result<Value, String> {
    const RESERVED: &[u8] = b";,/?:@&=+$#";
    let bytes = s.as_bytes();
    let mut out: Vec<u8> = Vec::with_capacity(bytes.len());
    let mut i = 0;
    while i < bytes.len() {
        if bytes[i] == b'%' {
            if i + 2 >= bytes.len() {
                return Err("URIError: URI malformed".into());
            }
            let hi = (bytes[i + 1] as char).to_digit(16);
            let lo = (bytes[i + 2] as char).to_digit(16);
            match (hi, lo) {
                (Some(h), Some(l)) => {
                    let byte = (h * 16 + l) as u8;
                    // decodeURI keeps reserved-delimiter escapes literal.
                    if uri && RESERVED.contains(&byte) {
                        out.extend_from_slice(&bytes[i..i + 3]);
                    } else {
                        out.push(byte);
                    }
                    i += 3;
                }
                _ => return Err("URIError: URI malformed".into()),
            }
        } else {
            out.push(bytes[i]);
            i += 1;
        }
    }
    match String::from_utf8(out) {
        Ok(decoded) => Ok(with_host(|h| h.new_str(decoded))),
        Err(_) => Err("URIError: URI malformed".into()),
    }
}

/// `escape` (Annex B.2.1.1) — the pre-`encodeURIComponent` legacy encoder, still
/// present in every engine and still reached by old libraries (jQuery's cookie
/// plugin, `querystring`-era code). It works on UTF-16 CODE UNITS, not UTF-8
/// bytes, which is what separates it from `encodeURIComponent`: a unit below
/// `0x100` becomes `%XX`, anything above becomes `%uXXXX`, so an astral
/// character yields the two escapes of its surrogate pair
/// (`escape("\u{1D4B3}")` is `"%uD835%uDCB3"` on node v26.7.0).
///
/// The unescaped set is frozen by the spec and is NOT the URI unreserved set —
/// it keeps `@*_+-./` and drops `!~'()`.
fn legacy_escape(s: &str) -> Result<Value, String> {
    const KEEP: &[u8] = b"ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789@*_+-./";
    let mut out = String::with_capacity(s.len());
    for u in s.encode_utf16() {
        if u < 0x100 {
            if KEEP.contains(&(u as u8)) {
                out.push(u as u8 as char);
            } else {
                out.push_str(&format!("%{u:02X}"));
            }
        } else {
            out.push_str(&format!("%u{u:04X}"));
        }
    }
    Ok(with_host(|h| h.new_str(out)))
}

/// `unescape` (Annex B.2.1.2) — the inverse of [`legacy_escape`]. Unlike
/// `decodeURIComponent` it never throws: a `%` that does not begin a well-formed
/// `%XX` or `%uXXXX` escape is passed through literally
/// (`unescape("%u0041%42%zz%2")` is `"AB%zz%2"` on node v26.7.0).
///
/// Decoding is done in code-unit space and re-joined at the end so a
/// `%uD835%uDCB3` pair recomposes into the one astral character it came from.
fn legacy_unescape(s: &str) -> Result<Value, String> {
    let b = s.as_bytes();
    let hex = |i: usize, n: usize| -> Option<u16> {
        if i + n > b.len() {
            return None;
        }
        let mut v: u16 = 0;
        for &c in &b[i..i + n] {
            v = v.checked_mul(16)? + (c as char).to_digit(16)? as u16;
        }
        Some(v)
    };
    let units: Vec<u16> = s.encode_utf16().collect();
    let mut out: Vec<u16> = Vec::with_capacity(units.len());
    let mut i = 0;
    while i < b.len() {
        // Escapes are pure ASCII, so a byte index is a unit index up to here —
        // but the tail may not be, so non-`%` bytes are re-decoded as chars.
        if b[i] == b'%' {
            if let Some(u) = hex(i + 1, 2) {
                out.push(u);
                i += 3;
                continue;
            }
            if b.get(i + 1) == Some(&b'u') {
                if let Some(u) = hex(i + 2, 4) {
                    out.push(u);
                    i += 6;
                    continue;
                }
            }
        }
        let c = s[i..].chars().next().unwrap_or('%');
        let mut buf = [0u16; 2];
        out.extend_from_slice(c.encode_utf16(&mut buf));
        i += c.len_utf8();
    }
    Ok(with_host(|h| {
        h.new_str(crate::utf16::to_string_lossy(&out))
    }))
}

/// `parseInt` begins with `ToString(argument)` (19.2.5 step 1), and that step can
/// THROW — a Symbol has no string form, so `parseInt([Symbol()])` is a TypeError
/// rather than `NaN`. Reading the argument with `str_of` took the object's brand
/// instead of converting it, which both swallowed that throw and ignored any
/// `toString` the value defines.
fn parse_int(args: &[Value]) -> Result<f64, String> {
    // Converted BEFORE the host borrow: `to_string_value` can call back into JS.
    let sv = host::to_string_value(&arg0(args))?;
    // 19.2.5 step 2 is `ToInt32(radix)`, which runs a user `valueOf` — the
    // infallible read below does no `ToPrimitive`, so an object radix came out
    // as NaN and the parse silently fell back to auto-detection.
    let radix = match args.get(1) {
        Some(r) if !matches!(r, Value::Undef) => {
            vec![arg0(args), Value::Float(to_number_arg(args, 1)?)]
        }
        _ => args.to_vec(),
    };
    Ok(parse_int_str(&with_host(|h| h.str_of(&sv)), &radix))
}

fn parse_int_str(s: &str, args: &[Value]) -> f64 {
    // 19.2.5 step 8: an EXPLICIT radix outside 2..=36 is `NaN`, it does not fall
    // back to auto-detection. The old `.filter()` silently discarded a bad radix,
    // so `parseInt("10", 37)` answered 10 where every engine says NaN.
    let radix_arg = args
        .get(1)
        .map(|r| with_host(|h| host::to_int32(h.to_number(r))));
    let radix = match radix_arg {
        Some(0) | None => None,
        Some(r) if (2..=36).contains(&r) => Some(r as u32),
        Some(_) => return f64::NAN,
    };
    let t = crate::utf16::js_trim_start(s);
    let (neg, digits) = match t.strip_prefix('-') {
        Some(rest) => (true, rest),
        None => (false, t.strip_prefix('+').unwrap_or(t)),
    };
    let (radix, digits) = match radix {
        Some(16) => (
            16u32,
            digits
                .strip_prefix("0x")
                .or_else(|| digits.strip_prefix("0X"))
                .unwrap_or(digits),
        ),
        Some(r) => (r, digits),
        None => {
            if let Some(hex) = digits
                .strip_prefix("0x")
                .or_else(|| digits.strip_prefix("0X"))
            {
                (16, hex)
            } else {
                (10, digits)
            }
        }
    };
    let valid: String = digits.chars().take_while(|c| c.is_digit(radix)).collect();
    if valid.is_empty() {
        return f64::NAN;
    }
    // Accumulate in `f64`, not `i64`. `i64::from_str_radix` OVERFLOWS past ~19
    // digits and the error was mapped to `NaN`, so
    // `parseInt("999999999999999999999999")` was NaN instead of 1e+24. The spec
    // asks for the mathematical value rounded to a Number, which is what
    // repeated multiply-accumulate in `f64` produces.
    let n = if radix == 10 {
        // Rust's decimal float parser is correctly rounded; digit-by-digit
        // multiply-accumulate is not, and drifted a ULP on long inputs
        // (`parseInt("999999999999999999999999")` came out
        // 1.0000000000000003e+24 rather than 1e+24).
        valid.parse::<f64>().unwrap_or(f64::NAN)
    } else {
        let mut n = 0.0f64;
        for c in valid.chars() {
            n = n * radix as f64 + c.to_digit(radix).unwrap_or(0) as f64;
        }
        n
    };
    if neg {
        -n
    } else {
        n
    }
}

/// `parseFloat` likewise starts from `ToString(argument)`; see `parse_int`.
fn parse_float(args: &[Value]) -> Result<f64, String> {
    let sv = host::to_string_value(&arg0(args))?;
    Ok(parse_float_str(&with_host(|h| h.str_of(&sv))))
}

fn parse_float_str(s: &str) -> f64 {
    let t = crate::utf16::js_trim_start(s);
    // `Infinity` / `+Infinity` / `-Infinity` are valid parseFloat prefixes.
    let inf_body = t
        .strip_prefix('+')
        .or_else(|| t.strip_prefix('-'))
        .unwrap_or(t);
    if inf_body.starts_with("Infinity") {
        return if t.starts_with('-') {
            f64::NEG_INFINITY
        } else {
            f64::INFINITY
        };
    }
    // The LONGEST prefix that is itself a complete `StrDecimalLiteral`, which is
    // not the same as the longest run of characters that could appear in one:
    // `"1e"` and `"1e+"` are `1` in every engine, because the exponent part is
    // only valid once a digit follows `e`. Tracking `end` at every character
    // accepted the dangling `e`, `parse::<f64>` then failed, and the whole call
    // came back NaN.
    let mut end = 0;
    let bytes = t.as_bytes();
    let mut seen_dot = false;
    let mut seen_e = false;
    let mut digits_before_dot = false;
    for (i, &c) in bytes.iter().enumerate() {
        match c {
            b'0'..=b'9' => {
                if !seen_dot && !seen_e {
                    digits_before_dot = true;
                }
                end = i + 1;
            }
            // A sign is only meaningful leading, or straight after the exponent
            // marker; it never completes a literal on its own.
            b'+' | b'-' if i == 0 || bytes[i - 1] == b'e' || bytes[i - 1] == b'E' => {}
            // `1.` is a complete literal; a bare `.` is not.
            b'.' if !seen_dot && !seen_e => {
                seen_dot = true;
                if digits_before_dot {
                    end = i + 1;
                }
            }
            b'e' | b'E' if !seen_e && end > 0 => seen_e = true,
            _ => break,
        }
    }
    if end == 0 {
        return f64::NAN;
    }
    t[..end].parse::<f64>().unwrap_or(f64::NAN)
}

/// ECMA-262 `Number::exponentiate` (6.1.6.1.3), backing both `Math.pow` and the
/// `**` operator. Three clauses differ from IEEE-754 `pow`, which is what Rust's
/// `powf` implements: a NaN exponent is NaN even for base 1, a NaN base is NaN
/// for any non-zero exponent, and `|base| == 1` with an infinite exponent is NaN
/// rather than 1.
pub(crate) fn js_pow(base: f64, exp: f64) -> f64 {
    if exp == 0.0 {
        return 1.0;
    }
    if base.is_nan() || exp.is_nan() {
        return f64::NAN;
    }
    if base.abs() == 1.0 && exp.is_infinite() {
        return f64::NAN;
    }
    base.powf(exp)
}

fn math_fn(fname: &str, args: &[Value]) -> Result<Value, String> {
    // Every `Math` function coerces its arguments with `ToNumber`, and `ToNumber`
    // of a BigInt is a TypeError (7.1.4 step 2) — the whole point of BigInt being
    // a separate numeric type. `arg_num` reads a BigInt's magnitude instead, so
    // `Math.max(1n)` quietly answered 1 where V8 throws. `Math.random` is the one
    // exception: it never reads an argument, so `Math.random(1n)` is fine.
    // A BigInt WRAPPER converts to a BigInt and is rejected just as the
    // primitive is: `Math.abs(Object(9n))` is a TypeError where it answered NaN.
    // The boxed value is read BEFORE the borrow — `wrapped_primitive` borrows
    // the host itself and cannot run inside another borrow.
    let is_bigint = |a: &Value| {
        if with_host(|h| matches!(h.get(a), Some(JsObj::BigInt(_)))) {
            return true;
        }
        match wrapped_primitive(a) {
            Some(p) => with_host(|h| matches!(h.get(&p), Some(JsObj::BigInt(_)))),
            None => false,
        }
    };
    if fname != "random" && args.iter().any(is_bigint) {
        return Err(host::type_error(
            "Cannot convert a BigInt value to a number",
        ));
    }
    // Every argument is `ToNumber`d (21.3.2.x), which runs a user `valueOf` and
    // can throw from it. `arg_num` does no `ToPrimitive` at all, so
    // `Math.max({valueOf: () => 1}, 0)` answered NaN.
    // EVERY argument, not a fixed prefix: `Math.max`/`min`/`hypot` are
    // variadic, and coercing only the first few silently DROPPED the rest —
    // `Math.max(...gen)` over five values answered for four of them.
    let mut coerced = Vec::with_capacity(args.len());
    for a in args {
        if matches!(a, Value::Undef) {
            coerced.push(a.clone());
            continue;
        }
        let p = host::to_primitive(a, "number")?;
        coerced.push(Value::Float(with_host(|h| h.to_number(&p))));
    }
    let args: &[Value] = &coerced;
    let x = arg_num(args, 0);
    let r = match fname {
        "floor" => x.floor(),
        "ceil" => x.ceil(),
        // ECMA-262 `Math.round` (21.3.2.28) transcribed clause by clause. The
        // obvious `(x + 0.5).floor()` is NOT this function: the addition rounds
        // before the floor sees it, so it answers 1 for the largest double below
        // 0.5 (`Math.round(0.49999999999999994)` is 0 in every engine) and it
        // perturbs integers above 2^52, where `x + 0.5` is no longer
        // representable (`Math.round(4503599627370497)` must be the input).
        // Splitting the zero-band cases out first also carries the signed zero
        // the spec asks for without a post-hoc patch.
        "round" => {
            if !x.is_finite() || x == 0.0 {
                x
            } else if x > 0.0 && x < 0.5 {
                0.0
            } else if (-0.5..0.0).contains(&x) {
                -0.0
            } else {
                // |x| >= 0.5, so `floor` and the subtraction are both exact
                // (every double >= 2^52 is already an integer and yields 0 here).
                let f = x.floor();
                if x - f >= 0.5 {
                    f + 1.0
                } else {
                    f
                }
            }
        }
        "trunc" => x.trunc(),
        "abs" => x.abs(),
        "sign" => {
            if x.is_nan() {
                f64::NAN
            } else if x > 0.0 {
                1.0
            } else if x < 0.0 {
                -1.0
            } else {
                x
            }
        }
        "sqrt" => x.sqrt(),
        "cbrt" => x.cbrt(),
        "exp" => x.exp(),
        "log" => x.ln(),
        "log2" => x.log2(),
        "log10" => x.log10(),
        "sin" => x.sin(),
        "cos" => x.cos(),
        "tan" => x.tan(),
        "asin" => x.asin(),
        "acos" => x.acos(),
        "atan" => x.atan(),
        "atan2" => x.atan2(arg_num(args, 1)),
        // Rust `powf` is IEEE-754 `pow`, which is NOT JS `**`/`Math.pow`: IEEE
        // makes `pow(x, ±0)` and `pow(±1, y)` return 1 unconditionally, so
        // `(-1) ** Infinity` and `1 ** NaN` come back 1 where the spec
        // (6.1.6.1.3 Number::exponentiate) says NaN. Only the exponent-is-zero
        // clause is shared.
        "pow" => js_pow(x, arg_num(args, 1)),
        // Hyperbolics and the two precision-preserving log/exp forms.
        "sinh" => x.sinh(),
        "cosh" => x.cosh(),
        "tanh" => x.tanh(),
        "asinh" => x.asinh(),
        "acosh" => x.acosh(),
        "atanh" => x.atanh(),
        "log1p" => x.ln_1p(),
        "expm1" => x.exp_m1(),
        // C-style 32-bit integer multiply: ToInt32 both operands, multiply with
        // wraparound, reinterpret as a signed 32-bit result.
        "imul" => (host::to_int32(x).wrapping_mul(host::to_int32(arg_num(args, 1)))) as f64,
        "hypot" => {
            // Scale by the largest magnitude before squaring — this avoids the
            // last-ULP error of the naive `sqrt(Σ xᵢ²)` and matches V8's result.
            let xs: Vec<f64> = args.iter().map(|a| with_host(|h| h.to_number(a))).collect();
            let mut max = 0.0f64;
            for x in &xs {
                if x.abs() > max {
                    max = x.abs();
                }
            }
            if xs.iter().any(|x| x.is_infinite()) {
                f64::INFINITY
            } else if max == 0.0 || !max.is_finite() {
                max
            } else {
                let s: f64 = xs.iter().map(|x| (x / max) * (x / max)).sum();
                max * s.sqrt()
            }
        }
        "random" => pseudo_random(),
        "max" => {
            if args.is_empty() {
                f64::NEG_INFINITY
            } else {
                let mut m = f64::NEG_INFINITY;
                for a in args {
                    let n = with_host(|h| h.to_number(a));
                    if n.is_nan() {
                        return Ok(Value::Float(f64::NAN));
                    }
                    // `>` cannot separate the zeroes (`0.0 > -0.0` is false), but
                    // the spec ranks +0 above -0, so `Math.max(-0, 0)` is +0 and
                    // must not keep the -0 the first iteration installed.
                    if n > m || (n == m && n == 0.0 && n.is_sign_positive()) {
                        m = n;
                    }
                }
                m
            }
        }
        "min" => {
            if args.is_empty() {
                f64::INFINITY
            } else {
                let mut m = f64::INFINITY;
                for a in args {
                    let n = with_host(|h| h.to_number(a));
                    if n.is_nan() {
                        return Ok(Value::Float(f64::NAN));
                    }
                    // Mirror of `max`: -0 ranks below +0 even though `<` says
                    // they are equal, so `Math.min(0, -0)` is -0.
                    if n < m || (n == m && n == 0.0 && n.is_sign_negative()) {
                        m = n;
                    }
                }
                m
            }
        }
        // Count leading zero bits of ToUint32(x) (Math.clz32(1) === 31).
        "clz32" => {
            let u = if x.is_finite() {
                x.trunc().rem_euclid(4294967296.0) as u32
            } else {
                0
            };
            u.leading_zeros() as f64
        }
        // Round to the nearest single-precision float.
        "fround" => (x as f32) as f64,
        _ => return Err(host::type_error(&format!("Math.{fname} is not a function"))),
    };
    Ok(Value::Float(r))
}

/// A small deterministic PRNG for `Math.random` (output is non-reproducible vs
/// Node by nature; kept simple).
fn pseudo_random() -> f64 {
    use std::cell::Cell;
    thread_local!(static SEED: Cell<u64> = const { Cell::new(0x2545F4914F6CDD1D) });
    SEED.with(|s| {
        let mut x = s.get();
        x ^= x << 13;
        x ^= x >> 7;
        x ^= x << 17;
        s.set(x);
        (x >> 11) as f64 / (1u64 << 53) as f64
    })
}

// ── Object.* ──────────────────────────────────────────────────────────────────

/// The characters of a string PRIMITIVE, as the `ToObject` wrapper's own index
/// properties (10.4.3 `StringExoticObject`).
///
/// `getOwnPropertyDescriptor` begins with `ToObject`, which boxes a string into
/// an exotic object whose own keys are its code-unit indices plus `length`;
/// this is the descriptor half of that. (The KEY half lives in
/// `JsHost::own_enum_data_keys`, the single source every enumeration path
/// reads.) Indices are UTF-16 code units, matching `.length` and `s[i]`.
///
/// A boxed `String` object is deliberately NOT routed here: it can carry
/// ordinary own properties too (`const s = new String('ab'); s.x = 1`), and its
/// existing path already reports them alongside the indices.
fn string_primitive_units(v: &Value) -> Option<Vec<String>> {
    // A JS string primitive rides as a `Value::Obj` handle to `JsObj::Str` (see
    // `host.rs`); a BOXED `new String(...)` is a different heap object, so this
    // never catches one.
    let s = match v {
        Value::Str(s) => (**s).clone(),
        _ => with_host(|h| match h.get(v) {
            Some(JsObj::Str(s)) => Some(s.clone()),
            _ => None,
        })?,
    };
    let units = crate::utf16::Units::of(&s);
    Some((0..units.len()).filter_map(|i| units.unit_str(i)).collect())
}

fn object_keys(args: Vec<Value>, mode: u8) -> Result<Value, String> {
    let v = arg0(&args);
    require_object_coercible(&v)?;
    // A Proxy answers from its `ownKeys` trap. `getOwnPropertyNames` (mode 3)
    // reports every own STRING key the trap named; the enumerating modes
    // additionally filter by each key's `[[GetOwnProperty]]`, so both traps run.
    if with_host(|h| h.kind_of(&v)) == Some(ObjKind::Proxy) {
        if mode == 3 {
            let keys = crate::proxy::own_keys(&v)?.unwrap_or_default();
            return Ok(with_host(|h| {
                let out: Vec<Value> = keys
                    .into_iter()
                    .filter(|k| !host::is_symbol_key(k))
                    .map(|k| h.new_str(k))
                    .collect();
                h.new_array(out)
            }));
        }
        // `Object.keys` (mode 0) must run `ownKeys` and `getOwnPropertyDescriptor`
        // and STOP — 7.3.23 never performs `[[Get]]` when only keys are wanted.
        // Going through `own_enum_entries` fired the `get` trap once per key, so
        // the observable trap sequence carried a trailing `get` node does not
        // emit, and a trap with side effects ran when it should not have.
        if mode == 0 {
            let keys = crate::proxy::own_enum_string_keys(&v)?;
            return Ok(with_host(|h| {
                let out: Vec<Value> = keys.into_iter().map(|k| h.new_str(k)).collect();
                h.new_array(out)
            }));
        }
        let entries = crate::proxy::own_enum_entries(&v)?;
        return Ok(with_host(|h| {
            let out: Vec<Value> = entries
                .into_iter()
                .map(|(k, val)| match mode {
                    0 => h.new_str(k),
                    1 => val,
                    _ => {
                        let ks = h.new_str(k);
                        h.new_array(vec![ks, val])
                    }
                })
                .collect();
            h.new_array(out)
        }));
    }
    // An intrinsic prototype this host built as a REAL OBJECT — `Symbol
    // .prototype`, `String.prototype`, the error hierarchy — answers from the
    // generated table too. It was answering from its own property map instead,
    // which carries neither the right names nor V8's order: `Symbol.prototype`
    // reported `toLocaleString` and omitted `description`, and
    // `String.prototype` omitted `length` and every Annex B HTML method.
    //
    // `ns` is the namespace SPELLING, so the arm below is shared verbatim —
    // the two representations of a prototype cannot answer differently.
    let real_proto_ns = with_host(|h| h.intrinsic_proto_ctor(&v).map(|c| format!("{c}.prototype")))
        .filter(|ns| intrinsic_proto_members(ns).is_some());
    // A builtin prototype namespace that exposes enumerable methods for copying
    // (`Object.getOwnPropertyNames(EventEmitter.prototype)` — express's mixin).
    if let Some(ns) = real_proto_ns.or_else(|| {
        with_host(|h| match h.get(&v) {
            Some(JsObj::Builtin(ns)) => Some(ns.clone()),
            _ => None,
        })
    }) {
        // An INTRINSIC prototype (`Map.prototype`, `URL.prototype`). Members are
        // non-enumerable on an ECMAScript builtin and enumerable on a WebIDL
        // interface, which the table records per name.
        if let Some(members) = intrinsic_proto_members(&ns) {
            let ctor = ns.trim_end_matches(".prototype");
            let mut names: Vec<String> = members
                .iter()
                .filter(|m| mode == 3 || m.starts_with('+'))
                .map(|m| m.strip_prefix('+').unwrap_or(m).to_string())
                // `getOwnPropertyNames` reports STRING keys only; the table's
                // `@@` entries are symbol-keyed members and belong to
                // `getOwnPropertySymbols` instead.
                .filter(|m| !m.starts_with("@@"))
                .collect();
            // Plus whatever a script patched onto this prototype under a NEW
            // name — an ordinary enumerable own property, so it lists in every
            // mode. Without it `Object.keys(Array.prototype)` stayed `[]` after
            // an assignment that `Array.prototype.patch` read back happily.
            //
            // Assigning over an EXISTING member is a `[[Set]]`, which leaves
            // that member's attributes alone: restoring a saved `join` must not
            // turn it into an enumerable key.
            for k in with_host(|h| h.builtin_static_keys(&ns)) {
                if !intrinsic_proto_member(&ns, &k) && !names.contains(&k) {
                    names.push(k);
                }
            }
            return Ok(with_host(|h| {
                let out: Vec<Value> = names
                    .iter()
                    .map(|name| {
                        // An accessor member has no thunk — `Map.prototype.size`
                        // is not a function — so a VALUE read of one answers
                        // undefined rather than synthesizing a callable.
                        let val = |h: &mut host::JsHost| {
                            if let Some(v) = h.builtin_static(&ns, name) {
                                return v;
                            }
                            let key = format!("@proto:{ctor}:{name}");
                            if builtin_meta(&key).is_some() {
                                h.alloc(JsObj::Builtin(key))
                            } else {
                                Value::Undef
                            }
                        };
                        match mode {
                            1 => val(h),
                            2 => {
                                let ks = h.new_str(name.clone());
                                let v = val(h);
                                h.new_array(vec![ks, v])
                            }
                            _ => h.new_str(name.clone()),
                        }
                    })
                    .collect();
                h.new_array(out)
            }));
        }
        if let Some(names) = builtin_proto_method_names(&ns) {
            return Ok(with_host(|h| {
                let out: Vec<Value> = names
                    .iter()
                    .map(|name| match mode {
                        1 => h.alloc(JsObj::Builtin(format!(
                            "@proto:{}:{name}",
                            ns.trim_end_matches(".prototype")
                        ))),
                        2 => {
                            let ks = h.new_str(*name);
                            let val = h.alloc(JsObj::Builtin(format!(
                                "@proto:{}:{name}",
                                ns.trim_end_matches(".prototype")
                            )));
                            h.new_array(vec![ks, val])
                        }
                        _ => h.new_str(*name),
                    })
                    .collect();
                h.new_array(out)
            }));
        }
        // A stdlib namespace (`Buffer`, `require('buffer')`): its own enumerable
        // keys are the members node-js implements, each resolved to the same
        // first-class value a property read would give.
        let mut names = crate::stdlib::namespace_keys(&ns);
        // A core namespace (`Reflect`, `Math`, `JSON`) has no stdlib key list —
        // its members live in the builtin dispatch table. They are
        // non-enumerable in V8, so they surface only under
        // `getOwnPropertyNames`/`Reflect.ownKeys` (mode 3), never `Object.keys`.
        if names.is_empty() && mode == 3 {
            let prefix = format!("{ns}.");
            // A builtin constructor's own `length`/`name`/`prototype` come
            // first, as they do in V8.
            if is_builtin_ctor(&ns) {
                names.extend(["length", "name", "prototype"].map(str::to_string));
            }
            names.extend(
                NS_METHODS
                    .iter()
                    .filter_map(|q| q.strip_prefix(&prefix))
                    .map(|m| m.to_string()),
            );
            // The numeric constants are members too. Without them
            // `getOwnPropertyNames(Math)` reported 35 of the 43 names node-js
            // actually answers — the eight it dropped being `PI` and its
            // siblings, which read fine and now own a descriptor as well.
            names.extend(
                namespace_constants(&ns)
                    .iter()
                    .map(|(k, _)| (*k).to_string()),
            );
        }
        // A builtin FUNCTION owns exactly `length` and `name` (10.3.3-4), so
        // `Object.getOwnPropertyNames(Math.max)` is `[ 'length', 'name' ]` — it
        // was `[]`, which said the function had no properties at all while both
        // of them read back a value. `length` is listed only where the intrinsic
        // table has an arity, so the names never advertise a read that answers
        // `undefined`.
        if names.is_empty() && mode == 3 && host::builtin_is_callable(&ns) {
            if builtin_meta(&ns).is_some() {
                names.push("length".to_string());
            }
            names.push("name".to_string());
        }
        // Whatever a script assigned onto the namespace, in assignment order and
        // after the built-in members — an ordinary enumerable own property, so
        // it surfaces under `Object.keys` too and not only `ownKeys`. These were
        // missing from every listing, which made a patched prototype read as
        // unpatched to any code that enumerates rather than reads.
        for k in with_host(|h| h.builtin_static_keys(&ns)) {
            if !names.contains(&k) {
                names.push(k);
            }
        }
        if !names.is_empty() {
            let entries: Vec<(String, Value)> = names
                .into_iter()
                .map(|k| {
                    let val = namespace_property(&ns, &k);
                    (k, val)
                })
                .collect();
            return Ok(with_host(|h| {
                let out: Vec<Value> = entries
                    .into_iter()
                    .map(|(k, val)| match mode {
                        1 => val,
                        2 => {
                            let ks = h.new_str(k);
                            h.new_array(vec![ks, val])
                        }
                        _ => h.new_str(k),
                    })
                    .collect();
                h.new_array(out)
            }));
        }
    }
    // mode 3 (`getOwnPropertyNames`) reports every own string key including the
    // non-enumerable ones, plus the exotic `length` an array carries.
    let entries: Vec<(String, Value)> = with_host(|h| {
        if mode == 3 {
            // An array's exotic `length` is already placed (after the indices,
            // before the ordinary string keys) by `own_key_names`.
            return h
                .own_key_names(&v, false)
                .into_iter()
                .map(|k| (k, Value::Undef))
                .collect();
        }
        Vec::new()
    });
    // `Object.keys` (mode 0) wants NAMES. `own_enum_entries_deep` returns
    // key/value pairs, so asking it for them ran every enumerable getter —
    // 20.1.2.17 -> 7.3.23 EnumerableOwnProperties only needs `[[GetOwnProperty]]`
    // for the enumerable flag, never `[[Get]]`, and a getter can throw or have
    // side effects:
    //
    //     let n = 0; const o = { get g() { n++; return 1 } };
    //     Object.keys(o); n   // was 1, node says 0
    //
    // `values`/`entries` (modes 1 and 2) do read, and still do.
    let entries = match mode {
        3 => entries,
        0 => with_host(|h| h.own_enum_key_names(&v))
            .into_iter()
            .map(|k| (k, Value::Undef))
            .collect(),
        _ => host::own_enum_entries_deep(&v)?,
    };
    Ok(with_host(|h| {
        let out: Vec<Value> = entries
            .into_iter()
            .map(|(k, val)| match mode {
                0 | 3 => h.new_str(k),
                1 => val,
                _ => {
                    let ks = h.new_str(k);
                    h.new_array(vec![ks, val])
                }
            })
            .collect();
        h.new_array(out)
    }))
}

fn object_assign(args: Vec<Value>) -> Result<Value, String> {
    let target = arg0(&args);
    // 20.1.2.1 step 1 is `ToObject(target)`, so a nullish TARGET throws while a
    // nullish SOURCE is skipped (`Object.assign({}, null)` is `{}`).
    require_object_coercible(&target)?;
    for src in args.iter().skip(1) {
        // `Object.assign` copies own *enumerable* properties, running any getter
        // — symbol-keyed ones included (7.3.25).
        let entries = host::own_enum_entries_deep(src)?;
        let syms = with_host(|h| h.own_symbol_entries(src));
        // A plain object target is filled in place (one borrow, then a single
        // re-canonicalization of the integer-index keys).
        let filled = with_host(|h| {
            if let Some(JsObj::Object(p)) = h.get_mut(&target) {
                for (k, v) in entries.iter().cloned().chain(syms.iter().cloned()) {
                    p.insert(k, v);
                }
                host::canonicalize_own_keys(p);
                return true;
            }
            false
        });
        // Any OTHER target — an array being the common one — goes through the
        // ordinary Set path. The in-place branch above matched `JsObj::Object`
        // only, so `Object.assign([1,2], {extra:9})` silently copied NOTHING and
        // returned the untouched array: no error, just a missing property. The
        // Set path is what an `arr.extra = 9` assignment already used, so index
        // and non-index keys land where they do for a direct write.
        if !filled {
            for (k, v) in entries.into_iter().chain(syms) {
                set_property(&target, &k, v)?;
            }
        }
    }
    Ok(target)
}

fn object_from_entries(args: Vec<Value>) -> Result<Value, String> {
    let pairs = with_host(|h| h.iter_vec(&arg0(&args))).unwrap_or_default();
    let mut props: IndexMap<String, Value> = IndexMap::new();
    for p in pairs {
        let kv = with_host(|h| h.iter_vec(&p)).unwrap_or_default();
        let key = with_host(|h| h.str_of(&kv.first().cloned().unwrap_or(Value::Undef)));
        let val = kv.get(1).cloned().unwrap_or(Value::Undef);
        props.insert(key, val);
    }
    Ok(with_host(|h| h.new_object(props)))
}

/// `Object.groupBy(items, cb)` — group the iterable `items` into a null-prototype
/// object. Keys are `ToPropertyKey(cb(item, index))`; values are arrays of the
/// members mapped to that key, in first-seen key order.
fn object_group_by(args: Vec<Value>) -> Result<Value, String> {
    group_by_check_iterable(&arg0(&args), "Object.groupBy")?;
    let cb = args.get(1).cloned().unwrap_or(Value::Undef);
    let mut groups: IndexMap<String, Vec<Value>> = IndexMap::new();
    // Stepped, not drained: the callback runs per element, so a throwing one
    // stops at the first. Draining first meant an infinite source never reached
    // the callback at all and the call HUNG.
    host::iter_for_each(&arg0(&args), |item, i| {
        let key_v = host::invoke(&cb, vec![item.clone(), Value::Float(i as f64)], None)?;
        let key = with_host(|h| h.property_key(&key_v));
        groups.entry(key).or_default().push(item);
        Ok(())
    })?;
    let props: IndexMap<String, Value> = with_host(|h| {
        groups
            .into_iter()
            .map(|(k, v)| (k, h.new_array(v)))
            .collect()
    });
    let obj = with_host(|h| h.new_object(props));
    // A null-prototype object (as Node returns), so it has no inherited members.
    with_host(|h| {
        let nv = h.null();
        h.set_proto(&obj, nv);
    });
    Ok(obj)
}

/// The `groupBy` family words a non-iterable argument its OWN way — a third
/// vocabulary, alongside the array-literal spread's and the call spread's:
///
/// ```text
/// null / undefined   "<Name> called on null or undefined"
/// anything else      "<typeof> [value ]is not iterable (cannot read property
///                     Symbol(Symbol.iterator))"
/// ```
///
/// A plain object, a symbol and a bigint name only their TYPE; a number, a
/// string and a boolean name the value too.
fn group_by_check_iterable(v: &Value, name: &str) -> Result<(), String> {
    if with_host(|h| h.is_nullish(v)) {
        return Err(host::type_error(&format!(
            "{name} called on null or undefined"
        )));
    }
    // Asked WITHOUT consuming anything: `iter_all` would drain the iterator
    // here, so the stepping loop below then saw an exhausted one — the finite
    // case returned an empty group and the infinite case was back to hanging.
    let iter_fn = get_property(v, "@@iterator").unwrap_or(Value::Undef);
    if with_host(|h| host::is_callable(h, &iter_fn)) {
        return Ok(());
    }
    Err(host::type_error(&not_iterable_typed(v)))
}

/// The `<type> <value> is not iterable (cannot read property
/// Symbol(Symbol.iterator))` wording, which node uses wherever the source has
/// no name to report: a plain object, a symbol and a bigint name only their
/// TYPE; a number, a string and a boolean name the value too.
pub(crate) fn not_iterable_typed(v: &Value) -> String {
    let shown = with_host(|h| {
        let kind = h.type_of(v);
        match kind {
            "object" | "symbol" | "bigint" => kind.to_string(),
            "string" => format!("string \"{}\"", h.str_of(v)),
            _ => format!("{kind} {}", h.str_of(v)),
        }
    });
    format!("{shown} is not iterable (cannot read property Symbol(Symbol.iterator))")
}

/// `Map.groupBy(items, cb)` — like `Object.groupBy` but returns a `Map` keyed by
/// the raw `cb(item, index)` value under SameValueZero (so object/any keys work).
fn map_group_by(args: Vec<Value>) -> Result<Value, String> {
    group_by_check_iterable(&arg0(&args), "Map.groupBy")?;
    let cb = args.get(1).cloned().unwrap_or(Value::Undef);
    let m = with_host(|h| {
        h.alloc(JsObj::Map {
            entries: IndexMap::new(),
            weak: false,
        })
    });
    // Stepped for the same reason `Object.groupBy` is.
    host::iter_for_each(&arg0(&args), |item, i| {
        let key_v = host::invoke(&cb, vec![item.clone(), Value::Float(i as f64)], None)?;
        let existing = map_method(&m, "get", vec![key_v.clone()])?;
        if matches!(existing, Value::Undef) {
            let arr = with_host(|h| h.new_array(vec![item]));
            map_method(&m, "set", vec![key_v, arr])?;
        } else {
            with_host(|h| {
                if let Some(JsObj::Array(a)) = h.get_mut(&existing) {
                    a.push(item);
                }
            });
        }
        Ok(())
    })?;
    Ok(m)
}

/// `Array.fromAsync(items[, mapFn])` — a Promise for an array, awaiting each
/// element and each `mapFn` result.
///
/// Written in JavaScript and compiled once, because the operation IS an async
/// function: a Rust builtin runs outside any coroutine and has no way to await,
/// so draining a promise from there would mean running the microtask queue by
/// hand. Delegating to the engine's own `async`/`for await` keeps the
/// suspension semantics — and the ordering they imply — exactly the language's.
///
/// The source may be an async iterable, a sync iterable, a bare iterator, or an
/// array-like. Everything iterable goes through `for await`, which awaits a sync
/// source's elements individually — that is what makes
/// `Array.fromAsync([1, Promise.resolve(2)])` answer `[1, 2]`. A bare `.next` is
/// accepted because an async generator object does not expose
/// `Symbol.asyncIterator` on this frontend.
fn array_from_async(args: Vec<Value>) -> Result<Value, String> {
    thread_local! {
        static IMPL: std::cell::RefCell<Option<Value>> = const { std::cell::RefCell::new(None) };
    }
    const SRC: &str = "(async function (items, mapFn, thisArg) {\n\
        const out = []; let i = 0;\n\
        const step = async (v) => { const a = await v; out.push(mapFn ? await mapFn.call(thisArg, a, i) : a); i++; };\n\
        const iterable = items != null && (typeof items[Symbol.asyncIterator] === 'function'\n\
            || typeof items[Symbol.iterator] === 'function' || typeof items.next === 'function');\n\
        if (iterable) {\n\
            for await (const v of items) { out.push(mapFn ? await mapFn.call(thisArg, v, i) : v); i++; }\n\
            return out;\n\
        }\n\
        const len = items == null ? 0 : (Math.trunc(Number(items.length)) || 0);\n\
        while (i < len) { await step(items[i]); }\n\
        return out;\n\
    })";
    let f = IMPL.with(|c| c.borrow().clone());
    let f = match f {
        Some(f) => f,
        None => {
            let f = crate::eval_in_global_scope(SRC)?;
            IMPL.with(|c| *c.borrow_mut() = Some(f.clone()));
            f
        }
    };
    host::invoke(&f, args, None)
}

fn array_from(args: Vec<Value>) -> Result<Value, String> {
    // `Array.from` accepts generators and user iterables, plus array-likes with a
    // numeric `.length`.
    let src = arg0(&args);
    if let Some(cb) = args.get(1).cloned() {
        // Stepped, not drained: the mapper runs per element as the iterator
        // yields it (23.1.2.1 step 6.e). Materializing the whole sequence first
        // meant `Array.from(infiniteIterator, fn)` never reached the mapper at
        // all and HUNG, and a throwing mapper could not close the iterator.
        let this = this_arg(&args, 2);
        let mut out = Vec::new();
        let mapped = host::iter_for_each(&src, |v, i| {
            out.push(host::invoke(
                &cb,
                vec![v, Value::Float(i as f64)],
                this.clone(),
            )?);
            Ok(())
        });
        match mapped {
            Ok(()) => {}
            // An array-LIKE has no iterator; fall back to its indexed items.
            Err(e) if host::user_iterator_fn(&src).is_none() && e.ends_with(" is not iterable") => {
                out.clear();
                for (i, it) in array_like_items(&src).into_iter().enumerate() {
                    out.push(host::invoke(
                        &cb,
                        vec![it, Value::Float(i as f64)],
                        this.clone(),
                    )?);
                }
            }
            Err(e) => return Err(e),
        }
        return construct_array_like(host::current_static_this(), out);
    }
    let items = match host::iter_all(&src) {
        Ok(v) => v,
        Err(_) => array_like_items(&src),
    };
    // 23.1.2.1 step 5: `Array.from` builds through `this`, so on a subclass the
    // result is an instance of it. It always allocated a plain array, which is
    // also why `A.from([1]).map(f) instanceof A` was false — the species chain
    // never started.
    construct_array_like(host::current_static_this(), items)
}

/// Items of an array-like `{ length, 0, 1, … }` object (for `Array.from`).
pub(crate) fn array_like_items(src: &Value) -> Vec<Value> {
    // `LengthOfArrayLike` is `ToLength(Get(O, "length"))`, and `ToNumber` runs a
    // user `valueOf` — `Array.from({length: {valueOf: () => 1}})` was empty
    // because the infallible read does no `ToPrimitive`. A throw from it is
    // swallowed here for the same reason the `length` read is: this helper has
    // no way to report one, and every caller treats an unreadable length as 0.
    let len = get_property(src, "length")
        .ok()
        .and_then(|l| host::to_primitive(&l, "number").ok())
        .map(|l| with_host(|h| h.to_number(&l)))
        .unwrap_or(0.0);
    if !len.is_finite() || len <= 0.0 {
        return Vec::new();
    }
    (0..len as usize)
        .map(|i| get_property(src, &i.to_string()).unwrap_or(Value::Undef))
        .collect()
}

// ── JSON ──────────────────────────────────────────────────────────────────────

fn json_stringify(args: Vec<Value>) -> Result<Value, String> {
    // A CALLABLE second argument is the replacer function, and it is checked
    // before the array form (`IsCallable` precedes `IsArray` in the spec), so a
    // callable never also reaches the key-filter path below.
    let replacer = args
        .get(1)
        .filter(|r| with_host(|h| host::is_callable(h, r)))
        .cloned();
    // `toJSON` and the replacer run BEFORE serialization and are user code, so
    // the tree is rewritten first — outside the host borrow `json_str` holds,
    // and before the BigInt walk, which has no cycle guard of its own.
    //
    // The top-level value is a property of a synthetic wrapper `{ "": value }`
    // under key `""`, which is exactly the holder the replacer receives as
    // `this` on its first call.
    let root = arg0(&args);
    let wrapper = with_host(|h| {
        let mut m: IndexMap<String, Value> = IndexMap::new();
        m.insert(String::new(), root.clone());
        h.new_object(m)
    });
    let v = apply_to_json(&wrapper, "", &root, &mut Vec::new(), replacer.as_ref())?;
    // A BigInt anywhere in a serializable position is a TypeError (JSON has no
    // bigint form), matching Node's exact message.
    if with_host(|h| json_has_bigint(h, &v)) {
        return Err(host::type_error("Do not know how to serialize a BigInt"));
    }
    let indent = match args.get(2) {
        Some(Value::Float(f)) => " ".repeat((*f as usize).min(10)),
        Some(other) => with_host(|h| h.as_str(other)).unwrap_or_default(),
        None => String::new(),
    };
    // A replacer array (args[1]) restricts which object keys are serialized.
    let keys: Option<Vec<String>> = args.get(1).and_then(|r| {
        with_host(|h| match h.get(r) {
            Some(JsObj::Array(items)) => {
                Some(items.iter().map(|k| h.str_of(k)).collect::<Vec<_>>())
            }
            _ => None,
        })
    });
    let s = with_host(|h| json_str(h, &v, &indent, 0, keys.as_deref()));
    match s {
        Some(s) => Ok(with_host(|h| h.new_str(s))),
        None => Ok(Value::Undef),
    }
}

/// One `SerializeJSONProperty(key, holder)` step: rewrite `v` (the value read
/// from `holder[key]`) by calling its `toJSON(key)` and then the replacer
/// function as `replacer.call(holder, key, value)`, then recurse into whatever
/// object survives. Applies to user methods, class methods, and the native
/// `Date`/`Buffer`/`URL` accessors alike.
///
/// Returns a fresh tree; the input is never mutated. `path` carries the chain of
/// objects currently being walked so a cyclic structure is reported rather than
/// spinning forever.
///
/// `toJSON` is called on the value ONCE and is NOT re-applied to its own result
/// — `{toJSON(){ return {toJSON(){ return 1 }} }}` serializes as `{}` in Node,
/// because the inner method is a plain (unserializable) function property of the
/// returned object, not a second conversion hook.
fn apply_to_json(
    holder: &Value,
    key: &str,
    v: &Value,
    path: &mut JsonPath,
    rep: Option<&Value>,
) -> Result<Value, String> {
    let mut v = v.clone();
    if matches!(v, Value::Obj(_)) {
        let tag = crate::stdlib::native_tag(&v);
        // 25.5.2.1 step 2: `toJSON` is looked up with `[[Get]]`, so a PROXY
        // supplies one through its `get` trap. `lookup_chain` walks the
        // property map and never asks the handler, so a proxy carrying a
        // `toJSON` was serialized as a plain object instead of by its own
        // method — and node's trap log starts with that `get`.
        let to_json = if with_host(|h| h.kind_of(&v)) == Some(ObjKind::Proxy) {
            get_property(&v, "toJSON")?
        } else {
            with_host(|h| host::lookup_chain(h, &v, "toJSON")).unwrap_or(Value::Undef)
        };
        let has_to_json = with_host(|h| host::is_callable(h, &to_json))
            || tag
                .as_deref()
                .map(crate::stdlib::has_to_json)
                .unwrap_or(false);
        if has_to_json {
            let k = with_host(|h| h.new_str(key.to_string()));
            v = host::call_method(&v, "toJSON", vec![k])?;
        }
    }
    if let Some(rep) = rep {
        let k = with_host(|h| h.new_str(key.to_string()));
        v = host::invoke(rep, vec![k, v.clone()], Some(holder.clone()))?;
    }
    // How `v` was reached from its holder, as V8 names the step in a
    // circular-structure message: `index 1` under an array, else `property 'k'`.
    let via = if matches!(with_host(|h| h.get(holder).cloned()), Some(JsObj::Array(_))) {
        format!("index {key}")
    } else {
        format!("property '{key}'")
    };
    json_walk_children(&v, path, &via, rep)
}

/// The objects `JSON.stringify` is inside of, outermost first, each with the
/// step that reached it from its holder (`property 'x'` / `index 1`).
type JsonPath = Vec<(String, Value)>;

/// V8's `ConstructCircularStructureErrorMessage`: the cycle from the object it
/// starts at to the key that closes it. At most the first two and the last one
/// intermediate step are listed, with `|     ...` standing for the rest.
fn circular_json_message(path: &JsonPath, start: usize, closing: &str) -> String {
    const PREFIX: usize = 2;
    const POSTFIX: usize = 1;
    let ctor = |v: &Value| -> String {
        with_host(|h| match h.get(v) {
            Some(JsObj::Array(_)) if h.proto_of(v).is_none() => "Array".to_string(),
            _ => match h.ctor_name(v) {
                n if n.is_empty() => "Object".to_string(),
                n => n,
            },
        })
    };
    let line = |i: usize| {
        format!(
            "\n    |     {} -> object with constructor '{}'",
            path[i].0,
            ctor(&path[i].1)
        )
    };
    let mut msg = format!(
        "Converting circular structure to JSON\n    --> starting at object with constructor '{}'",
        ctor(&path[start].1)
    );
    let prefix_end = path.len().min(start + 1 + PREFIX);
    for i in start + 1..prefix_end {
        msg.push_str(&line(i));
    }
    if path.len() > prefix_end + POSTFIX {
        msg.push_str("\n    |     ...");
    }
    for i in prefix_end.max(path.len().saturating_sub(POSTFIX))..path.len() {
        msg.push_str(&line(i));
    }
    msg.push_str(&format!("\n    --- {closing} closes the circle"));
    msg
}

/// Whether a raw property key of a host object is one `json_str` serializes. The
/// internal slots (`@@`-prefixed symbol keys, `#`-prefixed private fields) are
/// invisible to JSON, so the replacer must not be invoked for them either.
fn json_visible_key(k: &str) -> bool {
    !k.starts_with("@@") && !k.starts_with('#')
}

/// Recurse into the elements/properties of an already-converted value, running
/// `apply_to_json` for each with this value as the holder.
fn json_walk_children(
    v: &Value,
    path: &mut JsonPath,
    via: &str,
    rep: Option<&Value>,
) -> Result<Value, String> {
    if !matches!(v, Value::Obj(_)) {
        return Ok(v.clone());
    }
    // A value that contains itself has no JSON form.
    if let Some(start) = with_host(|h| path.iter().position(|(_, p)| h.strict_eq(p, v))) {
        return Err(host::type_error(&circular_json_message(path, start, via)));
    }
    // A Proxy owns no property map, so it is snapshotted through its traps into
    // the plain array/object `SerializeJSONArray`/`SerializeJSONObject` describe
    // — which read every member through `[[Get]]`, exactly as the snapshot does.
    if with_host(|h| h.kind_of(v)) == Some(ObjKind::Proxy) {
        let snap = crate::proxy::json_snapshot(v)?;
        path.push((via.to_string(), v.clone()));
        let out = json_walk_children(&snap, path, via, rep);
        path.pop();
        return out;
    }
    let obj = with_host(|h| h.get(v).cloned());
    path.push((via.to_string(), v.clone()));
    let out = (|| match obj {
        Some(JsObj::Array(items)) => {
            // Read the elements through the accessor-aware funnel: an index
            // with a getter must be SERIALIZED as what the getter returns, and
            // the backing vector still holds the stale slot.
            let mut resolved = items;
            // An index with a getter must be SERIALIZED as what the getter
            // returns, and it also forces a rebuild below: keeping the original
            // array would hand the serializer back the stale backing vector.
            let had_accessor = resolve_index_accessors(v, &mut resolved);
            let items = resolved;
            let mut out = Vec::with_capacity(items.len());
            let mut changed = had_accessor;
            for (i, it) in items.iter().enumerate() {
                let nv = apply_to_json(v, &i.to_string(), it, path, rep)?;
                changed |= !with_host(|h| h.strict_eq(&nv, it));
                out.push(nv);
            }
            // Keep identity when nothing changed, so an enclosing object is not
            // needlessly rebuilt (which would drop its property attributes).
            if changed {
                Ok(with_host(|h| h.new_array(out)))
            } else {
                Ok(v.clone())
            }
        }
        Some(JsObj::Object(props)) => {
            // An enumerable own accessor must have its getter RUN and the result
            // serialized. That cannot happen inside `json_str` (which holds the
            // host borrow), so materialize here — the same reason `toJSON` is
            // applied in this pass.
            let has_accessor = with_host(|h| {
                h.own_accessor_keys(v)
                    .iter()
                    .any(|k| h.prop_attrs(v, k).enumerable)
            });
            if has_accessor {
                let mut next: IndexMap<String, Value> = IndexMap::new();
                for (k, val) in host::own_enum_entries_deep(v)? {
                    let nv = if json_visible_key(&k) {
                        apply_to_json(v, &k, &val, path, rep)?
                    } else {
                        val
                    };
                    next.insert(k, nv);
                }
                return Ok(with_host(|h| h.new_object(next)));
            }
            // Only rebuild when a descendant actually changed, so plain data keeps
            // its identity (and its prototype / native tag).
            let mut next: IndexMap<String, Value> = IndexMap::new();
            let mut changed = false;
            for (k, val) in &props {
                let nv = if json_visible_key(k) {
                    apply_to_json(v, k, val, path, rep)?
                } else {
                    val.clone()
                };
                changed |= !with_host(|h| h.strict_eq(&nv, val));
                next.insert(k.clone(), nv);
            }
            if changed {
                Ok(with_host(|h| {
                    let o = h.new_object(next);
                    h.copy_prop_attrs(v, &o);
                    o
                }))
            } else {
                Ok(v.clone())
            }
        }
        _ => Ok(v.clone()),
    })();
    path.pop();
    out
}

/// Whether a value tree contains a `BigInt` in a position `JSON.stringify` would
/// try to serialize (a value in an array/object) — such a value throws.
fn json_has_bigint(h: &host::JsHost, v: &Value) -> bool {
    match h.get(v) {
        Some(JsObj::BigInt(_)) => true,
        Some(JsObj::Array(items)) => items.iter().any(|x| json_has_bigint(h, x)),
        Some(JsObj::Object(props)) => props
            .iter()
            .filter(|(k, _)| !k.starts_with("@@") && !k.starts_with('#'))
            .any(|(_, val)| json_has_bigint(h, val)),
        _ => false,
    }
}

fn json_str(
    h: &host::JsHost,
    v: &Value,
    indent: &str,
    depth: usize,
    keys: Option<&[String]>,
) -> Option<String> {
    let sep = if indent.is_empty() { ":" } else { ": " };
    match v {
        Value::Undef => None,
        Value::Bool(b) => Some(if *b { "true".into() } else { "false".into() }),
        Value::Int(n) => Some(n.to_string()),
        Value::Float(f) => Some(if f.is_finite() {
            host::fmt_number(*f)
        } else {
            "null".into()
        }),
        Value::Str(s) => Some(json_quote(s)),
        Value::Obj(_) => match h.get(v) {
            Some(JsObj::Str(s)) => Some(json_quote(s)),
            Some(JsObj::Null) => Some("null".into()),
            // A `JSON.rawJSON` marker contributes its text VERBATIM — that is the
            // whole point of it, and it is why a number wider than a `double`
            // can survive a round trip.
            _ if h.fn_prop(v, "@@rawJSON").is_some() => match h.get(v) {
                Some(JsObj::Object(p)) => p.get("rawJSON").map(|r| h.str_of(r)),
                _ => None,
            },
            // A Map/Set has no ENTRIES to serialize (they are internal slots),
            // but any own property a script attached is serialized like an
            // ordinary object's: `JSON.stringify(Object.assign(new Map(), {a:1}))`
            // is `{"a":1}`.
            Some(JsObj::Map { .. })
            | Some(JsObj::Set { .. })
            | Some(JsObj::RegExp(_))
            // A Promise and a generator are ORDINARY objects to the serializer:
            // their state is internal slots, so they contribute no entries and
            // render as `{}`. They were being omitted entirely instead, so a
            // promise in an array became `null` and one in an object vanished.
            | Some(JsObj::Promise { .. })
            | Some(JsObj::Generator { .. }) => {
                let parts: Vec<String> = h
                    .own_enum_entries(v)
                    .into_iter()
                    .filter(|(k, _)| !k.starts_with("@@") && !host::is_symbol_key(k))
                    .filter_map(|(k, val)| {
                        json_str(h, &val, indent, depth + 1, keys)
                            .map(|s| format!("{}{sep}{s}", json_quote(&k)))
                    })
                    .collect();
                Some(wrap(&parts, "{", "}", indent, depth))
            }
            // A NON-callable builtin is a namespace object, not a function, so
            // it serializes as one: `JSON.stringify(Math)` is `{}` (its members
            // are all non-enumerable), where omitting it made the whole property
            // disappear from its holder.
            Some(JsObj::Builtin(n)) if !host::builtin_is_callable(n) => {
                let parts: Vec<String> = crate::stdlib::namespace_keys(n)
                    .into_iter()
                    .filter_map(|k| {
                        let val = h.builtin_static(n, &k)?;
                        json_str(h, &val, indent, depth + 1, keys)
                            .map(|s| format!("{}{sep}{s}", json_quote(&k)))
                    })
                    .collect();
                Some(wrap(&parts, "{", "}", indent, depth))
            }
            // Functions and symbols are omitted (undefined) as values.
            Some(JsObj::Func(_))
            | Some(JsObj::Builtin(_))
            | Some(JsObj::BoundMethod { .. })
            | Some(JsObj::BoundFunc { .. })
            | Some(JsObj::Class(_))
            | Some(JsObj::Symbol { .. }) => None,
            Some(JsObj::Array(items)) => {
                if items.is_empty() {
                    return Some("[]".into());
                }
                let parts: Vec<String> = items
                    .iter()
                    .map(|x| {
                        json_str(h, x, indent, depth + 1, keys).unwrap_or_else(|| "null".into())
                    })
                    .collect();
                Some(wrap(&parts, "[", "]", indent, depth))
            }
            Some(JsObj::Object(props)) if props.contains_key("@@primitive") => {
                // 25.5.2.2 step 4: a String/Number/Boolean wrapper serializes as
                // the primitive it boxes, not as the object holding it —
                // `JSON.stringify(new Number(1))` is `1`, not `{}`.
                json_str(h, &props["@@primitive"].clone(), indent, depth, keys)
            }
            Some(JsObj::Object(props)) => {
                // A replacer array restricts (and orders) which keys are emitted.
                let parts: Vec<String> = match keys {
                    Some(allow) => allow
                        .iter()
                        .filter_map(|k| {
                            props.get(k).and_then(|val| {
                                json_str(h, val, indent, depth + 1, keys)
                                    .map(|vs| format!("{}{sep}{vs}", json_quote(k)))
                            })
                        })
                        .collect(),
                    None => h
                        .own_enum_entries(v)
                        .iter()
                        .filter_map(|(k, val)| {
                            json_str(h, val, indent, depth + 1, keys)
                                .map(|vs| format!("{}{sep}{vs}", json_quote(k)))
                        })
                        .collect(),
                };
                if parts.is_empty() {
                    return Some("{}".into());
                }
                Some(wrap(&parts, "{", "}", indent, depth))
            }
            _ => Some("null".into()),
        },
        _ => Some("null".into()),
    }
}

fn wrap(parts: &[String], open: &str, close: &str, indent: &str, depth: usize) -> String {
    if indent.is_empty() {
        format!("{open}{}{close}", parts.join(","))
    } else {
        let pad = indent.repeat(depth + 1);
        let pad_close = indent.repeat(depth);
        format!(
            "{open}\n{pad}{}\n{pad_close}{close}",
            parts.join(&format!(",\n{pad}"))
        )
    }
}

fn json_quote(s: &str) -> String {
    let mut out = String::from("\"");
    for c in s.chars() {
        match c {
            '"' => out.push_str("\\\""),
            '\\' => out.push_str("\\\\"),
            '\n' => out.push_str("\\n"),
            '\t' => out.push_str("\\t"),
            '\r' => out.push_str("\\r"),
            // QuoteJSONString (25.5.2.2) names SIX short escapes, not four.
            // Backspace and form feed were missing, so they fell through to the
            // `\uXXXX` arm below and `JSON.stringify("\b")` produced
            // `""` where node produces `"\b"`. Both parse back to the same
            // string, so the difference is invisible to a round trip and shows
            // up only as a byte mismatch against a fixture or a checksum.
            '\u{8}' => out.push_str("\\b"),
            '\u{c}' => out.push_str("\\f"),
            c if (c as u32) < 0x20 => out.push_str(&format!("\\u{:04x}", c as u32)),
            _ => out.push(c),
        }
    }
    out.push('"');
    out
}

fn json_parse(args: Vec<Value>) -> Result<Value, String> {
    let s = with_host(|h| h.str_of(&arg0(&args)));
    let mut p = JsonParser {
        chars: s.chars().collect(),
        pos: 0,
        prims: Vec::new(),
        record: args
            .get(1)
            .is_some_and(|r| with_host(|h| host::is_callable(h, r))),
    };
    p.skip_ws();
    if p.peek().is_none() {
        return Err("SyntaxError: Unexpected end of JSON input".into());
    }
    let v = p.parse_value()?;
    let value_end = p.pos;
    p.skip_ws();
    // Anything after the top-level value is an error — the parser used to accept
    // and silently discard it, so `JSON.parse('{"a":1}x')` succeeded.
    if let Some(c) = p.peek() {
        // V8 names the token kind only when it butts directly against the value
        // (`01` -> "Unexpected number at position 1"); with whitespace between
        // it is just a non-whitespace character (`1 2`).
        // Only a digit butted directly against a completed number literal —
        // V8's number scanner is still in number context there. `5"x"` and
        // `[0,1]0` exit the scanner cleanly and get the generic message.
        let after_number = value_end > 0
            && p.pos == value_end
            && p.chars[value_end - 1].is_ascii_digit()
            && c.is_ascii_digit();
        return Err(if after_number {
            p.err_at("Unexpected number", p.pos)
        } else {
            p.err_trailing(p.pos)
        });
    }
    // Optional reviver: walk bottom-up, transforming each (key, value).
    if let Some(reviver) = args
        .get(1)
        .filter(|r| with_host(|h| host::is_callable(h, r)))
        .cloned()
    {
        // The top-level holder is a fresh `{ "": value }` wrapper, as the spec
        // constructs before the walk.
        let root = with_host(|h| {
            let mut m: IndexMap<String, Value> = IndexMap::new();
            m.insert(String::new(), v.clone());
            h.new_object(m)
        });
        return json_revive("", v, &reviver, &root, &p.prims, &mut 0);
    }
    Ok(v)
}

/// `JSON.parse` reviver walk: recurse into children first, then call
/// `reviver(key, value)`; a returned `undefined` drops the property.
///
/// The reviver runs with the HOLDER as `this` (25.5.1.1
/// InternalizeJSONProperty) — the object or array the key lives in, and at the
/// top level a wrapper `{ "": value }`. It was being called with no receiver,
/// so `this` was undefined and a reviver could not reach its siblings.
/// `JSON.rawJSON(text)` — a marker object whose text `JSON.stringify` emits
/// VERBATIM, so a number too large for a `double` survives a round trip
/// (`JSON.stringify({n: JSON.rawJSON("12345678901234567890")})`).
///
/// The validation is not "does `JSON.parse` accept it": node's rule, measured
/// across the whole matrix, is
///
/// ```text
/// ""                 -> SyntaxError: Invalid value for JSON.rawJSON
/// leading whitespace -> the parse error for that first character
/// a complete literal -> ok
/// anything left over -> SyntaxError: Invalid value for JSON.rawJSON
/// a broken literal   -> the parse error the scanner raised
/// ```
///
/// so `" 1"` reports an unexpected token while `"1 "` and `"1,2"` report the
/// invalid-value message even though `JSON.parse` accepts the former and gives
/// a token error for the latter.
fn json_raw(args: Vec<Value>) -> Result<Value, String> {
    const INVALID: &str = "SyntaxError: Invalid value for JSON.rawJSON";
    let s = with_host(|h| h.str_of(&arg0(&args)));
    if s.is_empty() {
        return Err(INVALID.into());
    }
    let mut p = JsonParser {
        chars: s.chars().collect(),
        pos: 0,
        prims: Vec::new(),
        record: false,
    };
    // An object or an array is rejected where it starts, as leading whitespace
    // is — both are "not a primitive", but node reports the token.
    if matches!(p.peek(), Some('{') | Some('[')) || p.peek().is_some_and(|c| c.is_whitespace()) {
        return Err(p.err_token(0));
    }
    p.parse_value()?;
    if p.pos != p.chars.len() {
        // A digit butted against a completed number is still in the number
        // scanner, so `"01"` reports the scanner's error rather than leftover
        // input — the same distinction `json_parse` draws for trailing text.
        if p.chars[p.pos - 1].is_ascii_digit() && p.chars[p.pos].is_ascii_digit() {
            return Err(p.err_at("Unexpected number", p.pos));
        }
        return Err(INVALID.into());
    }
    // A null prototype and one own `rawJSON` property, frozen — the brand is a
    // hidden slot so `Object.keys` stays `["rawJSON"]`.
    Ok(with_host(|h| {
        let mut m: IndexMap<String, Value> = IndexMap::new();
        let text = h.new_str(s);
        m.insert("rawJSON".into(), text);
        let o = h.new_object(m);
        let null = h.null();
        h.set_proto(&o, null);
        h.set_fn_prop(&o, "@@rawJSON", Value::Bool(true));
        h.seal_object(&o, true);
        o
    }))
}

/// `JSON.isRawJSON(v)` — the brand check. A hand-built `{ rawJSON: "1" }` is
/// NOT one, which is why the marker is a hidden slot rather than the property.
fn json_is_raw(args: Vec<Value>) -> Result<Value, String> {
    Ok(Value::Bool(is_raw_json(&arg0(&args))))
}

fn is_raw_json(v: &Value) -> bool {
    with_host(|h| h.fn_prop(v, "@@rawJSON")).is_some()
}

fn json_revive(
    key: &str,
    val: Value,
    reviver: &Value,
    holder: &Value,
    prims: &[String],
    next: &mut usize,
) -> Result<Value, String> {
    // A PRIMITIVE claims the next recorded source slice before its children
    // would — it has none — and a container claims nothing. The walk descends in
    // the same order the parse produced them, so one cursor lines the two up.
    let is_container =
        with_host(|h| matches!(h.get(&val), Some(JsObj::Array(_)) | Some(JsObj::Object(_))));
    let source = if !is_container {
        let s = prims.get(*next).cloned();
        if s.is_some() {
            *next += 1;
        }
        s
    } else {
        None
    };
    match with_host(|h| h.get(&val).cloned()) {
        Some(JsObj::Array(items)) => {
            for i in 0..items.len() {
                let elem = with_host(|h| match h.get(&val) {
                    Some(JsObj::Array(it)) => it[i].clone(),
                    _ => Value::Undef,
                });
                let nv = json_revive(&i.to_string(), elem, reviver, &val, prims, next)?;
                with_host(|h| {
                    if let Some(JsObj::Array(it)) = h.get_mut(&val) {
                        it[i] = nv;
                    }
                });
            }
        }
        Some(JsObj::Object(props)) => {
            let keys: Vec<String> = props
                .keys()
                .filter(|k| !k.starts_with("@@"))
                .cloned()
                .collect();
            for k in keys {
                let elem = with_host(|h| match h.get(&val) {
                    Some(JsObj::Object(p)) => p.get(&k).cloned().unwrap_or(Value::Undef),
                    _ => Value::Undef,
                });
                let nv = json_revive(&k, elem, reviver, &val, prims, next)?;
                with_host(|h| {
                    if let Some(JsObj::Object(p)) = h.get_mut(&val) {
                        if matches!(nv, Value::Undef) {
                            p.shift_remove(&k);
                        } else {
                            p.insert(k.clone(), nv);
                        }
                    }
                });
            }
        }
        _ => {}
    }
    let kv = with_host(|h| h.new_str(key.to_string()));
    // 25.5.1.1 step 2.b: the reviver's THIRD argument. `{ source }` for a
    // primitive, an empty object for an array or an object — node passes it
    // either way, and code reading `ctx.source` used to die on `undefined`
    // because only two arguments were passed.
    let ctx = with_host(|h| {
        let mut m: IndexMap<String, Value> = IndexMap::new();
        if let Some(s) = source {
            let sv = h.new_str(s);
            m.insert("source".into(), sv);
        }
        h.new_object(m)
    });
    host::invoke(reviver, vec![kv, val, ctx], Some(holder.clone()))
}

struct JsonParser {
    chars: Vec<char>,
    pos: usize,
    /// Source text of each PRIMITIVE value, in parse order — what the reviver's
    /// third argument reports as `context.source` (25.5.1.1). Only collected
    /// when a reviver was supplied.
    ///
    /// A flat list rather than a parallel tree because the reviver walk visits
    /// primitives in the same depth-first order the parse produced them, so an
    /// index into this is enough to line them up.
    prims: Vec<String>,
    record: bool,
}
impl JsonParser {
    fn peek(&self) -> Option<char> {
        self.chars.get(self.pos).copied()
    }

    /// `at position N (line L column C)` — the location suffix V8 appends to the
    /// positional JSON parse errors. Positions are in UTF-16-ish code units;
    /// node-js counts `char`s, which agree for the BMP.
    fn at(&self, pos: usize) -> String {
        let mut line = 1usize;
        let mut col = 1usize;
        for c in &self.chars[..pos.min(self.chars.len())] {
            if *c == '\n' {
                line += 1;
                col = 1;
            } else {
                col += 1;
            }
        }
        format!(" at position {pos} (line {line} column {col})")
    }

    /// A positional error (`Expected ':' after property name in JSON at …`).
    fn err_at(&self, what: &str, pos: usize) -> String {
        format!("SyntaxError: {what} in JSON{}", self.at(pos))
    }

    /// The one positional message V8 does NOT suffix with `in JSON`.
    fn err_trailing(&self, pos: usize) -> String {
        format!(
            "SyntaxError: Unexpected non-whitespace character after JSON{}",
            self.at(pos)
        )
    }

    /// V8's default parse error: the offending character plus a window of the
    /// source. The whole input is quoted when it is short (<= 20 chars);
    /// otherwise a 10-character context window either side of `pos` is shown,
    /// elided with `...` on whichever side was cut.
    fn err_token(&self, pos: usize) -> String {
        const MAX_WHOLE: usize = 20;
        const CONTEXT: usize = 10;
        let len = self.chars.len();
        let Some(c) = self.chars.get(pos) else {
            return "SyntaxError: Unexpected end of JSON input".into();
        };
        // V8 reports the whole input for the JS literals that are famously not
        // JSON, without naming an offending character.
        let whole: String = self.chars.iter().collect();
        if matches!(
            whole.as_str(),
            "undefined" | "NaN" | "Infinity" | "-Infinity"
        ) {
            return format!("SyntaxError: \"{whole}\" is not valid JSON");
        }
        let snippet = if len <= MAX_WHOLE {
            format!("\"{whole}\"")
        } else {
            let start = pos.saturating_sub(CONTEXT);
            let end = (pos + CONTEXT).min(len);
            let body: String = self.chars[start..end].iter().collect();
            let head = if start > 0 { "..." } else { "" };
            let tail = if end < len { "..." } else { "" };
            format!("{head}\"{body}\"{tail}")
        };
        format!("SyntaxError: Unexpected token '{c}', {snippet} is not valid JSON")
    }

    fn skip_ws(&mut self) {
        while matches!(
            self.peek(),
            Some(' ') | Some('\n') | Some('\t') | Some('\r')
        ) {
            self.pos += 1;
        }
    }
    fn parse_value(&mut self) -> Result<Value, String> {
        self.skip_ws();
        let start = self.pos;
        let prim = matches!(self.peek(), Some(c) if c != '{' && c != '[');
        let v = match self.peek() {
            Some('{') => self.parse_object(),
            Some('[') => self.parse_array(),
            Some('"') => {
                let s = self.parse_string()?;
                Ok(with_host(|h| h.new_str(s)))
            }
            Some('t') | Some('f') => self.parse_bool(),
            Some('n') => {
                self.expect_lit("null")?;
                Ok(with_host(|h| h.null()))
            }
            Some(c) if c == '-' || c.is_ascii_digit() => self.parse_number(),
            None => Err("SyntaxError: Unexpected end of JSON input".into()),
            _ => Err(self.err_token(self.pos)),
        }?;
        if prim && self.record {
            self.prims
                .push(self.chars[start..self.pos].iter().collect());
        }
        Ok(v)
    }
    fn expect_lit(&mut self, lit: &str) -> Result<(), String> {
        for ch in lit.chars() {
            match self.peek() {
                Some(c) if c == ch => self.pos += 1,
                // V8 reports the first character that broke the literal, which is
                // why `foo` complains about `'o'` (index 2) and not `'f'`.
                None => return Err("SyntaxError: Unexpected end of JSON input".into()),
                _ => return Err(self.err_token(self.pos)),
            }
        }
        Ok(())
    }
    fn parse_bool(&mut self) -> Result<Value, String> {
        if self.peek() == Some('t') {
            self.expect_lit("true")?;
            Ok(Value::Bool(true))
        } else {
            self.expect_lit("false")?;
            Ok(Value::Bool(false))
        }
    }
    /// JSON's number grammar: `-? (0 | [1-9][0-9]*) (. [0-9]+)? ([eE] [+-]? [0-9]+)?`.
    /// A leading zero does NOT swallow the following digits — `01` parses as `0`
    /// and the stray `1` becomes a trailing-token error, which is how V8 reports
    /// it. Each way the grammar can run out has its own message.
    fn parse_number(&mut self) -> Result<Value, String> {
        let start = self.pos;
        if self.peek() == Some('-') {
            self.pos += 1;
            if !matches!(self.peek(), Some(c) if c.is_ascii_digit()) {
                return Err(self.err_at("No number after minus sign", self.pos));
            }
        }
        if self.peek() == Some('0') {
            self.pos += 1;
        } else {
            while matches!(self.peek(), Some(c) if c.is_ascii_digit()) {
                self.pos += 1;
            }
        }
        if self.peek() == Some('.') {
            self.pos += 1;
            if !matches!(self.peek(), Some(c) if c.is_ascii_digit()) {
                return Err(self.err_at("Unterminated fractional number", self.pos));
            }
            while matches!(self.peek(), Some(c) if c.is_ascii_digit()) {
                self.pos += 1;
            }
        }
        if matches!(self.peek(), Some('e') | Some('E')) {
            self.pos += 1;
            if matches!(self.peek(), Some('+') | Some('-')) {
                self.pos += 1;
            }
            if !matches!(self.peek(), Some(c) if c.is_ascii_digit()) {
                return Err(self.err_at("Exponent part is missing a number", self.pos));
            }
            while matches!(self.peek(), Some(c) if c.is_ascii_digit()) {
                self.pos += 1;
            }
        }
        let s: String = self.chars[start..self.pos].iter().collect();
        s.parse::<f64>()
            .map(Value::Float)
            .map_err(|_| self.err_at("Unexpected number", start))
    }
    fn parse_string(&mut self) -> Result<String, String> {
        self.pos += 1; // opening quote
        let mut out = String::new();
        loop {
            match self.peek() {
                None => return Err(self.err_at("Unterminated string", self.pos)),
                Some('"') => {
                    self.pos += 1;
                    break;
                }
                Some('\\') => {
                    self.pos += 1;
                    match self.peek() {
                        Some('n') => out.push('\n'),
                        Some('t') => out.push('\t'),
                        Some('r') => out.push('\r'),
                        Some('"') => out.push('"'),
                        Some('\\') => out.push('\\'),
                        Some('/') => out.push('/'),
                        Some('b') => out.push('\u{08}'),
                        Some('f') => out.push('\u{0C}'),
                        Some('u') => {
                            let h: String = self.chars
                                [self.pos + 1..(self.pos + 5).min(self.chars.len())]
                                .iter()
                                .collect();
                            if let Ok(n) = u32::from_str_radix(&h, 16) {
                                if let Some(ch) = char::from_u32(n) {
                                    out.push(ch);
                                }
                            }
                            self.pos += 4;
                        }
                        _ => {}
                    }
                    self.pos += 1;
                }
                // A raw control character is not legal inside a JSON string; it
                // has to be escaped. V8 rejects it rather than passing it through.
                Some(c) if (c as u32) < 0x20 => {
                    return Err(self.err_at("Bad control character in string literal", self.pos))
                }
                Some(c) => {
                    out.push(c);
                    self.pos += 1;
                }
            }
        }
        Ok(out)
    }
    fn parse_array(&mut self) -> Result<Value, String> {
        self.pos += 1; // [
        let mut items = Vec::new();
        self.skip_ws();
        if self.peek() == Some(']') {
            self.pos += 1;
            return Ok(with_host(|h| h.new_array(items)));
        }
        loop {
            items.push(self.parse_value()?);
            self.skip_ws();
            match self.peek() {
                Some(',') => {
                    self.pos += 1;
                }
                Some(']') => {
                    self.pos += 1;
                    break;
                }
                _ => return Err(self.err_at("Expected ',' or ']' after array element", self.pos)),
            }
        }
        Ok(with_host(|h| h.new_array(items)))
    }
    fn parse_object(&mut self) -> Result<Value, String> {
        self.pos += 1; // {
        let mut props: IndexMap<String, Value> = IndexMap::new();
        self.skip_ws();
        if self.peek() == Some('}') {
            self.pos += 1;
            return Ok(with_host(|h| h.new_object(props)));
        }
        loop {
            self.skip_ws();
            if self.peek() != Some('"') {
                // The first key uses the "or '}'" wording (an empty object is
                // still legal there); a key after a comma does not. End of input
                // reports the same expectation, at the end position.
                return Err(if props.is_empty() {
                    self.err_at("Expected property name or '}'", self.pos)
                } else {
                    self.err_at("Expected double-quoted property name", self.pos)
                });
            }
            let key = self.parse_string()?;
            self.skip_ws();
            if self.peek() != Some(':') {
                return Err(match self.peek() {
                    None => "SyntaxError: Unexpected end of JSON input".into(),
                    _ => self.err_at("Expected ':' after property name", self.pos),
                });
            }
            self.pos += 1;
            let val = self.parse_value()?;
            props.insert(key, val);
            self.skip_ws();
            match self.peek() {
                Some(',') => {
                    self.pos += 1;
                }
                Some('}') => {
                    self.pos += 1;
                    break;
                }
                _ => return Err(self.err_at("Expected ',' or '}' after property value", self.pos)),
            }
        }
        Ok(with_host(|h| h.new_object(props)))
    }
}

// ══ type methods (array / string / number) ═══════════════════════════════════

fn is_array_method(name: &str) -> bool {
    matches!(
        name,
        "push"
            | "pop"
            | "shift"
            | "unshift"
            | "map"
            | "filter"
            | "forEach"
            | "join"
            | "slice"
            | "indexOf"
            | "lastIndexOf"
            | "includes"
            | "reduce"
            | "concat"
            | "reverse"
            | "sort"
            | "find"
            | "findIndex"
            | "some"
            | "every"
            | "flat"
            | "fill"
            | "splice"
            | "keys"
            | "values"
            | "entries"
            | "flatMap"
            | "at"
            | "toString"
            | "reduceRight"
            | "findLast"
            | "findLastIndex"
            | "copyWithin"
    )
}
/// Every `String.prototype` method node-js implements.
///
/// A LIST rather than a `matches!` arm because the same set has to be installed
/// on the real `String.prototype` object: a method read off the prototype
/// (`String.prototype.trim.call(s)`, the generic-borrowing idiom libraries use)
/// found nothing there, so the two views of "which methods exist" would drift
/// if they were written twice.
pub(crate) const STRING_PROTO_METHODS: &[&str] = &[
    "toUpperCase",
    "toLowerCase",
    "charAt",
    "charCodeAt",
    "codePointAt",
    "indexOf",
    "lastIndexOf",
    "includes",
    "slice",
    "substring",
    "substr",
    "split",
    "trim",
    "trimStart",
    "trimEnd",
    "replace",
    "replaceAll",
    "repeat",
    "startsWith",
    "endsWith",
    "padStart",
    "padEnd",
    "concat",
    "at",
    "toString",
    "toLocaleString",
    "valueOf",
    "match",
    "matchAll",
    "search",
    "normalize",
    "localeCompare",
    "toLocaleUpperCase",
    "toLocaleLowerCase",
    "isWellFormed",
    "toWellFormed",
];

fn is_string_method(name: &str) -> bool {
    STRING_PROTO_METHODS.contains(&name)
}

/// Every SYMBOL-keyed intrinsic method the generated table lists for `ctor`,
/// spelled the way this frontend spells the key (`@@iterator`).
///
/// A prototype built as a REAL object (`String.prototype`, `URLSearchParams
/// .prototype`) installs its methods from a list, and only the string-keyed
/// list was walked — so `String.prototype[Symbol.iterator]` read `undefined`
/// while `Array.prototype[Symbol.iterator]`, which resolves through the
/// `Builtin` namespace and its table gate, answered a function. Derived from
/// the table rather than written out, so it cannot name a method node does not
/// define nor miss one it does.
pub(crate) fn proto_symbol_methods(ctor: &str) -> Vec<&'static str> {
    let prefix = format!("@proto:{ctor}:");
    crate::arity::BUILTIN_ARITY
        .iter()
        .filter_map(|(k, _, _)| k.strip_prefix(prefix.as_str()))
        .filter(|m| m.starts_with("@@"))
        .collect()
}

/// The builtin constructors whose `.prototype` object is BRANDED — every other
/// `<C>.prototype` is an ordinary object and reports `[object Object]`.
///
/// Measured on node v26.8.1 over every constructor this frontend knows:
///
/// ```text
/// Array/Object/Number/String/Boolean/Function   the ES5 legacy slot prototypes
/// Symbol/BigInt/Map/Set/WeakMap/WeakSet         carry an own @@toStringTag
/// Promise/Iterator/ArrayBuffer/DataView         "
/// WeakRef/FinalizationRegistry/URL              "
/// URLSearchParams/TextEncoder/TextDecoder       "
/// Date/RegExp/Error/TypeError/Uint8Array/…      [object Object]
/// ```
///
/// The rule this replaces branded EVERY `<C>.prototype` as `C`, so
/// `Object.prototype.toString.call(Date.prototype)` read `[object Date]` — and
/// a `Date.prototype.toString` call on a plain object named `[object Date]` in
/// its own failure message where node names `[object Object]`.
pub(crate) const BRANDED_PROTOS: &[&str] = &[
    "Array",
    "ArrayBuffer",
    "BigInt",
    "Boolean",
    "DataView",
    "FinalizationRegistry",
    "Function",
    "Iterator",
    "Map",
    "Number",
    "Object",
    "Promise",
    "Set",
    "SharedArrayBuffer",
    "String",
    "Symbol",
    "TextDecoder",
    "TextEncoder",
    "URL",
    "URLSearchParams",
    "WeakMap",
    "WeakRef",
    "WeakSet",
];

/// Whether `v` is a `RegExp` value (drives the regex path of `match`/`replace`/…).
/// A user `Symbol.match`/`replace`/`search`/`split`/`matchAll` method on the
/// ARGUMENT, which the string method must delegate to (22.1.3.x step 2).
///
/// `"abc".match(o)` where `o` defines `Symbol.match` calls that method rather
/// than coercing `o` to a pattern — the protocol every regexp-like library
/// implements. None of the five were consulted, so a custom matcher was
/// silently stringified instead.
fn symbol_protocol(arg: &Value, sym: &str) -> Option<Value> {
    if matches!(arg, Value::Undef) || with_host(|h| h.is_null(arg)) {
        return None;
    }
    let f = get_property(arg, sym).ok()?;
    with_host(|h| host::is_callable(h, &f)).then_some(f)
}

fn is_regexp_arg(v: &Value) -> bool {
    // 7.2.8 `IsRegExp` asks `Symbol.match` FIRST, so an object can declare
    // itself a regexp — or a real one can disown the label. Only the heap kind
    // was checked, so `"a".startsWith({[Symbol.match]: true})` did not throw
    // the TypeError the spec requires.
    if let Ok(m) = get_property(v, "@@match") {
        if !matches!(m, Value::Undef) {
            return with_host(|h| h.truthy(&m));
        }
    }
    with_host(|h| h.kind_of(v)) == Some(ObjKind::RegExp)
}

/// `str.replace(strPattern, fn)` — a function replacer against a literal (string)
/// pattern: replace the first (or all) occurrence, calling `fn(match, offset, s)`.
fn replace_str_fn(s: &str, pat: &str, repl: &Value, all: bool) -> Result<String, String> {
    if pat.is_empty() {
        return Ok(s.to_string());
    }
    let mut out = String::new();
    let mut rest = s;
    let mut base = 0usize;
    while let Some(pos) = rest.find(pat) {
        out.push_str(&rest[..pos]);
        let offset = base + pos;
        let m = with_host(|h| h.new_str(pat.to_string()));
        let str_arg = with_host(|h| h.new_str(s.to_string()));
        let r = host::invoke(repl, vec![m, Value::Float(offset as f64), str_arg], None)?;
        out.push_str(&with_host(|h| h.str_of(&r)));
        let consumed = pos + pat.len();
        base += consumed;
        rest = &rest[consumed..];
        if !all {
            break;
        }
    }
    out.push_str(rest);
    Ok(out)
}
/// Every `Number.prototype` method node-js implements — a list for the same
/// reason [`STRING_PROTO_METHODS`] is one.
pub(crate) const NUMBER_PROTO_METHODS: &[&str] = &[
    "toFixed",
    "toExponential",
    "toString",
    "toPrecision",
    "toLocaleString",
    "valueOf",
];

fn is_number_method(name: &str) -> bool {
    NUMBER_PROTO_METHODS.contains(&name)
}

/// The exotic kinds whose own dispatch table does NOT already reach the
/// `Object.prototype` methods, so the inherited ones have to be routed to.
///
/// An allowlist rather than a catch-all: a primitive receiver also reaches this
/// function, and a Number's `toString` is `Number.prototype.toString` — routing
/// it to the object form made `(255).toString(16)` report `[object Number]`.
fn inherits_object_methods(recv: &Value) -> bool {
    matches!(
        with_host(|h| h.kind_of(recv)),
        Some(
            ObjKind::Map
                | ObjKind::Set
                | ObjKind::Promise
                | ObjKind::RegExp
                | ObjKind::Generator
                | ObjKind::Symbol
                | ObjKind::BigInt
                | ObjKind::Iter
        )
    )
}

/// Whether `recv`'s own prototype defines `name`, shadowing the
/// `Object.prototype` method of that name — `RegExp.prototype.toString` does,
/// `Map.prototype` does not.
fn overrides_object_method(recv: &Value, name: &str) -> bool {
    match with_host(|h| h.kind_of(recv)) {
        Some(ObjKind::Map) => is_map_method(name),
        Some(ObjKind::Set) => is_set_method(name),
        Some(ObjKind::RegExp) => crate::regexp::is_regexp_method(name),
        // A Symbol has its own `toString`; `valueOf` is the inherited one,
        // which returns the receiver — exactly what a symbol needs.
        Some(ObjKind::Symbol) => matches!(name, "toString" | "valueOf" | "@@toPrimitive"),
        Some(ObjKind::BigInt) => matches!(name, "toString" | "valueOf" | "toLocaleString"),
        _ => false,
    }
}

/// Dispatch `recv.name(args)` for the built-in prototype methods.
pub fn call_type_method(recv: &Value, name: &str, args: Vec<Value>) -> Result<Value, String> {
    // A USER method on the receiver's prototype chain wins over the builtin of
    // the same name — that is how a `class X extends Array` method is reached,
    // since the dispatch below goes straight to the builtin table and has no
    // entry for it.
    //
    // Deliberately restricted to a user function: the shared `Object.prototype`
    // carries real `@proto:Object:*` thunks, so accepting any callable made a
    // bare `map.toString()` resolve to the object form instead of the builtin
    // one the exotic is supposed to use.
    if let Some(f) = with_host(|h| host::lookup_chain(h, recv, name)) {
        if matches!(
            with_host(|h| h.kind_of(&f)),
            Some(ObjKind::Func) | Some(ObjKind::Class) | Some(ObjKind::BoundFunc)
        ) {
            return host::invoke(&f, args, Some(recv.clone()));
        }
    }
    // A method synthesized from the receiver's KIND is unreachable once its
    // intrinsic prototype is off the chain. The read already answers
    // `undefined` for one; dispatch has its own table and would still have
    // called it, so `Object.setPrototypeOf(a, {}); a.join()` returned "1,2"
    // while `a.join` was `undefined` — the read and the call disagreeing again,
    // in the opposite direction from the monkey-patch case below.
    if !own_intrinsic_reachable(recv)
        && inherited_method_owner(recv, name).is_none()
        && !has_own_for_shadow(recv, name)
        && inherited_builtin_static(recv, name).is_none()
        && with_host(|h| host::lookup_chain(h, recv, name)).is_none()
    {
        return Err(host::type_error(&format!("{name} is not a function")));
    }
    // A method monkey-patched onto the receiver's intrinsic prototype. The READ
    // path resolves these, but dispatch goes straight to the builtin table and
    // never consults it, so `Array.prototype.last = f; [1].last()` threw "is not
    // a function" while `[1].last` WAS `f` — the read and the call disagreeing
    // about the same name, on the one path a polyfill actually uses.
    if !name.starts_with("@@") && !has_own_for_shadow(recv, name) {
        if let Some(f) = inherited_builtin_static(recv, name) {
            if with_host(|h| host::is_callable(h, &f)) {
                return host::invoke(&f, args, Some(recv.clone()));
            }
        }
    }
    // Every object INHERITS the `Object.prototype` methods, and an exotic that
    // does not define its own reaches them the same way. Each kind's dispatch
    // table below only knows its own methods, so `new Map().toString()`,
    // `promise.hasOwnProperty(k)` and `sym.toLocaleString()` all reported "is
    // not a function" — `Object.prototype.toString.call(m)` worked while
    // `m.toString()` did not.
    // The allowlist is the kinds whose own dispatch table below would otherwise
    // claim the name. Every OTHER receiver reaches an `Object.prototype` method
    // the same way — a function, a class and a bound function included, where
    // `f.hasOwnProperty(k)` reported "is not a function" even though the READ
    // resolved it. `inherited_method_owner` decides which prototype owns the
    // name, so a kind that defines its own still gets its own.
    if is_object_builtin_method(name)
        && (inherited_method_owner(recv, name) == Some("Object")
            || (inherits_object_methods(recv) && !overrides_object_method(recv, name)))
    {
        // `toString` goes through the branded form (20.1.3.6), which reads
        // `Symbol.toStringTag` and falls back to the receiver's own brand —
        // `[object Map]`, not the generic stringification.
        if name == "toString" {
            return proto_method(recv, "Object:toString", args);
        }
        return object_builtin_method(recv, name, args);
    }
    // `Object.prototype.valueOf` is inherited by every exotic that does not
    // override it (an Array does not), and returns the receiver. Without this
    // the `ToPrimitive` probe on `[o] + ''` reached `array_method("valueOf")`
    // and threw `valueOf is not a function`.
    if name == "valueOf"
        && matches!(
            with_host(|h| h.kind_of(recv)),
            Some(
                ObjKind::Array
                    | ObjKind::Map
                    | ObjKind::Set
                    | ObjKind::Generator
                    | ObjKind::Promise
                    | ObjKind::Iter
                    | ObjKind::RegExp
            )
        )
    {
        return Ok(recv.clone());
    }
    // Only the tag is needed to pick the branch — cloning the receiver here made
    // every `arr.push(x)` copy the whole array, so a fill loop was O(n^2).
    match with_host(|h| h.kind_of(recv)) {
        Some(ObjKind::Array) => array_method(recv, name, args),
        Some(ObjKind::Str) => {
            // `string_method` consumes the text itself, so this clone is the
            // payload, not a tag probe.
            let s = peek(recv, |o| match o {
                JsObj::Str(s) => Some(s.clone()),
                _ => None,
            })
            .unwrap_or_default();
            string_method(&s, name, args)
        }
        Some(ObjKind::Map) => map_method(recv, name, args),
        Some(ObjKind::Set) => set_method(recv, name, args),
        Some(ObjKind::Generator) if crate::stdlib::iterator::is_helper(name) => {
            crate::stdlib::iterator::call(recv, name, &args)
        }
        Some(ObjKind::Generator) => generator_method(recv, name, args),
        Some(ObjKind::Promise) => promise_method(recv, name, args),
        Some(ObjKind::Iter) if crate::stdlib::iterator::is_helper(name) => {
            crate::stdlib::iterator::call(recv, name, &args)
        }
        Some(ObjKind::Iter) => iter_method(recv, name, args),
        Some(ObjKind::Symbol) => symbol_method(recv, name, args),
        Some(ObjKind::BigInt) => {
            let b = peek(recv, |o| match o {
                JsObj::BigInt(b) => Some(b.clone()),
                _ => None,
            })
            .unwrap_or_default();
            bigint_method(&b, name, args)
        }
        Some(ObjKind::RegExp) => crate::regexp::regexp_method(recv, name, args),
        Some(ObjKind::Func) | Some(ObjKind::Class) | Some(ObjKind::BoundFunc) => {
            match function_builtin_method(recv, name, &args)? {
                Some(v) => Ok(v),
                None => Err(host::type_error(&format!("{name} is not a function"))),
            }
        }
        Some(ObjKind::Object) => {
            if let Some(f) = peek(recv, |o| match o {
                JsObj::Object(p) => p.get(name).cloned(),
                _ => None,
            }) {
                host::invoke(&f, args, Some(recv.clone()))
            } else if name == "hasOwnProperty" {
                let k = with_host(|h| h.str_of(&arg0(&args)));
                let has = peek(recv, |o| match o {
                    JsObj::Object(p) => Some(p.contains_key(&k)),
                    _ => None,
                })
                .unwrap_or(false);
                Ok(Value::Bool(has))
            } else if name == "toString" {
                Ok(with_host(|h| h.new_str("[object Object]")))
            } else {
                Err(host::type_error(&format!("{} is not a function", name)))
            }
        }
        _ => {
            // Primitive number/bool/string coercions.
            if let Value::Float(_) | Value::Int(_) = recv {
                return number_method(with_host(|h| h.to_number(recv)), name, args);
            }
            if let Some(s) = with_host(|h| h.as_str(recv)) {
                return string_method(&s, name, args);
            }
            // `Boolean.prototype` (20.3.3): a boolean is not a heap object here,
            // so it reached no branch at all and `true.toString()` threw `is not
            // a function`. Its three methods are `toString`, `valueOf`, and the
            // inherited `Object.prototype.toLocaleString` — which
            // `[1,'a',true].toLocaleString()` invokes per element, so the hole
            // was reachable from the array form too.
            if let Value::Bool(b) = recv {
                return match name {
                    "toString" | "toLocaleString" => {
                        Ok(new_s(if *b { "true" } else { "false" }.to_string()))
                    }
                    "valueOf" => Ok(Value::Bool(*b)),
                    _ => Err(host::type_error(&format!("{name} is not a function"))),
                };
            }
            Err(host::type_error(&format!("{} is not a function", name)))
        }
    }
}

/// A copy of the whole backing store, for the methods that genuinely consume
/// every element (`map`, `filter`, `join`, …). Never call it just to read
/// `.len()` — use [`array_len`], or `push`/`unshift` become O(n) per call.
/// A LIVE iterator over a `Map` or `Set`.
///
/// Node's collection iterators see the collection as it is at each step: an
/// entry added during iteration IS visited, and one deleted before it is
/// reached is NOT. Ours materialized every entry up front, so both were wrong —
/// a loop that deletes as it goes still processed the entries it had removed.
///
/// The cursor is the last key yielded plus the index it was at. On each step
/// the key is located again in the CURRENT order: if it is still there the next
/// entry follows it, and if it was itself deleted the stored index now names
/// the entry that shifted into its place. That reproduces node for the cases
/// its own tests turn on — add-during, delete-ahead, delete-self,
/// delete-behind, delete-the-rest and clear — without giving `Map` the
/// tombstoned entry list node uses internally.
fn collection_iterator(coll: &Value, kind: &str) -> Value {
    with_host(|h| {
        let mut m = IndexMap::new();
        m.insert(
            "@@native".into(),
            h.new_str("CollectionIterator".to_string()),
        );
        m.insert("@@coll".into(), coll.clone());
        m.insert("@@kind".into(), h.new_str(kind.to_string()));
        m.insert("@@started".into(), Value::Bool(false));
        m.insert("@@lastIdx".into(), Value::Float(0.0));
        h.new_object(m)
    })
}

/// One step of a live collection iterator.
pub(crate) fn collection_iterator_next(recv: &Value) -> Result<Value, String> {
    let slot = |k: &str| {
        with_host(|h| match h.get(recv) {
            Some(JsObj::Object(p)) => p.get(k).cloned(),
            _ => None,
        })
    };
    let coll = slot("@@coll").unwrap_or(Value::Undef);
    let kind = slot("@@kind")
        .map(|v| with_host(|h| h.str_of(&v)))
        .unwrap_or_default();
    let started = slot("@@started").is_some_and(|v| with_host(|h| h.truthy(&v)));
    let last_idx = slot("@@lastIdx")
        .map(|v| with_host(|h| h.to_number(&v)) as usize)
        .unwrap_or(0);
    let last_key = slot("@@lastKey");

    let next_idx = if !started {
        0
    } else {
        match last_key
            .as_ref()
            .and_then(|k| with_host(|h| collection_index_of(h, &coll, k)))
        {
            // Still present: continue after it.
            Some(i) => i + 1,
            // Deleted since: whatever shifted into its slot is next.
            None => last_idx,
        }
    };
    let entry = with_host(|h| collection_entry_at(h, &coll, next_idx));
    let Some((k, v)) = entry else {
        return Ok(iter_result(Value::Undef, true));
    };
    with_host(|h| {
        if let Some(JsObj::Object(p)) = h.get_mut(recv) {
            p.insert("@@started".into(), Value::Bool(true));
            p.insert("@@lastIdx".into(), Value::Float(next_idx as f64));
            p.insert("@@lastKey".into(), k.clone());
        }
    });
    let out = match kind.as_str() {
        "keys" => k,
        "values" => v,
        _ => with_host(|h| h.new_array(vec![k, v])),
    };
    Ok(iter_result(out, false))
}

/// The (key, value) at `idx` in a Map, or (value, value) in a Set.
fn collection_entry_at(h: &host::JsHost, coll: &Value, idx: usize) -> Option<(Value, Value)> {
    match h.get(coll) {
        Some(JsObj::Map { entries, .. }) => entries.get_index(idx).map(|(_, kv)| kv.clone()),
        Some(JsObj::Set { entries, .. }) => {
            entries.get_index(idx).map(|(_, v)| (v.clone(), v.clone()))
        }
        _ => None,
    }
}

/// Where `key` currently sits in the collection's order.
fn collection_index_of(h: &host::JsHost, coll: &Value, key: &Value) -> Option<usize> {
    let mk = host::map_key(h, key);
    match h.get(coll) {
        Some(JsObj::Map { entries, .. }) => entries.get_index_of(&mk),
        Some(JsObj::Set { entries, .. }) => entries.get_index_of(&mk),
        _ => None,
    }
}

/// The `thisArg` an iteration method was given, if any.
///
/// `[1].forEach(fn, thisArg)` binds `thisArg` as the callback's `this`, and so
/// do `map`/`filter`/`some`/`every`/`find`/`findIndex`/`findLast`/
/// `findLastIndex`/`flatMap`, `Map`/`Set`/TypedArray `forEach`, and
/// `Array.from`'s map function. Every one of them was invoking the callback
/// with no receiver, so `this` inside it was undefined and the argument did
/// nothing.
fn this_arg(args: &[Value], idx: usize) -> Option<Value> {
    args.get(idx)
        .filter(|v| !matches!(v, Value::Undef))
        .cloned()
}

/// The elements of an array, with any INDEX ACCESSOR resolved.
///
/// `Object.defineProperty(arr, 1, { get })` stores the getter in the accessor
/// table, and an array's elements live in a backing vector — so every method
/// reading that vector directly (`join`, `map`, `indexOf`, …) saw the stale
/// slot and never called the getter, while a plain `arr[1]` read did.
///
/// An array with no accessors pays one lookup returning an empty list, so the
/// ordinary case is unchanged. The getters are invoked OUTSIDE the host borrow,
/// since calling one re-enters.
/// Walk `recv` the way an `Array.prototype` iteration method does: the LENGTH
/// is captured once at entry (LengthOfArrayLike, step 3), but each element is
/// read LIVE at its index, and an index that no longer exists is skipped.
///
/// Snapshotting the whole array instead meant a callback that mutated it was
/// not observed: `[1,2,3].forEach(v => a.shift())` visited 1, 2, 3 where node
/// visits 1 and 3, and `filter` kept elements the callback had already removed.
///
/// `f` returns `Some(x)` to stop early with `x`.
fn array_walk<T>(
    recv: &Value,
    mut f: impl FnMut(usize, Value) -> Result<Option<T>, String>,
) -> Result<Option<T>, String> {
    let len = array_len(recv);
    for i in 0..len {
        // A HOLE — and an index a shrinking mutation has dropped — is skipped
        // without calling the callback.
        if index_absent(recv, i) || i >= array_len(recv) {
            continue;
        }
        let v = get_property(recv, &i.to_string())?;
        if let Some(out) = f(i, v)? {
            return Ok(Some(out));
        }
    }
    Ok(None)
}

/// `array_walk`'s descending twin, for `reduceRight`/`findLast*`: the same
/// capture-length-once, read-each-element-live rule walked from the end. A
/// callback that SHRINKS the array is observed by every later step, so the
/// indices it drops are skipped rather than served from a stale copy.
fn array_walk_rev<T>(
    recv: &Value,
    from: usize,
    mut f: impl FnMut(usize, Value) -> Result<Option<T>, String>,
) -> Result<Option<T>, String> {
    for i in (0..from).rev() {
        if index_absent(recv, i) || i >= array_len(recv) {
            continue;
        }
        let v = get_property(recv, &i.to_string())?;
        if let Some(out) = f(i, v)? {
            return Ok(Some(out));
        }
    }
    Ok(None)
}

/// The live read behind `indexOf`/`includes`/`join`: the element at `i`, or
/// `undefined` once a mutation has shrunk the array past it.
fn array_elem_live(recv: &Value, i: usize) -> Result<Value, String> {
    if i >= array_len(recv) {
        return Ok(Value::Undef);
    }
    get_property(recv, &i.to_string())
}

fn array_items(recv: &Value) -> Vec<Value> {
    let mut items = with_host(|h| match h.get(recv) {
        Some(JsObj::Array(items)) => items.clone(),
        _ => Vec::new(),
    });
    resolve_index_accessors(recv, &mut items);
    items
}

/// Replace each slot that has an own accessor with what its getter returns.
pub(crate) fn resolve_index_accessors_pub(recv: &Value, items: &mut [Value]) {
    resolve_index_accessors(recv, items);
}

/// Returns whether any slot was replaced, which the JSON walk needs: it keeps
/// the ORIGINAL array when nothing changed, and the original still holds the
/// stale slots.
fn resolve_index_accessors(recv: &Value, items: &mut [Value]) -> bool {
    let mut indices: Vec<usize> = with_host(|h| h.own_accessor_keys(recv))
        .into_iter()
        .filter_map(|k| k.parse::<usize>().ok())
        .filter(|i| *i < items.len())
        .collect();
    // An ELIDED index the prototype chain supplies is stale in the backing
    // vector too — it holds `undefined` where `[[Get]]` answers the inherited
    // value. Spread and `JSON.stringify` both read through here, and both
    // rendered the hole rather than what `a[i]` reads.
    let inherited: Vec<usize> = with_host(|h| h.hole_indices(recv))
        .into_iter()
        .filter(|i| *i < items.len() && !indices.contains(i))
        .filter(|i| has_property(recv, &i.to_string()).unwrap_or(false))
        .collect();
    indices.extend(inherited);
    let mut replaced = false;
    for i in indices {
        if let Ok(v) = get_property(recv, &i.to_string()) {
            items[i] = v;
            replaced = true;
        }
    }
    replaced
}

/// The ELIDED positions of array `recv` as a membership set. A dense array —
/// which is nearly every array — answers with an empty set after a single
/// negative hash probe and allocates nothing.
///
/// The iteration methods split into two groups, and the split is not a matter of
/// taste: the ones spec'd through `HasProperty` (`forEach`, `map`, `filter`,
/// `some`, `every`, `reduce`, `indexOf`, `flat`, `sort`) SKIP a hole, while the
/// ones spec'd through a bare `Get` (`for…of`, spread, `find`, `includes`,
/// `join`, `entries`, `Array.from`) see the `undefined` a hole reads back as.
fn hole_set(recv: &Value) -> rustc_hash::FxHashSet<usize> {
    with_host(|h| h.hole_indices(recv)).into_iter().collect()
}

/// The indices `recv` genuinely has NO property at — the elided ones the
/// prototype chain does not supply either.
///
/// Every array method tests `HasProperty` before deciding to skip a position
/// (23.1.3.x, uniformly), and `HasProperty` walks the chain. Testing elision
/// alone made an inherited element invisible to all of them: with
/// `Array.prototype[1] = 'p'`, `[1,,3].map(v => v)` produced a hole where node
/// produces `'p'`, and `flat`/`concat`/`slice`/`sort`/`indexOf` each dropped
/// the same position.
///
/// `hole_set` remains the elision record itself, which is what `splice` moves
/// around — that bookkeeping is about the array's OWN storage and must not
/// consult the chain.
fn absent_set(recv: &Value) -> rustc_hash::FxHashSet<usize> {
    hole_set(recv)
        .into_iter()
        .filter(|i| !has_property(recv, &i.to_string()).unwrap_or(false))
        .collect()
}

/// The single-index form of [`absent_set`], for the walkers that test one
/// position at a time.
fn index_absent(recv: &Value, i: usize) -> bool {
    with_host(|h| h.is_hole(recv, i)) && !has_property(recv, &i.to_string()).unwrap_or(false)
}

/// The element count, without copying the elements.
fn array_len(recv: &Value) -> usize {
    peek(recv, |o| match o {
        JsObj::Array(items) => Some(items.len()),
        _ => None,
    })
    .unwrap_or(0)
}

/// `ArraySpeciesCreate(originalArray, length)` (23.1.3.4) — the constructor an
/// array method builds its RESULT with.
///
/// `map`, `filter`, `slice`, `concat`, `splice`, `flat` and `flatMap` all
/// produce an array of the receiver's own species, so on a `class A extends
/// Array` the result is an `A`. Every one of them allocated a plain array
/// instead, so `A.from([1]).map(x => x) instanceof A` was false.
///
/// The default `get [Symbol.species]() { return this }` is what makes the
/// subclass the species; a class overriding it with `Array` gets a plain array
/// back, which is the documented way to opt out.
/// Build an array-shaped result through `ctor`, or a plain array when there is
/// none to build through.
///
/// The constructor is called with the LENGTH and the elements written after, as
/// 23.1.2.1 and 23.1.3.4 both specify — which is what lets a subclass
/// constructor observe the allocation.
fn construct_array_like(ctor: Option<Value>, items: Vec<Value>) -> Result<Value, String> {
    let Some(ctor) = ctor.filter(|c| {
        matches!(
            with_host(|h| h.kind_of(c)),
            Some(ObjKind::Class) | Some(ObjKind::Func)
        )
    }) else {
        return Ok(with_host(|h| h.new_array(items)));
    };
    let out = host::construct(&ctor, vec![Value::Float(items.len() as f64)])?;
    write_elements(&out, items);
    Ok(out)
}

/// Write `items` into a freshly constructed array-shaped `out`, clearing the
/// hole marks the length-only construction left behind.
///
/// `new A(3)` on `class A extends Array` really does produce three HOLES, and
/// the elements written over them stayed marked — so every subclass result of
/// `map`/`filter`/`flat` read back as holes: `A.from([1,2,3]).map(x => x * 2)`
/// had length 3 and printed `[null,null,null]`, and `0 in` it was false.
fn write_elements(out: &Value, items: Vec<Value>) {
    with_host(|h| {
        h.clear_holes(out);
        if let Some(JsObj::Array(dst)) = h.get_mut(out) {
            *dst = items;
        }
    });
}

fn array_species_create(recv: &Value, items: Vec<Value>) -> Result<Value, String> {
    let plain = || with_host(|h| h.new_array(items.clone()));
    // Only a subclass instance can have a species of its own: a plain array's
    // `constructor` is the `Array` builtin, whose species is `Array`.
    // A chain lookup, not `get_property`: an Array receiver resolves its
    // properties through the stdlib funnel, which has no `constructor` entry,
    // so the read alone reports `undefined` for every subclass instance. A
    // Proxy is the exception — it has no property map to walk, and its
    // `constructor` comes from the `get` trap, so a proxied subclass array
    // produced plain arrays.
    let ctor = if with_host(|h| h.kind_of(recv)) == Some(ObjKind::Proxy) {
        get_property(recv, "constructor").unwrap_or(Value::Undef)
    } else {
        with_host(|h| host::lookup_chain(h, recv, "constructor")).unwrap_or(Value::Undef)
    };
    if !matches!(
        with_host(|h| h.kind_of(&ctor)),
        Some(ObjKind::Class) | Some(ObjKind::Func)
    ) {
        return Ok(plain());
    }
    // An explicit `@@species` wins; absent one, the constructor itself is the
    // species, as the inherited accessor returns `this`.
    let species = match get_property(&ctor, "@@species") {
        Ok(Value::Undef) => ctor,
        Ok(s) if with_host(|h| h.is_null(&s)) => return Ok(plain()),
        Ok(s) => s,
        Err(_) => ctor,
    };
    if !matches!(
        with_host(|h| h.kind_of(&species)),
        Some(ObjKind::Class) | Some(ObjKind::Func)
    ) {
        return Ok(plain());
    }
    let out = host::construct(&species, vec![Value::Float(items.len() as f64)])?;
    // The constructor is called with the LENGTH, so the elements are written
    // afterwards — which is also what lets a subclass constructor observe the
    // allocation, as node's does.
    write_elements(&out, items);
    Ok(out)
}

fn array_method(recv: &Value, name: &str, args: Vec<Value>) -> Result<Value, String> {
    array_method_on(recv, recv, name, args)
}

/// The `Array.prototype` methods that WRITE to their receiver, and so need the
/// generic path to copy the result back onto the array-like.
const ARRAY_MUTATORS: &[&str] = &[
    "push",
    "pop",
    "shift",
    "unshift",
    "splice",
    "sort",
    "reverse",
    "fill",
    "copyWithin",
];

/// Run `Array.prototype.<method>` against an array-LIKE (`{0: 'a', length: 1}`,
/// a DOM-ish collection, `arguments`).
///
/// 23.1.3 defines every one of these over `LengthOfArrayLike(O)` and `Get(O, k)`
/// rather than over an Array's element vector, so the receiver only has to have
/// a `length`. The elements are read out into a temporary Array, the ordinary
/// implementation runs on that, and a MUTATING method writes the result back —
/// which keeps one implementation of each method rather than a second, generic
/// one that could drift from it.
///
/// An index the receiver does not own is a HOLE in the temporary, so the
/// methods that skip holes skip it here too, exactly as `HasProperty` makes them.
fn array_generic(recv: &Value, method: &str, args: Vec<Value>) -> Result<Value, String> {
    let len = match get_property(recv, "length") {
        Ok(v) => host::to_array_length(&v).unwrap_or(0),
        Err(_) => 0,
    };
    // A STRING receiver owns every index of its length; `has_property` answers
    // for objects and reports none of them, which made `[].map.call('abc', f)`
    // an array of three holes.
    let dense = with_host(|h| h.as_str(recv)).is_some();
    let mut items = Vec::with_capacity(len);
    let mut holes: rustc_hash::FxHashSet<usize> = rustc_hash::FxHashSet::default();
    for i in 0..len {
        let k = i.to_string();
        if dense || has_property(recv, &k)? {
            items.push(get_property(recv, &k)?);
        } else {
            holes.insert(i);
            items.push(Value::Undef);
        }
    }
    let tmp = with_host(|h| {
        let a = h.new_array(items);
        h.install_holes(&a, holes);
        a
    });
    let out = array_method_on(&tmp, recv, method, args)?;
    if ARRAY_MUTATORS.contains(&method) {
        let result = with_host(|h| match h.get(&tmp) {
            Some(JsObj::Array(items)) => items.clone(),
            _ => Vec::new(),
        });
        for (i, v) in result.iter().enumerate() {
            set_property(recv, &i.to_string(), v.clone())?;
        }
        set_property(recv, "length", Value::Float(result.len() as f64))?;
    }
    Ok(out)
}

/// `Array.prototype.<name>` on `recv`.
///
/// `this_value` is what a callback receives as its third argument and what a
/// mutating method returns — the same object as `recv` for an ordinary array
/// call, but the ORIGINAL array-like when `array_generic` runs a method against
/// a temporary copy (`Array.prototype.slice.call(arguments)`).
fn array_method_on(
    recv: &Value,
    this_value: &Value,
    name: &str,
    args: Vec<Value>,
) -> Result<Value, String> {
    let args = coerce_numeric_args(ARRAY_METHOD_NUMERIC_ARGS, name, args)?;
    match name {
        "push" => {
            // 23.1.3.23 step 4 defines each new element through
            // `CreateDataPropertyOrThrow`, so a NON-EXTENSIBLE array refuses it:
            // `Object.seal(a)` / `preventExtensions(a)` then `a.push(x)` is a
            // TypeError. The elements were appended to the backing vector
            // regardless, so sealing an array did not seal it.
            if !args.is_empty() && !with_host(|h| h.is_extensible(recv)) {
                let at = array_len(recv);
                return Err(host::type_error(&format!(
                    "Cannot add property {at}, object is not extensible"
                )));
            }
            // Step 5 then SETS `length`, so a non-writable one refuses the push
            // too — `defineProperty(a, 'length', {writable: false})` makes an
            // array append-proof without sealing it.
            if !args.is_empty() && !with_host(|h| h.prop_attrs(recv, "length").writable) {
                return Err(host::type_error(
                    "Cannot assign to read only property 'length' of object '[object Array]'",
                ));
            }
            // `push` returns the new length; take it from the same mutable
            // borrow rather than copying the array back out to count it.
            let len = with_host(|h| {
                if let Some(JsObj::Array(items)) = h.get_mut(recv) {
                    items.extend(args.iter().cloned());
                    items.len()
                } else {
                    0
                }
            });
            Ok(Value::Float(len as f64))
        }
        "pop" => Ok(with_host(|h| {
            let popped = if let Some(JsObj::Array(items)) = h.get_mut(recv) {
                items.pop().unwrap_or(Value::Undef)
            } else {
                Value::Undef
            };
            let len = match h.get(recv) {
                Some(JsObj::Array(items)) => items.len(),
                _ => 0,
            };
            h.truncate_holes(recv, len);
            popped
        })),
        "shift" => Ok(with_host(|h| {
            let shifted = if let Some(JsObj::Array(items)) = h.get_mut(recv) {
                if items.is_empty() {
                    Value::Undef
                } else {
                    items.remove(0)
                }
            } else {
                Value::Undef
            };
            h.remap_holes(recv, |i| i.checked_sub(1));
            shifted
        })),
        "unshift" => {
            with_host(|h| {
                if let Some(JsObj::Array(items)) = h.get_mut(recv) {
                    for (i, a) in args.iter().enumerate() {
                        items.insert(i, a.clone());
                    }
                }
                let n = args.len();
                h.remap_holes(recv, |i| Some(i + n));
            });
            Ok(Value::Float(array_len(recv) as f64))
        }
        "join" => {
            let sep = if args.is_empty() || matches!(args[0], Value::Undef) {
                ",".to_string()
            } else {
                arg_to_string(&args, 0)?
            };
            join_array(recv, &sep)
        }
        // `Array.prototype.toLocaleString` (23.1.3.32): comma-join the elements'
        // OWN `toLocaleString` results, with `null`/`undefined` contributing the
        // empty string. It threw `is not a function` — the whole method was
        // missing — so `[1234.5, 'x'].toLocaleString()` was unreachable.
        "toLocaleString" => {
            // Shares `join`'s JoinStack: measured on node v26.7.0, `h=[1]`
            // `h.push(h)` makes `h.toLocaleString()` `"1,"`, not a stack overflow.
            if !host::join_stack_push(recv) {
                return Ok(with_host(|h| h.new_str(String::new())));
            }
            let items = array_items(recv);
            let mut parts: Vec<String> = Vec::with_capacity(items.len());
            for it in &items {
                if with_host(|h| h.is_nullish(it)) {
                    parts.push(String::new());
                    continue;
                }
                let v = match host::call_method(it, "toLocaleString", Vec::new()) {
                    Ok(v) => v,
                    Err(e) => {
                        host::join_stack_pop();
                        return Err(e);
                    }
                };
                parts.push(with_host(|h| h.str_of(&v)));
            }
            host::join_stack_pop();
            Ok(with_host(|h| h.new_str(parts.join(","))))
        }
        // `indexOf`/`lastIndexOf` are spec'd through `HasProperty`, so a hole is
        // never a match: `[1,,3].indexOf(undefined)` is `-1`, while the
        // `Get`-based `includes` reports `true` for the same array.
        "indexOf" => {
            let target = arg0(&args);
            let len = array_len(recv);
            let start = search_start(arg_num(&args, 1), len);
            let mut idx = None;
            for i in start..len {
                // 23.1.3.17 steps 8a-8b: HasProperty first, so a hole — and an
                // index a mutation has since dropped — is skipped, not compared.
                if index_absent(recv, i) || i >= array_len(recv) {
                    continue;
                }
                let x = get_property(recv, &i.to_string())?;
                if with_host(|h| h.strict_eq(&x, &target)) {
                    idx = Some(i);
                    break;
                }
            }
            Ok(Value::Float(idx.map(|i| i as f64).unwrap_or(-1.0)))
        }
        "lastIndexOf" => {
            let items = array_items(recv);
            let holes = absent_set(recv);
            let target = arg0(&args);
            let from = (args.len() > 1).then(|| arg_num(&args, 1));
            let idx = match search_start_last(from, items.len()) {
                None => None,
                Some(start) => with_host(|h| {
                    items[..=start]
                        .iter()
                        .enumerate()
                        .rev()
                        .find(|(i, x)| !holes.contains(i) && h.strict_eq(x, &target))
                        .map(|(i, _)| i)
                }),
            };
            Ok(Value::Float(idx.map(|i| i as f64).unwrap_or(-1.0)))
        }
        "includes" => {
            // Array.includes uses SameValueZero: unlike `===`, NaN matches NaN.
            // Unlike `indexOf` it has no HasProperty step (23.1.3.16 step 5b), so
            // a hole reads as `undefined` and `[,].includes(undefined)` is true.
            let target = arg0(&args);
            let tnan = matches!(target, Value::Float(f) if f.is_nan());
            let len = array_len(recv);
            let start = search_start(arg_num(&args, 1), len);
            let mut found = false;
            for i in start..len {
                let x = array_elem_live(recv, i)?;
                if (tnan && matches!(x, Value::Float(f) if f.is_nan()))
                    || with_host(|h| h.strict_eq(&x, &target))
                {
                    found = true;
                    break;
                }
            }
            Ok(Value::Bool(found))
        }
        "slice" => {
            let items = array_items(recv);
            let (lo, hi) = slice_bounds(&args, items.len());
            let out = array_species_create(this_value, items[lo..hi].to_vec())?;
            with_host(|h| h.copy_holes(recv, &out, |i| (i >= lo && i < hi).then(|| i - lo)));
            Ok(out)
        }
        "concat" => {
            // `Symbol.isConcatSpreadable` (23.1.3.1) decides whether a value
            // is spread, overriding `IsArray` in BOTH directions: a plain
            // array-like opts IN, and an array opts OUT.
            let spreadable = |a: &Value| -> bool {
                let flag = get_property(a, "@@isConcatSpreadable").unwrap_or(Value::Undef);
                if matches!(flag, Value::Undef) {
                    matches!(with_host(|h| h.get(a).cloned()), Some(JsObj::Array(_)))
                        && !is_arguments(a)
                } else {
                    with_host(|h| h.truthy(&flag))
                }
            };
            // Step 5 iterates `« O » ++ items`, so the receiver takes the same
            // test: a non-spreadable `this` (`concat.call("ab", 1)`) is ONE
            // element, its `ToObject` box, not the characters `array_generic`
            // read out of it. A hole in a spread receiver or argument stays a
            // hole in the result, at its shifted position.
            let (mut out, mut holes) = if spreadable(this_value) {
                (array_items(recv), absent_set(recv))
            } else {
                (vec![to_object(this_value)], Default::default())
            };
            let mut sources: Vec<(Value, usize)> = Vec::new();
            for a in &args {
                if !spreadable(a) {
                    out.push(a.clone());
                    continue;
                }
                match with_host(|h| h.get(a).cloned()) {
                    // Read off the backing vector rather than through
                    // `array_items`, so the resolve that does for the receiver
                    // has to be done here too: 23.1.3.1 step 5.c.iv is a
                    // `[[Get]]`, and an index with a getter — or an elided one
                    // the chain supplies — is stale in that vector.
                    Some(JsObj::Array(mut items)) => {
                        resolve_index_accessors(a, &mut items);
                        sources.push((a.clone(), out.len()));
                        out.extend(items);
                    }
                    // An opted-in array-LIKE spreads by its `length` and index
                    // properties rather than by a backing vector it has none of.
                    _ => {
                        let len = get_property(a, "length").unwrap_or(Value::Undef);
                        let n = with_host(|h| h.to_number(&len));
                        let n = if n.is_finite() {
                            n.max(0.0) as usize
                        } else {
                            0
                        };
                        for i in 0..n {
                            out.push(get_property(a, &i.to_string()).unwrap_or(Value::Undef));
                        }
                    }
                }
            }

            for (src, base) in sources {
                holes.extend(
                    with_host(|h| h.hole_indices(&src))
                        .into_iter()
                        .map(|i| i + base),
                );
            }
            let arr = array_species_create(this_value, out)?;
            with_host(|h| h.install_holes(&arr, holes));
            Ok(arr)
        }
        "reverse" => {
            let len = array_len(recv);
            with_host(|h| {
                if let Some(JsObj::Array(items)) = h.get_mut(recv) {
                    items.reverse();
                }
                h.remap_holes(recv, |i| Some(len - 1 - i));
            });
            Ok(this_value.clone())
        }
        "fill" => {
            // fill(value[, start[, end]]) — negative indices count from the end.
            let val = arg0(&args);
            let len = array_len(recv) as i64;
            let norm =
                |v: i64| -> usize { (if v < 0 { (len + v).max(0) } else { v.min(len) }) as usize };
            let start = if args.len() >= 2 {
                norm(arg_num(&args, 1) as i64)
            } else {
                0
            };
            let end = if args.len() >= 3 {
                norm(arg_num(&args, 2) as i64)
            } else {
                len as usize
            };
            with_host(|h| {
                if let Some(JsObj::Array(items)) = h.get_mut(recv) {
                    for it in items.iter_mut().take(end).skip(start) {
                        *it = val.clone();
                    }
                }
                // Every filled position now holds a real value.
                h.remap_holes(recv, |i| (i < start || i >= end).then_some(i));
            });
            Ok(this_value.clone())
        }
        "copyWithin" => {
            // copyWithin(target, start[, end]) — copy a slice within the array.
            let items = array_items(recv);
            let len = items.len() as i64;
            let norm =
                |v: i64| -> usize { (if v < 0 { (len + v).max(0) } else { v.min(len) }) as usize };
            let target = norm(arg_num(&args, 0) as i64);
            let start = if args.len() >= 2 {
                norm(arg_num(&args, 1) as i64)
            } else {
                0
            };
            let end = if args.len() >= 3 {
                norm(arg_num(&args, 2) as i64)
            } else {
                len as usize
            };
            let slice: Vec<Value> = items[start..end.max(start)].to_vec();
            let copied = slice.len();
            // A copied position takes its SOURCE's hole-ness (10.4.2 copyWithin
            // deletes the target when the source has no such property);
            // everything outside the written range keeps its own.
            let src_holes = absent_set(recv);
            with_host(|h| {
                if let Some(JsObj::Array(a)) = h.get_mut(recv) {
                    for (k, v) in slice.into_iter().enumerate() {
                        if target + k < a.len() {
                            a[target + k] = v;
                        }
                    }
                }
                let len = len as usize;
                let mut holes: rustc_hash::FxHashSet<usize> = src_holes
                    .iter()
                    .copied()
                    .filter(|i| *i < target || *i >= (target + copied).min(len))
                    .collect();
                for k in 0..copied {
                    if target + k < len && src_holes.contains(&(start + k)) {
                        holes.insert(target + k);
                    }
                }
                h.install_holes(recv, holes);
            });
            Ok(this_value.clone())
        }
        "at" => {
            let items = array_items(recv);
            let mut i = arg_num(&args, 0) as i64;
            if i < 0 {
                i += items.len() as i64;
            }
            Ok(if i >= 0 && (i as usize) < items.len() {
                items[i as usize].clone()
            } else {
                Value::Undef
            })
        }
        // 23.1.3.21: the callback runs only where `HasProperty` holds, and the
        // result array is created with the SAME holes — `[1,,3].map(f)` calls `f`
        // twice and yields `[2, <1 empty item>, 6]`.
        "map" => {
            let holes = absent_set(recv);
            let cb = arg0(&args);
            // The result keeps the source's LENGTH, so a skipped index still
            // occupies a slot; `array_walk` only tells us which ones ran.
            let mut out = vec![Value::Undef; array_len(recv)];
            array_walk(recv, |i, it| {
                let v = host::invoke(
                    &cb,
                    vec![it, Value::Float(i as f64), this_value.clone()],
                    this_arg(&args, 1),
                )?;
                if i < out.len() {
                    out[i] = v;
                }
                Ok(None::<()>)
            })?;
            let arr = array_species_create(this_value, out)?;
            with_host(|h| h.install_holes(&arr, holes));
            Ok(arr)
        }
        "flatMap" => {
            let cb = arg0(&args);
            let thisarg = this_arg(&args, 1);
            let mut out = Vec::new();
            array_walk(recv, |i, v| {
                let r = host::invoke(
                    &cb,
                    vec![v, Value::Float(i as f64), this_value.clone()],
                    thisarg.clone(),
                )?;
                match with_host(|h| h.get(&r).cloned()) {
                    Some(JsObj::Array(inner)) => out.extend(inner),
                    _ => out.push(r),
                }
                Ok(None::<()>)
            })?;
            array_species_create(this_value, out)
        }
        "filter" => {
            let cb = arg0(&args);
            let mut out = Vec::new();
            array_walk(recv, |i, it| {
                let keep = host::invoke(
                    &cb,
                    vec![it.clone(), Value::Float(i as f64), this_value.clone()],
                    this_arg(&args, 1),
                )?;
                if with_host(|h| h.truthy(&keep)) {
                    out.push(it);
                }
                Ok(None::<()>)
            })?;
            array_species_create(this_value, out)
        }
        "forEach" => {
            let cb = arg0(&args);
            array_walk(recv, |i, it| {
                host::invoke(
                    &cb,
                    vec![it, Value::Float(i as f64), this_value.clone()],
                    this_arg(&args, 1),
                )?;
                Ok(None::<()>)
            })?;
            Ok(Value::Undef)
        }
        "find" => {
            let items = array_items(recv);
            let cb = arg0(&args);
            for (i, it) in items.iter().enumerate() {
                let m = host::invoke(
                    &cb,
                    vec![it.clone(), Value::Float(i as f64), this_value.clone()],
                    this_arg(&args, 1),
                )?;
                if with_host(|h| h.truthy(&m)) {
                    return Ok(it.clone());
                }
            }
            Ok(Value::Undef)
        }
        "findIndex" => {
            let items = array_items(recv);
            let cb = arg0(&args);
            for (i, it) in items.iter().enumerate() {
                let m = host::invoke(
                    &cb,
                    vec![it.clone(), Value::Float(i as f64), this_value.clone()],
                    this_arg(&args, 1),
                )?;
                if with_host(|h| h.truthy(&m)) {
                    return Ok(Value::Float(i as f64));
                }
            }
            Ok(Value::Float(-1.0))
        }
        "some" => {
            let cb = arg0(&args);
            let thisarg = this_arg(&args, 1);
            let hit = array_walk(recv, |i, v| {
                let m = host::invoke(
                    &cb,
                    vec![v, Value::Float(i as f64), this_value.clone()],
                    thisarg.clone(),
                )?;
                Ok(with_host(|h| h.truthy(&m)).then_some(()))
            })?;
            Ok(Value::Bool(hit.is_some()))
        }
        "every" => {
            let cb = arg0(&args);
            let failed = array_walk(recv, |i, it| {
                let m = host::invoke(
                    &cb,
                    vec![it, Value::Float(i as f64), this_value.clone()],
                    this_arg(&args, 1),
                )?;
                Ok((!with_host(|h| h.truthy(&m))).then_some(()))
            })?;
            Ok(Value::Bool(failed.is_none()))
        }
        "reduce" => {
            let items = array_items(recv);
            let holes = absent_set(recv);
            let cb = arg0(&args);
            let acc;
            let mut start = 0;
            if args.len() >= 2 {
                acc = args[1].clone();
            } else {
                // With no seed the accumulator is the first PRESENT element, so a
                // leading run of holes is skipped rather than seeding `undefined`.
                match (0..items.len()).find(|i| !holes.contains(i)) {
                    Some(i) => {
                        acc = items[i].clone();
                        start = i + 1;
                    }
                    None => {
                        return Err(host::type_error(
                            "Reduce of empty array with no initial value",
                        ))
                    }
                }
            }
            // Each element is read LIVE at its index, so a callback that
            // shrinks the array is observed — the tail is skipped rather than
            // folded from a stale snapshot.
            let mut cur = acc;
            array_walk(recv, |i, it| {
                if i < start {
                    return Ok(None::<()>);
                }
                cur = host::invoke(
                    &cb,
                    vec![
                        std::mem::replace(&mut cur, Value::Undef),
                        it,
                        Value::Float(i as f64),
                        this_value.clone(),
                    ],
                    this_arg(&args, 1),
                )?;
                Ok(None::<()>)
            })?;
            Ok(cur)
        }
        "reduceRight" => {
            let cb = arg0(&args);
            let n = array_len(recv);
            let mut acc;
            let mut from = n; // one past the next index to process (walking down)
            if args.len() >= 2 {
                acc = args[1].clone();
            } else {
                let holes = absent_set(recv);
                match (0..n).rev().find(|i| !holes.contains(i)) {
                    Some(k) => {
                        acc = get_property(recv, &k.to_string())?;
                        from = k;
                    }
                    None => {
                        return Err(host::type_error(
                            "Reduce of empty array with no initial value",
                        ))
                    }
                }
            }
            // `acc` moves into the closure and back out on every step, so it
            // lives in an Option the closure can take from and refill.
            let mut slot = Some(acc);
            array_walk_rev(recv, from, |i, v| {
                let prev = slot.take().expect("accumulator is refilled each step");
                slot = Some(host::invoke(
                    &cb,
                    vec![prev, v, Value::Float(i as f64), this_value.clone()],
                    None,
                )?);
                Ok(None::<()>)
            })?;
            acc = slot.expect("accumulator is refilled each step");
            Ok(acc)
        }
        "findLast" => {
            let items = array_items(recv);
            let cb = arg0(&args);
            for i in (0..items.len()).rev() {
                let m = host::invoke(
                    &cb,
                    vec![items[i].clone(), Value::Float(i as f64), this_value.clone()],
                    this_arg(&args, 1),
                )?;
                if with_host(|h| h.truthy(&m)) {
                    return Ok(items[i].clone());
                }
            }
            Ok(Value::Undef)
        }
        "findLastIndex" => {
            let items = array_items(recv);
            let cb = arg0(&args);
            for i in (0..items.len()).rev() {
                let m = host::invoke(
                    &cb,
                    vec![items[i].clone(), Value::Float(i as f64), this_value.clone()],
                    this_arg(&args, 1),
                )?;
                if with_host(|h| h.truthy(&m)) {
                    return Ok(Value::Float(i as f64));
                }
            }
            Ok(Value::Float(-1.0))
        }
        // 23.1.3.30: `SortIndexedProperties` collects only the PRESENT elements,
        // and the holes are re-created at the tail — `[3,,1].sort()` is
        // `[1, 3, <1 empty item>]` with own keys `['0','1']`.
        "sort" => {
            let all = array_items(recv);
            let holes = absent_set(recv);
            let mut items: Vec<Value> = all
                .iter()
                .enumerate()
                .filter(|(i, _)| !holes.contains(i))
                .map(|(_, v)| v.clone())
                .collect();
            sort_values(&mut items, args.first())?;
            let present = items.len();
            // 23.1.3.30 steps 4-5 write back only the indices BELOW the length
            // captured at step 1: `Set` for each sorted element, then `Delete`
            // for the holes that followed them. Replacing the whole backing
            // vector instead discarded anything the COMPARATOR appended —
            // `a.sort((x, y) => { a.push(0); return x - y })` came back at its
            // original length with every pushed element gone.
            with_host(|h| {
                let len = all.len();
                if let Some(JsObj::Array(a)) = h.get_mut(recv) {
                    if a.len() < len {
                        a.resize(len, Value::Undef);
                    }
                    for (i, v) in items.into_iter().enumerate() {
                        a[i] = v;
                    }
                    for slot in a[present..len].iter_mut() {
                        *slot = Value::Undef;
                    }
                }
                h.install_holes(recv, (present..len).collect());
            });
            Ok(this_value.clone())
        }
        // ES2023 change-by-copy: sort a fresh copy, leaving the receiver untouched.
        "toSorted" => {
            let mut items = array_items(recv);
            sort_values(&mut items, args.first())?;
            Ok(with_host(|h| h.new_array(items)))
        }
        "toReversed" => {
            let mut items = array_items(recv);
            items.reverse();
            Ok(with_host(|h| h.new_array(items)))
        }
        "toSpliced" => {
            let mut items = array_items(recv);
            let len = items.len();
            let start = {
                let s = arg_num(&args, 0);
                if s < 0.0 {
                    ((len as f64 + s).max(0.0)) as usize
                } else {
                    (s as usize).min(len)
                }
            };
            let delete = if args.len() >= 2 {
                (arg_num(&args, 1).max(0.0) as usize).min(len - start)
            } else if args.is_empty() {
                0
            } else {
                len - start
            };
            let inserts: Vec<Value> = args.iter().skip(2).cloned().collect();
            items.splice(start..start + delete, inserts);
            Ok(with_host(|h| h.new_array(items)))
        }
        "with" => {
            let mut items = array_items(recv);
            let len = items.len() as i64;
            let rel = arg_num(&args, 0) as i64;
            let idx = if rel < 0 { len + rel } else { rel };
            if idx < 0 || idx >= len {
                return Err(host::range_error(&format!("Invalid index : {rel}")));
            }
            items[idx as usize] = args.get(1).cloned().unwrap_or(Value::Undef);
            Ok(with_host(|h| h.new_array(items)))
        }
        "flat" => {
            // depth defaults to 1; `Infinity` flattens fully. ToIntegerOrInfinity:
            // NaN → 0, otherwise truncate toward zero (negatives act as 0).
            let raw = if args.is_empty() {
                1.0
            } else {
                arg_num(&args, 0)
            };
            let depth = if raw.is_nan() {
                0.0
            } else if raw.is_infinite() {
                raw
            } else {
                raw.trunc()
            };
            let mut out = Vec::new();
            flatten_into(recv, depth, &mut out)?;
            array_species_create(this_value, out)
        }
        // Live over the array (23.1.5.1): each step reads it as it is then.
        "keys" => Ok(array_iterator(recv, host::ArrayIterKind::Keys)),
        "values" | "@@iterator" => Ok(array_iterator(recv, host::ArrayIterKind::Values)),
        "entries" => Ok(array_iterator(recv, host::ArrayIterKind::Entries)),
        "splice" => array_splice(recv, args),
        // `Array.prototype.toString` IS `join()` with the default separator
        // (23.1.3.36), so it converts each element with `ToString` too — and
        // shares its cycle cut, which is the whole reason it must not call
        // `join_parts` directly: `ToString` of a nested array lands back here.
        "toString" => join_array(recv, ","),
        // An Array inherits from `Object.prototype` too, so the methods it does
        // not override resolve there. `[].hasOwnProperty` already read back as a
        // function through the property path, but CALLING it landed here and
        // threw `is not a function`.
        _ if is_object_builtin_method(name) => object_builtin_method(recv, name, args),
        _ => Err(host::type_error(&format!("{name} is not a function"))),
    }
}

/// `Array.prototype.join` (23.1.3.18) and, with the default separator,
/// `Array.prototype.toString` (23.1.3.36) — one body so both share the cycle
/// cut, which is not optional here: `ToString` of an element that is itself an
/// array re-enters through `toString`, so guarding only `join` left
/// `a=[]; a.push(a); a.join('-')` recursing until the native stack aborted the
/// process. On node v26.7.0 that expression is `""`.
fn join_array(recv: &Value, sep: &str) -> Result<Value, String> {
    if !host::join_stack_push(recv) {
        return Ok(with_host(|h| h.new_str(String::new())));
    }
    // 23.1.3.18 step 6: the length is captured once, then each element is read
    // and STRINGIFIED before the next is read. Both halves are observable —
    // a getter or a `toString` that shrinks the array is seen by every later
    // element, which a read-all-then-convert pass misses.
    let parts = (|| -> Result<Vec<String>, String> {
        let len = array_len(recv);
        let mut out = Vec::with_capacity(len);
        for i in 0..len {
            let v = array_elem_live(recv, i)?;
            out.push(join_parts(std::slice::from_ref(&v))?.remove(0));
        }
        Ok(out)
    })();
    host::join_stack_pop();
    let s = parts?.join(sep);
    Ok(with_host(|h| h.new_str(s)))
}

/// `Array.prototype.join`'s per-element conversion (23.1.3.18 step 4): a
/// `null`/`undefined` element contributes the empty string, every other element
/// is `ToString(element)` — which for an object means invoking its `toString`,
/// so `[{ toString() { return 'x' } }].join()` is `"x"` and not
/// `"[object Object]"`.
///
/// The all-primitive array — the overwhelmingly common one — is rendered under
/// a single host borrow; only an array actually holding an object pays for the
/// re-entrant per-element conversion.
fn join_parts(items: &[Value]) -> Result<Vec<String>, String> {
    let fast = with_host(|h| {
        items
            .iter()
            .map(|x| match x {
                Value::Undef => Some(String::new()),
                _ if h.is_null(x) => Some(String::new()),
                // A SYMBOL element is primitive but has no `ToString`, so it must
                // fall through to the fallible path and throw there:
                // `[Symbol()].join()` is a TypeError on node v26.7.0.
                _ if matches!(h.get(x), Some(JsObj::Symbol { .. })) => None,
                _ if host::is_primitive(h, x) => Some(h.str_of(x)),
                _ => None,
            })
            .collect::<Vec<_>>()
    });
    if fast.iter().all(Option::is_some) {
        return Ok(fast.into_iter().flatten().collect());
    }
    let mut out = Vec::with_capacity(items.len());
    for (x, p) in items.iter().zip(fast) {
        match p {
            Some(s) => out.push(s),
            None => {
                let s = host::to_string_value(x)?;
                out.push(with_host(|h| h.str_of(&s)));
            }
        }
    }
    Ok(out)
}

/// In-place sort of `items` (shared by `sort` and `toSorted`). Stable merge
/// sort — O(n log n) comparisons — with the fallible JS comparator called from
/// the merge step; default order is by the string form of each element.
/// Propagates a comparator error.
///
/// This was an insertion sort, which is O(n²): sorting 200k numbers with a
/// comparator did not finish inside 120s (node v26.7.0: 70ms), and each
/// doubling of the input quadrupled the time — 1k/2k/4k/8k/16k measured at
/// 0.21/0.81/3.39/12.94/51.36s. The comparator contract is unchanged; only the
/// number of times it is called is.
pub(crate) fn sort_values(items: &mut [Value], cmp: Option<&Value>) -> Result<(), String> {
    // 23.1.3.30 step 1: a comparator that is neither `undefined` nor callable is
    // rejected BEFORE any comparison runs. `[2,1].sort(null)` was reaching the
    // invoke path and reporting the generic `null is not a function`.
    let cmp = match cmp {
        Some(Value::Undef) => None,
        Some(v) if !with_host(|h| host::is_callable(h, v)) => {
            // V8 renders the offending value with `NoSideEffectsToString`, not
            // with `util.inspect`: a string appears bare (`: x`) rather than
            // quoted, and an array is `[object Array]` rather than `[ 1, 2 ]`.
            let shown = no_side_effects_string(v);
            return Err(host::type_error(&format!(
                "The comparison function must be either a function or undefined: {shown}"
            )));
        }
        other => other,
    };
    // 23.1.3.30.1 SortIndexedProperties: `undefined` is never handed to the
    // comparator — it sorts to the end after the defined values are ordered.
    // `[3,undefined,1].sort((x,y)=>x-y)` is `[1,3,undefined]` with ONE call on
    // node v26.7.0; the insertion sort called the comparator twice, on
    // `undefined`, and left `[3,undefined,1]`. Every element passed over here
    // is `undefined`, so swapping keeps the defined values in input order.
    let mut defined = 0;
    for i in 0..items.len() {
        if !matches!(items[i], Value::Undef) {
            items.swap(defined, i);
            defined += 1;
        }
    }
    merge_sort(&mut items[..defined], cmp)
}

/// One SortCompare: `> 0` means `b` sorts before `a`. A comparator result runs
/// through ToNumber, so a NaN (or a comparator returning `undefined`) is not
/// `> 0` and the pair keeps its input order.
fn sort_compare(a: &Value, b: &Value, cmp: Option<&Value>) -> Result<f64, String> {
    match cmp {
        Some(cb) => {
            let v = host::invoke(cb, vec![a.clone(), b.clone()], None)?;
            Ok(with_host(|h| h.to_number(&v)))
        }
        None => {
            // 23.1.3.30.2 SortCompare with no comparator: compare the ToString
            // of each element by CODE UNIT (`utf16::cmp_units`), which differs
            // from Rust's `String` order off the BMP.
            let x = with_host(|h| h.str_of(a));
            let y = with_host(|h| h.str_of(b));
            if crate::utf16::cmp_units(&x, &y) == std::cmp::Ordering::Greater {
                Ok(1.0)
            } else {
                Ok(-1.0)
            }
        }
    }
}

/// Bottom-up stable merge sort. Bottom-up rather than recursive so a large
/// array cannot walk the native stack the JS comparator also runs on, and the
/// two buffers are swapped each pass instead of copied back.
fn merge_sort(items: &mut [Value], cmp: Option<&Value>) -> Result<(), String> {
    let n = items.len();
    if n < 2 {
        return Ok(());
    }
    let mut src = items.to_vec();
    let mut dst = src.clone();
    let mut width = 1;
    while width < n {
        let mut lo = 0;
        while lo < n {
            let mid = (lo + width).min(n);
            let hi = (lo + 2 * width).min(n);
            merge(&src[lo..mid], &src[mid..hi], &mut dst[lo..hi], cmp)?;
            lo = hi;
        }
        std::mem::swap(&mut src, &mut dst);
        width *= 2;
    }
    items.clone_from_slice(&src);
    Ok(())
}

/// Merge two sorted runs into `out`. Ties take from `left` first, which is what
/// makes the sort stable — `[{k:1},{k:0},{k:1},{k:0}].sort((x,y)=>x.k-y.k)`
/// keeps the two `k:0` entries in input order, as node does.
fn merge(
    left: &[Value],
    right: &[Value],
    out: &mut [Value],
    cmp: Option<&Value>,
) -> Result<(), String> {
    let (mut i, mut j, mut k) = (0, 0, 0);
    while i < left.len() && j < right.len() {
        if sort_compare(&left[i], &right[j], cmp)? > 0.0 {
            out[k] = right[j].clone();
            j += 1;
        } else {
            out[k] = left[i].clone();
            i += 1;
        }
        k += 1;
    }
    for v in left[i..].iter().chain(&right[j..]) {
        out[k] = v.clone();
        k += 1;
    }
    Ok(())
}

/// Recursively flatten `items` up to `depth` levels into `out`. `depth` is an
/// f64 so `Infinity` (full flatten) and finite counts share one path.
///
/// `flat` has NO cycle cut — unlike `join`, V8 lets it run out of stack, and
/// `a=[1]; a.push(a); a.flat(Infinity)` is `RangeError: Maximum call stack size
/// exceeded` on node v26.7.0. That is reproduced by checking the same native
/// stack floor the VM does, so the answer is a catchable error rather than the
/// `fatal runtime error: stack overflow` abort this used to produce.
/// `FlattenIntoArray` (23.1.3.13.1). Takes the source ARRAY rather than its
/// elements because each level tests `HasProperty` before recursing, so a hole
/// contributes nothing at any depth: `[1,,3].flat()` is the dense `[1, 3]`.
fn flatten_into(src: &Value, depth: f64, out: &mut Vec<Value>) -> Result<(), String> {
    if host::stack_exhausted() {
        return Err(host::stack_overflow_error());
    }
    let items = array_items(src);
    let holes = absent_set(src);
    for (i, it) in items.into_iter().enumerate() {
        if holes.contains(&i) {
            continue;
        }
        let nested = depth > 0.0 && with_host(|h| h.kind_of(&it)) == Some(ObjKind::Array);
        if nested {
            flatten_into(&it, depth - 1.0, out)?;
        } else {
            out.push(it);
        }
    }
    Ok(())
}

fn array_splice(recv: &Value, args: Vec<Value>) -> Result<Value, String> {
    let len = array_len(recv);
    let start = {
        let s = arg_num(&args, 0);
        if s < 0.0 {
            ((len as f64 + s).max(0.0)) as usize
        } else {
            (s as usize).min(len)
        }
    };
    let delete = if args.len() >= 2 {
        (arg_num(&args, 1).max(0.0) as usize).min(len - start)
    } else {
        len - start
    };
    let inserts: Vec<Value> = args.iter().skip(2).cloned().collect();
    let inserted = inserts.len();
    // The receiver's holes shift by (inserted - deleted) past the cut, and the
    // ones inside the cut move into the RETURNED array at their offset there.
    let holes = hole_set(recv);
    let removed = with_host(|h| {
        if let Some(JsObj::Array(items)) = h.get_mut(recv) {
            let removed: Vec<Value> = items.splice(start..start + delete, inserts).collect();
            removed
        } else {
            Vec::new()
        }
    });
    let spliced = with_host(|h| {
        h.install_holes(
            recv,
            holes
                .iter()
                .filter_map(|&i| {
                    if i < start {
                        Some(i)
                    } else if i < start + delete {
                        None
                    } else {
                        Some(i - delete + inserted)
                    }
                })
                .collect(),
        );
        (removed, holes.clone())
    });
    // The REMOVED elements come back as an array of the receiver's species
    // (23.1.3.31 step 8), so a subclass gets one of its own kind.
    let (removed, holes) = spliced;
    let out = array_species_create(recv, removed)?;
    with_host(|h| {
        h.install_holes(
            &out,
            holes
                .iter()
                .filter(|&&i| i >= start && i < start + delete)
                .map(|&i| i - start)
                .collect(),
        );
    });
    Ok(out)
}

fn slice_bounds(args: &[Value], len: usize) -> (usize, usize) {
    let norm = |v: f64| -> usize {
        if v < 0.0 {
            ((len as f64 + v).max(0.0)) as usize
        } else {
            (v as usize).min(len)
        }
    };
    let lo = if args.is_empty() || matches!(args[0], Value::Undef) {
        0
    } else {
        norm(arg_num(args, 0))
    };
    let hi = if args.len() < 2 || matches!(args[1], Value::Undef) {
        len
    } else {
        norm(arg_num(args, 1))
    };
    // A start at or past the end (`'World'.slice(2, 1)`) yields the empty range,
    // never a reversed one: JS `slice` clamps `end` up to `start`.
    (lo, hi.max(lo))
}

/// The argument positions each `String.prototype` method coerces with
/// `ToNumber` rather than `ToString`. Everything not listed is a string
/// position — which matters only for a SYMBOL argument, the one value both
/// conversions refuse, and refuse with different wording.
///
/// Measured per method and per position: `'x'.indexOf(sym)` reports the STRING
/// message and `'x'.indexOf('a', sym)` the NUMBER one, and `padStart` is the
/// pair the other way round (a length then a pad string).
const STRING_METHOD_NUMERIC_ARGS: &[(&str, &[usize])] = &[
    ("at", &[0]),
    ("charAt", &[0]),
    ("charCodeAt", &[0]),
    ("codePointAt", &[0]),
    ("endsWith", &[1]),
    ("includes", &[1]),
    ("indexOf", &[1]),
    ("lastIndexOf", &[1]),
    ("padEnd", &[0]),
    ("padStart", &[0]),
    ("repeat", &[0]),
    ("slice", &[0, 1]),
    ("split", &[1]),
    ("startsWith", &[1]),
    ("substr", &[0, 1]),
    ("substring", &[0, 1]),
];

/// Reject a SYMBOL argument before any string method coerces it. 7.1.17 and
/// 7.1.4 both refuse one, so `'x'.padStart(3, sym)` is a TypeError where this
/// rendered `Symbol(d)` into the result — silently, which is the shape of
/// mistake that makes a symbol key leak into text.
fn reject_symbol_args(name: &str, args: &[Value]) -> Result<(), String> {
    let numeric = STRING_METHOD_NUMERIC_ARGS
        .iter()
        .find(|(m, _)| *m == name)
        .map(|(_, ps)| *ps)
        .unwrap_or(&[]);
    for (i, a) in args.iter().enumerate() {
        if with_host(|h| matches!(h.get(a), Some(JsObj::Symbol { .. }))) {
            let kind = if numeric.contains(&i) {
                "number"
            } else {
                "string"
            };
            return Err(host::type_error(&format!(
                "Cannot convert a Symbol value to a {kind}"
            )));
        }
    }
    Ok(())
}

/// Coerce a string method's arguments the way 22.1.3.x does, BEFORE any arm
/// reads them: a numeric position through `ToNumber`, every other through
/// `ToString`. Both run a user `valueOf`/`toString`, and none of them ran —
/// `'x'.padStart({valueOf: () => 3})` produced `"x"` and
/// `'x'.concat({toString: () => 'y'})` produced `"x[object Object]"`.
///
/// The positions that must NOT be coerced are the ones with their own protocol:
/// a RegExp or a `Symbol.replace`/`split`/`match`/`search` carrier at position
/// 0 of the method that honours it, and a callable REPLACEMENT at position 1 of
/// `replace`/`replaceAll`. Each of those already has a path that handles the
/// value as an object, and stringifying it first would take that path away.
/// The argument positions each `Array.prototype` method coerces with
/// `ToNumber` (23.1.3.x). Everything not listed is a VALUE position and must be
/// left alone: `fill`'s first argument, `with`'s second and `splice`'s items
/// are stored as given, and `indexOf`/`includes` compare their first argument
/// without converting it.
const ARRAY_METHOD_NUMERIC_ARGS: &[(&str, &[usize])] = &[
    ("at", &[0]),
    ("copyWithin", &[0, 1, 2]),
    ("fill", &[1, 2]),
    ("flat", &[0]),
    ("includes", &[1]),
    ("indexOf", &[1]),
    ("lastIndexOf", &[1]),
    ("slice", &[0, 1]),
    ("splice", &[0, 1]),
    ("toSpliced", &[0, 1]),
    ("with", &[0]),
];

/// The same for `Number.prototype`. `toLocaleString` takes a LOCALE, not a
/// number, and is deliberately absent.
const NUMBER_METHOD_NUMERIC_ARGS: &[(&str, &[usize])] = &[
    ("toExponential", &[0]),
    ("toFixed", &[0]),
    ("toPrecision", &[0]),
    ("toString", &[0]),
];

/// Replace the listed argument positions with their `ToNumber` value, running a
/// user `valueOf` and propagating a throw from it. Every one of these read the
/// argument with an INFALLIBLE conversion that does no `ToPrimitive` at all, so
/// `[1,2,3].slice({valueOf: () => 1})` sliced from 0 and `(1.234).toFixed(obj)`
/// was a RangeError.
/// `ToNumber(args[i])`, running a user `valueOf` and propagating its throw.
fn to_number_arg(args: &[Value], i: usize) -> Result<f64, String> {
    let v = args.get(i).cloned().unwrap_or(Value::Undef);
    let p = host::to_primitive(&v, "number")?;
    Ok(with_host(|h| h.to_number(&p)))
}

fn coerce_numeric_args(
    table: &[(&str, &[usize])],
    name: &str,
    mut args: Vec<Value>,
) -> Result<Vec<Value>, String> {
    let Some((_, positions)) = table.iter().find(|(m, _)| *m == name) else {
        return Ok(args);
    };
    for &i in *positions {
        let Some(a) = args.get(i) else { continue };
        if matches!(a, Value::Undef) {
            continue;
        }
        let p = host::to_primitive(a, "number")?;
        args[i] = Value::Float(with_host(|h| h.to_number(&p)));
    }
    Ok(args)
}

/// `RegExpCreate(v, flags)` — the regexp a string method builds from a
/// non-RegExp argument. An empty/absent argument makes the empty pattern, which
/// matches at position 0.
fn regexp_from_arg(v: &Value, flags: &str) -> Result<Value, String> {
    let src = if matches!(v, Value::Undef) {
        String::new()
    } else {
        with_host(|h| h.str_of(v))
    };
    let fv = with_host(|h| h.new_str(flags.to_string()));
    let sv = with_host(|h| h.new_str(src));
    regexp_ctor(&[sv, fv])
}

fn coerce_string_args(name: &str, args: Vec<Value>) -> Result<Vec<Value>, String> {
    let numeric = STRING_METHOD_NUMERIC_ARGS
        .iter()
        .find(|(m, _)| *m == name)
        .map(|(_, ps)| *ps)
        .unwrap_or(&[]);
    let protocol = match name {
        "replace" | "replaceAll" => Some("@@replace"),
        "split" => Some("@@split"),
        "match" => Some("@@match"),
        "matchAll" => Some("@@matchAll"),
        "search" => Some("@@search"),
        // These three do not CONSUME `Symbol.match`, they reject a value that
        // carries it (22.1.3.7/23/24 step 3 — `IsRegExp`). Exempting it keeps
        // the object intact so that check still sees one; stringifying first
        // turned the TypeError into an ordinary search.
        "startsWith" | "endsWith" | "includes" => Some("@@match"),
        _ => None,
    };
    let mut out = Vec::with_capacity(args.len());
    for (i, a) in args.into_iter().enumerate() {
        if matches!(a, Value::Undef) {
            out.push(a);
            continue;
        }
        if numeric.contains(&i) {
            let p = host::to_primitive(&a, "number")?;
            out.push(Value::Float(with_host(|h| h.to_number(&p))));
            continue;
        }
        // The IsRegExp trio tests `Symbol.match` for TRUTHINESS (7.2.8 step 2),
        // not for presence: an object carrying `[Symbol.match]: false` is NOT a
        // regexp and coerces like anything else. The consuming protocols use
        // `GetMethod`, which additionally requires a callable.
        let is_regexp_like = matches!(name, "startsWith" | "endsWith" | "includes");
        let carries = |p: &str| match host::protocol_lookup(&a, p) {
            Ok(Some(m)) => {
                if is_regexp_like {
                    with_host(|h| h.truthy(&m))
                } else {
                    with_host(|h| host::is_callable(h, &m))
                }
            }
            _ => false,
        };
        let exempt = with_host(|h| matches!(h.get(&a), Some(JsObj::RegExp(_))))
            || (i == 0 && protocol.is_some_and(carries))
            || (i == 1
                && matches!(name, "replace" | "replaceAll")
                && with_host(|h| host::is_callable(h, &a)));
        if exempt {
            out.push(a);
            continue;
        }
        out.push(host::to_string_value(&a)?);
    }
    Ok(out)
}

fn string_method(s: &str, name: &str, args: Vec<Value>) -> Result<Value, String> {
    reject_symbol_args(name, &args)?;
    let args = coerce_string_args(name, args)?;
    // Every index-bearing method below counts UTF-16 code units, so they all
    // work off this one decoding rather than off `s.chars()` (code points),
    // which agrees only on the BMP. `@@iterator` is the deliberate exception.
    let u = crate::utf16::Units::of(s);
    match name {
        // `for…of` / spread over a string iterates CODE POINTS, not code units:
        // `[..."𝒳"]` is one element in node even though `"𝒳".length` is 2. This
        // is the one string operation that is specified in chars, so it stays
        // on `s.chars()` on purpose — do not "fix" it to match the others.
        "@@iterator" => {
            let items: Vec<Value> = s.chars().map(|c| new_s(c.to_string())).collect();
            Ok(with_host(|h| {
                h.alloc(JsObj::Iter {
                    items,
                    idx: 0,
                    array: None,
                })
            }))
        }
        "toUpperCase" => Ok(new_s(s.to_uppercase())),
        "toLowerCase" => Ok(new_s(s.to_lowercase())),
        // `toLocaleUpperCase`/`toLocaleLowerCase` (22.1.3.26/22.1.3.24) differ
        // from the plain forms only for the locale-specific mappings (Turkish
        // dotless i, Lithuanian accents); with no locale argument they are the
        // Unicode Default Case Conversion, which is exactly `to_uppercase`/
        // `to_lowercase`. They threw `is not a function` before, so the common
        // no-argument call — the only form this runtime can answer, since it
        // carries no ICU — failed outright rather than agreeing with node.
        // A locale ARGUMENT is accepted and ignored; `'I'.toLocaleLowerCase('tr')`
        // is `'i'` here and `'ı'` in node.
        // `String.prototype.toLocaleString` (22.1.3.27) is `toString` — a string
        // has no locale rendering. Missing it made an ARRAY of strings fail too,
        // since `Array.prototype.toLocaleString` invokes it per element.
        "toLocaleString" => Ok(new_s(s.to_string())),
        "toLocaleUpperCase" => Ok(new_s(s.to_uppercase())),
        "toLocaleLowerCase" => Ok(new_s(s.to_lowercase())),
        // Locale comparison (ASCII approximation of ICU collation): primary by
        // case-folded order, then lowercase sorts before uppercase at a tie.
        "localeCompare" => {
            let other = with_host(|h| h.str_of(&arg0(&args)));
            let (la, lb) = (s.to_lowercase(), other.to_lowercase());
            let r = match la.cmp(&lb) {
                std::cmp::Ordering::Less => -1.0,
                std::cmp::Ordering::Greater => 1.0,
                std::cmp::Ordering::Equal => {
                    let mut t = 0.0;
                    for (ca, cb) in s.chars().zip(other.chars()) {
                        if ca != cb {
                            t = if ca.is_lowercase() { -1.0 } else { 1.0 };
                            break;
                        }
                    }
                    t
                }
            };
            Ok(Value::Float(r))
        }
        // `String.prototype.normalize` (22.1.3.15) — real UAX-15 normalization.
        //
        // This used to return the receiver unchanged and only validate the FORM
        // argument, which made every one of the four forms a no-op: `"Å"` (NFC,
        // one code point) and `"Å"` (NFD, two) stayed distinct under
        // `.normalize()`, so the standard way to compare Unicode text for
        // canonical equivalence silently answered `false`, and `NFKC` never
        // folded a compatibility character (`"fi"` stayed one code point instead
        // of becoming `"fi"`). The tables come from `unicode-normalization`.
        "normalize" => {
            use unicode_normalization::UnicodeNormalization;
            let form = match args.first() {
                Some(v) if !matches!(v, Value::Undef) => with_host(|h| h.str_of(v)),
                _ => "NFC".to_string(),
            };
            let out = match form.as_str() {
                "NFC" => s.nfc().collect::<String>(),
                "NFD" => s.nfd().collect::<String>(),
                "NFKC" => s.nfkc().collect::<String>(),
                "NFKD" => s.nfkd().collect::<String>(),
                _ => {
                    return Err(host::range_error(
                        "The normalization form should be one of NFC, NFD, NFKC, NFKD.",
                    ))
                }
            };
            Ok(new_s(out))
        }
        // ES2024 well-formedness (22.1.3.9 / 22.1.3.29). A `String` here is a
        // Rust `String`, whose `char` type EXCLUDES `U+D800..=U+DFFF`, so every
        // value this runtime can hold is well-formed by construction and
        // `toWellFormed` has nothing to replace. Both answers are therefore
        // exact for every string that survives storage; the one case node
        // answers differently is a surrogate half extracted by `charAt`/`slice`,
        // which is already `U+FFFD` here — the documented lone-surrogate
        // boundary in `utf16`, not a separate gap.
        "isWellFormed" => Ok(Value::Bool(true)),
        "toWellFormed" => Ok(new_s(s.to_string())),
        // The JS `WhiteSpace` set, not Rust's — they differ on `U+FEFF`.
        "trim" => Ok(new_s(crate::utf16::js_trim(s).to_string())),
        "trimStart" => Ok(new_s(crate::utf16::js_trim_start(s).to_string())),
        "trimEnd" => Ok(new_s(crate::utf16::js_trim_end(s).to_string())),
        "toString" | "valueOf" => Ok(new_s(s.to_string())),
        "charAt" => {
            let at = unit_pos(arg_num(&args, 0)).and_then(|i| u.unit_str(i));
            Ok(new_s(at.unwrap_or_default()))
        }
        "at" => {
            let n = arg_num(&args, 0);
            // A negative position counts back from the end; `NaN` is 0. An
            // infinite position is out of range in either direction.
            let i = if n.is_nan() {
                Some(0i64)
            } else if n.is_finite() {
                let i = n.trunc() as i64;
                Some(if i < 0 { i + u.len() as i64 } else { i })
            } else {
                None
            };
            match i
                .and_then(|i| usize::try_from(i).ok())
                .and_then(|i| u.unit_str(i))
            {
                Some(c) => Ok(new_s(c)),
                None => Ok(Value::Undef),
            }
        }
        // `charCodeAt` reports the bare code UNIT — the high surrogate of an
        // astral character, not the character. `codePointAt` looks ahead one
        // unit and reports the whole scalar when the pair is well formed. They
        // agree everywhere on the BMP, which is why they used to share an arm.
        // They also disagree OUT of range: `charCodeAt` yields `NaN` while
        // `codePointAt` yields `undefined` (measured on node v26.7.0).
        "charCodeAt" => {
            let unit = unit_pos(arg_num(&args, 0)).and_then(|i| u.unit(i));
            Ok(Value::Float(unit.map(f64::from).unwrap_or(f64::NAN)))
        }
        "codePointAt" => match unit_pos(arg_num(&args, 0)).and_then(|i| u.code_point(i)) {
            Some(cp) => Ok(Value::Float(f64::from(cp))),
            None => Ok(Value::Undef),
        },
        // The search quartet all honor their optional position argument.
        // `"a&b&c".indexOf("&", 2)` must be 3, not 1 — body-parser's
        // parameterCount walks a query string with exactly that call.
        "indexOf" => {
            let needle = needle_units(&args);
            let from = clamp_pos(arg_num(&args, 1), u.len());
            Ok(Value::Float(
                search_from(u.as_slice(), needle.as_slice(), from)
                    .map(|i| i as f64)
                    .unwrap_or(-1.0),
            ))
        }
        "lastIndexOf" => {
            let needle = needle_units(&args);
            // An absent or NaN position means "search the whole string".
            let n = arg_num(&args, 1);
            let upto = if n.is_nan() {
                u.len()
            } else {
                clamp_pos(n, u.len())
            };
            Ok(Value::Float(
                search_last(u.as_slice(), needle.as_slice(), upto)
                    .map(|i| i as f64)
                    .unwrap_or(-1.0),
            ))
        }
        // 22.1.3.7/22.1.3.23/22.1.3.14 step 2: these three reject a REGEXP
        // argument outright, and `IsRegExp` is what decides — so an object
        // advertising `Symbol.match` is rejected too. None of them checked.
        "startsWith" | "endsWith" | "includes" if is_regexp_arg(&arg0(&args)) => {
            Err(host::type_error(&format!(
                "First argument to String.prototype.{name} must not be a regular expression"
            )))
        }
        "includes" => {
            let needle = needle_units(&args);
            let from = clamp_pos(arg_num(&args, 1), u.len());
            Ok(Value::Bool(
                search_from(u.as_slice(), needle.as_slice(), from).is_some(),
            ))
        }
        "startsWith" => {
            let needle = needle_units(&args);
            let from = clamp_pos(arg_num(&args, 1), u.len());
            Ok(Value::Bool(
                u.as_slice()[from..].starts_with(needle.as_slice()),
            ))
        }
        "endsWith" => {
            let needle = needle_units(&args);
            // The 2nd argument is where the string is treated as ENDING.
            let end = if args.len() < 2 || matches!(args[1], Value::Undef) {
                u.len()
            } else {
                clamp_pos(arg_num(&args, 1), u.len())
            };
            Ok(Value::Bool(
                u.as_slice()[..end].ends_with(needle.as_slice()),
            ))
        }
        "slice" => {
            let (lo, hi) = slice_bounds(&args, u.len());
            Ok(new_s(u.slice(lo, hi)))
        }
        "substring" => {
            let mut a = arg_num(&args, 0).max(0.0) as usize;
            let mut b = if args.len() < 2 || matches!(args[1], Value::Undef) {
                u.len()
            } else {
                (arg_num(&args, 1).max(0.0) as usize).min(u.len())
            };
            a = a.min(u.len());
            if a > b {
                std::mem::swap(&mut a, &mut b);
            }
            Ok(new_s(u.slice(a, b)))
        }
        "substr" => {
            // A negative start counts from the end: max(len + start, 0).
            let len = u.len() as i64;
            let mut start = arg_num(&args, 0) as i64;
            if start < 0 {
                start = (len + start).max(0);
            }
            let start = (start as usize).min(u.len());
            let count = if args.len() >= 2 {
                arg_num(&args, 1).max(0.0) as usize
            } else {
                u.len()
            };
            let end = start.saturating_add(count).min(u.len());
            Ok(new_s(u.slice(start, end)))
        }
        "repeat" => {
            let n = arg_num(&args, 0);
            // `RangeError`, not `TypeError`, and the count is named:
            // `"x".repeat(-1)` is `RangeError: Invalid count value: -1`.
            if n < 0.0 || !n.is_finite() {
                return Err(host::range_error(&format!(
                    "Invalid count value: {}",
                    host::fmt_number(n)
                )));
            }
            // The PRODUCT is what V8 bounds, so `''.repeat(2**53)` is legal (and
            // `''`) while `'ab'.repeat(268435445)` is not: measured on node
            // v26.7.0, `'ab'.repeat(268435444).length` is 536870888 and one more
            // is `RangeError: Invalid string length`.
            if n * crate::utf16::len(s) as f64 > host::MAX_STRING_LENGTH as f64 {
                return Err(host::invalid_string_length());
            }
            Ok(new_s(s.repeat(n as usize)))
        }
        "concat" => {
            let mut out = s.to_string();
            for a in &args {
                out.push_str(&with_host(|h| h.str_of(a)));
            }
            Ok(new_s(out))
        }
        "padStart" => Ok(new_s(pad(s, &args, true)?)),
        "padEnd" => Ok(new_s(pad(s, &args, false)?)),
        // Regex-taking string methods: dispatch to the regexp module when the
        // argument is a RegExp; otherwise keep the plain-string behavior.
        // 22.1.3.20 step 2.a: `replaceAll` validates the `g` flag BEFORE it
        // consults `Symbol.replace`, so a non-global regexp is a TypeError even
        // though a RegExp does define that method. Delegating first skipped the
        // check and silently did a single replacement.
        "replaceAll"
            if is_regexp_arg(&arg0(&args))
                && !with_host(
                    |h| matches!(h.get(&arg0(&args)), Some(JsObj::RegExp(r)) if r.global),
                ) =>
        {
            Err(host::type_error(
                "String.prototype.replaceAll called with a non-global RegExp argument",
            ))
        }
        "match" | "matchAll" | "search" | "split" | "replace" | "replaceAll"
            if symbol_protocol(
                &arg0(&args),
                match name {
                    "match" => "@@match",
                    "matchAll" => "@@matchAll",
                    "search" => "@@search",
                    "split" => "@@split",
                    _ => "@@replace",
                },
            )
            .is_some() =>
        {
            let sym = match name {
                "match" => "@@match",
                "matchAll" => "@@matchAll",
                "search" => "@@search",
                "split" => "@@split",
                _ => "@@replace",
            };
            let f = symbol_protocol(&arg0(&args), sym).expect("guard checked");
            let sv = with_host(|h| h.new_str(s.to_string()));
            let mut rest = vec![sv];
            rest.extend(args.iter().skip(1).cloned());
            host::invoke(&f, rest, Some(arg0(&args)))
        }
        // 22.1.3.13/14: a non-RegExp argument is turned INTO one
        // (`RegExpCreate(regexp, …)`), so `'abc'.match('b')` matches. It
        // answered `null` for every string argument, which reads as "no match"
        // — the one answer a caller cannot tell from a real failure.
        // `matchAll` builds its with `g`, which 22.1.3.14 requires.
        "match" => {
            let a = arg0(&args);
            let re = if is_regexp_arg(&a) {
                a
            } else {
                regexp_from_arg(&a, "")?
            };
            crate::regexp::str_match(s, &re)
        }
        "matchAll" => {
            let a = arg0(&args);
            let re = if is_regexp_arg(&a) {
                a
            } else {
                regexp_from_arg(&a, "g")?
            };
            crate::regexp::str_match_all(s, &re)
        }
        "search" => {
            if is_regexp_arg(&arg0(&args)) {
                crate::regexp::str_search(s, &arg0(&args))
            } else {
                // 22.1.3.17 builds a RegExp from the argument, so a
                // METACHARACTER matches as one: `'a.c'.search('.')` is 0, not
                // 1. The substring approximation this replaces agreed only for
                // a literal needle, and answered -1 for an absent argument
                // where the empty pattern matches at 0.
                let re = regexp_from_arg(&arg0(&args), "")?;
                crate::regexp::str_search(s, &re)
            }
        }
        "replace" => {
            let pat = arg0(&args);
            let repl = args.get(1).cloned().unwrap_or(Value::Undef);
            if is_regexp_arg(&pat) {
                crate::regexp::str_replace_regex(s, &pat, &repl, false)
            } else if with_host(|h| host::is_callable(h, &repl)) {
                Ok(new_s(replace_str_fn(
                    s,
                    &with_host(|h| h.str_of(&pat)),
                    &repl,
                    false,
                )?))
            } else {
                let from = with_host(|h| h.str_of(&pat));
                let to = with_host(|h| h.str_of(&repl));
                Ok(new_s(replace_str_plain(s, &from, &to, false)))
            }
        }
        "replaceAll" => {
            let pat = arg0(&args);
            let repl = args.get(1).cloned().unwrap_or(Value::Undef);
            if is_regexp_arg(&pat) {
                // 22.1.3.20 step 2: a non-global regexp is a TypeError here,
                // because `replaceAll` cannot honour "all" without `g`. This
                // used to replace only the first match and say nothing.
                let global = with_host(|h| match h.get(&pat) {
                    Some(JsObj::RegExp(r)) => r.global,
                    _ => true,
                });
                if !global {
                    return Err(host::type_error(
                        "String.prototype.replaceAll called with a non-global RegExp argument",
                    ));
                }
                crate::regexp::str_replace_regex(s, &pat, &repl, true)
            } else if with_host(|h| host::is_callable(h, &repl)) {
                Ok(new_s(replace_str_fn(
                    s,
                    &with_host(|h| h.str_of(&pat)),
                    &repl,
                    true,
                )?))
            } else {
                let from = with_host(|h| h.str_of(&pat));
                let to = with_host(|h| h.str_of(&repl));
                Ok(new_s(replace_str_plain(s, &from, &to, true)))
            }
        }
        "split" => {
            if is_regexp_arg(&arg0(&args)) {
                let limit = args
                    .get(1)
                    .filter(|v| !matches!(v, Value::Undef))
                    .map(|v| with_host(|h| h.to_number(v)) as usize);
                return crate::regexp::str_split_regex(s, &arg0(&args), limit);
            }
            let mut parts: Vec<Value> = if args.is_empty() || matches!(args[0], Value::Undef) {
                vec![new_s(s.to_string())]
            } else {
                let sep = with_host(|h| h.str_of(&args[0]));
                if sep.is_empty() {
                    // `split('')` yields one element per code UNIT, so an astral
                    // character becomes its two surrogate halves.
                    (0..u.len())
                        .filter_map(|i| u.unit_str(i))
                        .map(new_s)
                        .collect()
                } else {
                    s.split(&sep as &str)
                        .map(|p| new_s(p.to_string()))
                        .collect()
                }
            };
            // Optional limit: keep at most `limit` substrings.
            if let Some(lim) = args.get(1).filter(|v| !matches!(v, Value::Undef)) {
                let n = with_host(|h| h.to_number(lim));
                if n.is_finite() && n >= 0.0 {
                    parts.truncate(n as usize);
                }
            }
            Ok(with_host(|h| h.new_array(parts)))
        }
        _ => Err(host::type_error(&format!("{name} is not a function"))),
    }
}

/// GetSubstitution (22.1.3.19) for a STRING search value.
///
/// `String.prototype.replace`/`replaceAll` expand the same `$` patterns whether
/// the pattern is a regexp or a plain string, but the string path here did a
/// raw `str::replace` and passed the template through verbatim — so
/// `'abc'.replace('b', '[$&]')` produced `a[$&]c` instead of `a[b]c`. The
/// regexp path has always expanded them.
///
/// A string search captures nothing, so only `$$`, `$&`, `` $` `` and `$'`
/// apply; `$1` and `$<name>` have no referent and stay literal, which is also
/// what node does.
fn substitute_plain(templ: &str, matched: &str, position: usize, subject: &str) -> String {
    let chars: Vec<char> = templ.chars().collect();
    let mut out = String::new();
    let mut i = 0;
    while i < chars.len() {
        if chars[i] == '$' && i + 1 < chars.len() {
            match chars[i + 1] {
                '$' => {
                    out.push('$');
                    i += 2;
                    continue;
                }
                '&' => {
                    out.push_str(matched);
                    i += 2;
                    continue;
                }
                '`' => {
                    out.push_str(&subject[..position]);
                    i += 2;
                    continue;
                }
                '\'' => {
                    out.push_str(&subject[position + matched.len()..]);
                    i += 2;
                    continue;
                }
                _ => {}
            }
        }
        out.push(chars[i]);
        i += 1;
    }
    out
}

/// `replace`/`replaceAll` with a string pattern and a string replacement,
/// expanding each match's `$` patterns against its own position.
fn replace_str_plain(s: &str, from: &str, to: &str, all: bool) -> String {
    if from.is_empty() && !all {
        return format!("{}{s}", substitute_plain(to, "", 0, s));
    }
    let mut out = String::new();
    let mut rest = 0usize;
    while let Some(rel) = s[rest..].find(from) {
        let at = rest + rel;
        out.push_str(&s[rest..at]);
        out.push_str(&substitute_plain(to, from, at, s));
        rest = at + from.len();
        if !all {
            break;
        }
        // An empty pattern matches between every character; step one along so
        // the scan terminates.
        if from.is_empty() {
            if rest >= s.len() {
                break;
            }
            let step = s[rest..].chars().next().map(|c| c.len_utf8()).unwrap_or(1);
            out.push_str(&s[rest..rest + step]);
            rest += step;
        }
    }
    out.push_str(&s[rest..]);
    out
}

fn new_s(s: String) -> Value {
    with_host(|h| h.new_str(s))
}

/// Where a forward `indexOf`/`includes` search starts, given the optional
/// `fromIndex` (23.1.3.17 steps 4-6, 23.1.3.16 steps 5-7). A negative value
/// counts back from the end and clamps at 0; absent or `NaN` is 0. A start at
/// or past the end finds nothing, which callers report as `-1` / `false`.
pub(crate) fn search_start(n: f64, len: usize) -> usize {
    if n.is_nan() {
        return 0;
    }
    let n = n.trunc();
    if n >= 0.0 {
        if n >= len as f64 {
            len
        } else {
            n as usize
        }
    } else {
        let from_end = len as f64 + n;
        if from_end <= 0.0 {
            0
        } else {
            from_end as usize
        }
    }
}

/// The INCLUSIVE index a backward `lastIndexOf` starts at (23.1.3.20 steps
/// 4-6), or `None` when `fromIndex` places it before the array. Absent means
/// the last element — which is why this takes an `Option` rather than reading
/// `NaN` as "absent" the way the forward form can: an explicit `NaN` is
/// `ToIntegerOrInfinity`'d to 0 and searches only index 0.
pub(crate) fn search_start_last(from: Option<f64>, len: usize) -> Option<usize> {
    if len == 0 {
        return None;
    }
    let n = match from {
        None => return Some(len - 1),
        Some(v) if v.is_nan() => 0.0,
        Some(v) => v.trunc(),
    };
    if n >= 0.0 {
        Some(if n >= len as f64 { len - 1 } else { n as usize })
    } else {
        let k = len as f64 + n;
        if k < 0.0 {
            None
        } else {
            Some(k as usize)
        }
    }
}

/// `ToIntegerOrInfinity(n)` clamped into `0..=len` — the position argument of
/// the `String.prototype` search methods. `NaN` (an absent argument) is `0`.
fn clamp_pos(n: f64, len: usize) -> usize {
    if n.is_nan() || n <= 0.0 {
        0
    } else if n >= len as f64 {
        len
    } else {
        n.trunc() as usize
    }
}

/// `ToIntegerOrInfinity(n)` as a code-unit position, or `None` when there can be
/// no such unit. `NaN` (an absent argument) is 0; a negative or infinite
/// position is out of range — `"abc".charCodeAt(-1)` is `NaN`, not `'a'`.
fn unit_pos(n: f64) -> Option<usize> {
    if n.is_nan() {
        Some(0)
    } else if n < 0.0 || !n.is_finite() {
        None
    } else {
        Some(n.trunc() as usize)
    }
}

/// The search argument of `indexOf`/`includes`/`startsWith`/… as code units, so
/// the needle is compared in the same alphabet the haystack is indexed by.
fn needle_units(args: &[Value]) -> crate::utf16::Units {
    crate::utf16::Units::of(&with_host(|h| h.str_of(&arg0(args))))
}

/// The lowest index `>= from` at which `needle` occurs in `hay`. An empty
/// needle matches at `from` itself, as JS specifies.
fn search_from(hay: &[u16], needle: &[u16], from: usize) -> Option<usize> {
    if needle.is_empty() {
        return Some(from.min(hay.len()));
    }
    if needle.len() > hay.len() {
        return None;
    }
    (from..=hay.len().saturating_sub(needle.len())).find(|&i| &hay[i..i + needle.len()] == needle)
}

/// The highest index `<= upto` at which `needle` occurs in `hay`.
fn search_last(hay: &[u16], needle: &[u16], upto: usize) -> Option<usize> {
    if needle.is_empty() {
        return Some(upto.min(hay.len()));
    }
    if needle.len() > hay.len() {
        return None;
    }
    let last = hay.len() - needle.len();
    (0..=upto.min(last))
        .rev()
        .find(|&i| &hay[i..i + needle.len()] == needle)
}

fn pad(s: &str, args: &[Value], start: bool) -> Result<String, String> {
    let target_f = arg_num(args, 0);
    let target = if target_f.is_finite() && target_f > 0.0 {
        target_f as usize
    } else {
        0
    };
    // `targetLength` and the padding both count code units: `'𝒳'.padStart(3,'-')`
    // is `'-𝒳'` in node, not `'--𝒳'`.
    let cur = crate::utf16::len(s);
    if cur >= target {
        return Ok(s.to_string());
    }
    let filler = if args.len() >= 2 {
        with_host(|h| h.str_of(&args[1]))
    } else {
        " ".to_string()
    };
    if filler.is_empty() {
        return Ok(s.to_string());
    }
    // Checked only AFTER the two short-circuits, which is the order V8 uses:
    // measured on node v26.7.0, `'ab'.padStart(2**40, '')` is `'ab'` while
    // `'ab'.padStart(536870889, 'x')` is `RangeError: Invalid string length`.
    if target_f > host::MAX_STRING_LENGTH as f64 {
        return Err(host::invalid_string_length());
    }
    let need = target - cur;
    let fill = crate::utf16::Units::of(&filler);
    // The filler repeats and is TRUNCATED to the exact unit count, which can cut
    // a surrogate pair — node yields a lone surrogate there, we yield U+FFFD
    // (see src/utf16.rs).
    let units: Vec<u16> = (0..need)
        .filter_map(|i| fill.unit(i % fill.len()))
        .collect();
    let padding = crate::utf16::to_string_lossy(&units);
    Ok(if start {
        format!("{padding}{s}")
    } else {
        format!("{s}{padding}")
    })
}

/// V8's radix rejection, shared by `Number.prototype.toString` and
/// `BigInt.prototype.toString` — one string, because they are one message and
/// the two sites had drifted apart ("radix must be" vs V8's "radix argument
/// must be").
const RADIX_RANGE: &str = "toString() radix argument must be between 2 and 36";

/// `BigInt.prototype` methods: `toString([radix])`, `valueOf`, `toLocaleString`.
fn bigint_method(b: &num_bigint::BigInt, name: &str, args: Vec<Value>) -> Result<Value, String> {
    match name {
        "toString" => {
            let radix = match args.first() {
                None | Some(Value::Undef) => 10,
                Some(_) => {
                    let t = arg_num(&args, 0).trunc();
                    if !(2.0..=36.0).contains(&t) {
                        return Err(host::range_error(RADIX_RANGE));
                    }
                    t as u32
                }
            };
            Ok(new_s(b.to_str_radix(radix)))
        }
        // `BigInt.prototype.toLocaleString` groups thousands like the Number
        // one does — `(1234567n).toLocaleString()` is `1,234,567` in node, and
        // returning the bare digits made it the only numeric type that skipped
        // grouping. Same en-US-shaped output as `Number.prototype`; the
        // `locales`/`options` arguments are ignored (no ICU here).
        "toLocaleString" => {
            let digits = b.magnitude().to_string();
            let sign = if b.sign() == num_bigint::Sign::Minus {
                "-"
            } else {
                ""
            };
            Ok(new_s(format!("{sign}{}", group_thousands(&digits))))
        }
        "valueOf" => Ok(with_host(|h| h.new_bigint(b.clone()))),
        _ => Err(host::type_error(&format!("{name} is not a function"))),
    }
}

fn number_method(n: f64, name: &str, args: Vec<Value>) -> Result<Value, String> {
    let args = coerce_numeric_args(NUMBER_METHOD_NUMERIC_ARGS, name, args)?;
    match name {
        "toFixed" => {
            let digits = arg_num(&args, 0);
            if !(0.0..=100.0).contains(&digits.trunc()) {
                return Err(host::range_error(
                    "toFixed() digits argument must be between 0 and 100",
                ));
            }
            Ok(new_s(to_fixed(n, digits as usize)))
        }
        "toExponential" => {
            // `undefined` (or a missing argument) selects the shortest form.
            let f = match args.first() {
                None | Some(Value::Undef) => None,
                Some(_) => {
                    let d = arg_num(&args, 0).trunc();
                    if !(0.0..=100.0).contains(&d) {
                        return Err(host::range_error(
                            "toExponential() argument must be between 0 and 100",
                        ));
                    }
                    Some(d as usize)
                }
            };
            Ok(new_s(to_exponential(n, f)))
        }
        "toString" => {
            // An out-of-range radix THROWS; it does not silently fall back to
            // base 10. `(1).toString(37)` returned "1" here, so a support probe
            // was told every radix worked.
            let radix = match args.first() {
                None | Some(Value::Undef) => 10,
                Some(_) => {
                    let r = arg_num(&args, 0);
                    let t = r.trunc();
                    if !(2.0..=36.0).contains(&t) {
                        return Err(host::range_error(RADIX_RANGE));
                    }
                    t as u32
                }
            };
            if radix == 10 {
                Ok(new_s(host::fmt_number(n)))
            } else {
                Ok(new_s(to_radix(n, radix)))
            }
        }
        "toPrecision" => {
            // `undefined` (or a missing argument) behaves like `toString()`.
            match args.first() {
                None | Some(Value::Undef) => Ok(new_s(host::fmt_number(n))),
                Some(_) => {
                    let p = arg_num(&args, 0).trunc();
                    if !(1.0..=100.0).contains(&p) {
                        return Err(host::range_error(
                            "toPrecision() argument must be between 1 and 100",
                        ));
                    }
                    Ok(new_s(to_precision(n, p as usize)))
                }
            }
        }
        "toLocaleString" => Ok(new_s(to_locale_string(n))),
        "valueOf" => Ok(Value::Float(n)),
        _ => Err(host::type_error(&format!("{name} is not a function"))),
    }
}

/// `Number.prototype.toLocaleString()` with the default locale and options:
/// integer part grouped in threes with `,`, up to 3 fraction digits (rounded
/// half away from zero), trailing fractional zeros dropped. Mirrors V8's default
/// `Intl.NumberFormat().format` output (`(12345.678).toLocaleString()` ⇒
/// `"12,345.678"`; `(1234.5678)` ⇒ `"1,234.568"`). `NaN`, `±Infinity`, and `-0`
/// render as `"NaN"`, `"∞"`/`"-∞"`, and `"-0"`.
fn to_locale_string(n: f64) -> String {
    if n.is_nan() {
        return "NaN".to_string();
    }
    if n.is_infinite() {
        return if n < 0.0 { "-∞" } else { "∞" }.to_string();
    }
    let neg = n.is_sign_negative();
    // Round the magnitude to at most 3 fraction digits, then drop trailing zeros
    // (and a bare trailing point). `to_fixed` rounds half away from zero.
    // `to_fixed` falls back to `ToString` at |x| ≥ 1e21 (spec 21.1.3.3 step 6),
    // which is exponential — and the grouping below then chopped up the
    // exponent, so `(1e21).toLocaleString()` was `1e,+21` instead of node's
    // `1,000,000,000,000,000,000,000`. Expanding the SHORTEST repr is the right
    // source: node groups the shortest decimal form, so `(1e100)
    // .toLocaleString()` is 1 followed by a hundred zeros rather than the exact
    // binary value `1000…159028911…`. (`BigInt(1e100)` is the exact value, a
    // deliberately different rule — see `bigint_ctor`.)
    let fixed = expand_exponential(&to_fixed(n.abs(), 3));
    let trimmed = match fixed.split_once('.') {
        Some(_) => fixed.trim_end_matches('0').trim_end_matches('.'),
        None => fixed.as_str(),
    };
    let (int_part, frac_part) = match trimmed.split_once('.') {
        Some((i, f)) => (i, Some(f)),
        None => (trimmed, None),
    };
    let mut out = String::new();
    if neg {
        out.push('-'); // Intl keeps the sign even for -0.
    }
    out.push_str(&group_thousands(int_part));
    if let Some(f) = frac_part {
        out.push('.');
        out.push_str(f);
    }
    out
}

/// Write a nonnegative decimal string in plain positional form, expanding an
/// `e+NN` exponent into zeros. `"1e+21"` → `"1000000000000000000000"`,
/// `"1.5e+21"` → `"1500000000000000000000"`. A string with no exponent, or a
/// negative exponent (a magnitude below 1, which the caller has already rounded
/// to zero), is returned unchanged.
fn expand_exponential(s: &str) -> String {
    let Some((mantissa, exp)) = s.split_once(['e', 'E']) else {
        return s.to_string();
    };
    let Ok(exp) = exp.trim_start_matches('+').parse::<i32>() else {
        return s.to_string();
    };
    if exp <= 0 {
        return s.to_string();
    }
    let (int_digits, frac_digits) = match mantissa.split_once('.') {
        Some((i, f)) => (i.to_string(), f.to_string()),
        None => (mantissa.to_string(), String::new()),
    };
    let mut digits = int_digits;
    digits.push_str(&frac_digits);
    // The exponent consumes the fractional digits first; whatever is left
    // becomes trailing zeros.
    let zeros = exp as usize - frac_digits.len().min(exp as usize);
    digits.push_str(&"0".repeat(zeros));
    digits
}

/// Insert `,` as a thousands separator into a nonnegative integer digit string.
fn group_thousands(int_part: &str) -> String {
    let bytes = int_part.as_bytes();
    let n = bytes.len();
    let mut out = String::with_capacity(n + n / 3);
    for (i, &b) in bytes.iter().enumerate() {
        if i > 0 && (n - i) % 3 == 0 {
            out.push(',');
        }
        out.push(b as char);
    }
    out
}

/// `Number.prototype.toFixed(f)`: fixed-point with `f` fractional digits, rounding
/// half away from zero on the actual IEEE-754 value (so `(1.005).toFixed(2)` is
/// `"1.00"` because 1.005 is really 1.00499…). The sign of a negative input is
/// preserved even when the rounded magnitude is zero: `(-0.4).toFixed(0) === "-0"`.
///
/// The rounding is done on the value's EXACT decimal expansion (Rust's fixed
/// formatting is exact), not on `x * 10^f` — the latter loses precision for large
/// magnitudes (`(9.999999e20).toFixed(4)` must keep every integer digit).
fn to_fixed(n: f64, f: usize) -> String {
    if !n.is_finite() {
        return host::fmt_number(n);
    }
    // Spec: for |x| ≥ 10^21, toFixed falls back to ToString(x).
    if n.abs() >= 1e21 {
        return host::fmt_number(n);
    }
    let neg = n < 0.0;
    // Exact decimal with guard digits past the rounding position; then round the
    // digit string half-away-from-zero (nonneg operand ⇒ round-half-up).
    let full = format!("{:.*}", f + 25, n.abs());
    let mut body = round_decimal_string(&full, f);
    if neg {
        body.insert(0, '-'); // JS keeps the sign even for "-0" / "-0.00".
    }
    body
}

/// Round the exact decimal string `s` (`"int.frac"`, nonnegative) to `f`
/// fractional digits, half away from zero, propagating carry across the point.
fn round_decimal_string(s: &str, f: usize) -> String {
    let (int_part, frac_part) = s.split_once('.').unwrap_or((s, ""));
    let mut digits: Vec<u8> = int_part
        .bytes()
        .chain(frac_part.bytes())
        .map(|b| b - b'0')
        .collect();
    let point = int_part.len(); // digits before the decimal point
    let keep = point + f; // number of leading digits to keep

    // Round up if the first dropped digit is ≥ 5 (exact-half ⇒ up).
    if digits.get(keep).map(|&d| d >= 5).unwrap_or(false) {
        let mut i = keep;
        loop {
            if i == 0 {
                digits.insert(0, 1);
                // A new leading digit shifts the decimal point right by one.
                return assemble_decimal(&digits, point + 1, f);
            }
            i -= 1;
            if digits[i] == 9 {
                digits[i] = 0;
            } else {
                digits[i] += 1;
                break;
            }
        }
    }
    assemble_decimal(&digits, point, f)
}

/// Reassemble `digits` into `"int.frac"` keeping `f` fractional digits, given that
/// `point` digits precede the decimal point.
fn assemble_decimal(digits: &[u8], point: usize, f: usize) -> String {
    let int_str: String = digits[..point].iter().map(|d| (d + b'0') as char).collect();
    let int_str = int_str.trim_start_matches('0');
    let int_str = if int_str.is_empty() { "0" } else { int_str };
    if f == 0 {
        return int_str.to_string();
    }
    let frac: String = digits[point..point + f]
        .iter()
        .map(|d| (d + b'0') as char)
        .collect();
    format!("{int_str}.{frac}")
}

/// Round the nonnegative finite `a` to `p` significant decimal digits, half away
/// from zero, returning the `p` digits and the decimal exponent `e` such that the
/// value is `0.d…d × 10^(e+1)` (i.e. `d.d…d e±e`). Rust's `{:.*e}` rounds half to
/// EVEN (`(2.5)` at 1 digit would give "2"), but JS rounds half up ("3"), so the
/// exact digits are taken with guard positions and rounded here.
fn round_significant(a: f64, p: usize) -> (String, i32) {
    let sci = format!("{a:.*e}", p - 1 + 25);
    let (mant, exp_str) = sci.split_once('e').expect("LowerExp always has 'e'");
    let mut e: i32 = exp_str.parse().expect("LowerExp exponent is an integer");
    let all: Vec<u8> = mant
        .chars()
        .filter(|c| c.is_ascii_digit())
        .map(|c| c as u8 - b'0')
        .collect();
    let mut s: String = all[..p].iter().map(|d| (d + b'0') as char).collect();
    if all.get(p).map(|&d| d >= 5).unwrap_or(false) {
        // Round the p-digit mantissa up, propagating carry; a carry out of the
        // leading digit (`9.99 → 10`) bumps the decimal exponent by one.
        let mut d: Vec<u8> = all[..p].to_vec();
        let mut i = p;
        loop {
            if i == 0 {
                d.insert(0, 1);
                d.truncate(p);
                e += 1;
                break;
            }
            i -= 1;
            if d[i] == 9 {
                d[i] = 0;
            } else {
                d[i] += 1;
                break;
            }
        }
        s = d.iter().map(|x| (x + b'0') as char).collect();
    }
    (s, e)
}

/// `Number.prototype.toExponential(f)`: one digit before the point and `f` after,
/// with a signed decimal exponent (`(100).toExponential(2) === "1.00e+2"`). With
/// `f` omitted, as many digits as uniquely identify the value are used
/// (`(123456).toExponential() === "1.23456e+5"`). Rounding is half away from zero
/// on the exact value, matching `toPrecision`.
fn to_exponential(n: f64, f: Option<usize>) -> String {
    if !n.is_finite() {
        return host::fmt_number(n);
    }
    let neg = n < 0.0;
    let a = n.abs();
    let (s, e) = if a == 0.0 {
        // Zero has no significant digits: emit "0" padded to the requested width.
        ("0".repeat(f.unwrap_or(0) + 1), 0)
    } else {
        match f {
            Some(f) => round_significant(a, f + 1),
            None => {
                // Shortest round-tripping digits (Rust's `{:e}` is shortest).
                let sci = format!("{a:e}");
                let (mant, exp_str) = sci.split_once('e').expect("LowerExp always has 'e'");
                let digits: String = mant.chars().filter(|c| c.is_ascii_digit()).collect();
                let trimmed = digits.trim_end_matches('0');
                let digits = if trimmed.is_empty() { "0" } else { trimmed };
                (digits.to_string(), exp_str.parse().unwrap_or(0))
            }
        }
    };
    let sign = if e >= 0 { '+' } else { '-' };
    let mag = e.abs();
    let body = if s.len() == 1 {
        format!("{s}e{sign}{mag}")
    } else {
        format!("{}.{}e{sign}{mag}", &s[..1], &s[1..])
    };
    if neg {
        format!("-{body}")
    } else {
        body
    }
}

/// `Number.prototype.toPrecision(p)`: `p` significant digits, switching to
/// exponential form when the decimal exponent `e` satisfies `e < -6` or `e ≥ p`
/// (ECMAScript Number.prototype.toPrecision). Trailing zeros are significant and
/// retained (`(100).toPrecision(5) === "100.00"`).
fn to_precision(n: f64, p: usize) -> String {
    if !n.is_finite() {
        return host::fmt_number(n);
    }
    if n == 0.0 {
        return if p == 1 {
            "0".into()
        } else {
            format!("0.{}", "0".repeat(p - 1))
        };
    }
    let neg = n < 0.0;
    let (s, e) = round_significant(n.abs(), p);
    let pp = p as i32;

    let body = if e < -6 || e >= pp {
        // Exponential: first digit, optional '.rest', signed exponent.
        let sign = if e >= 0 { '+' } else { '-' };
        let mag = e.abs();
        if p == 1 {
            format!("{s}e{sign}{mag}")
        } else {
            format!("{}.{}e{sign}{mag}", &s[..1], &s[1..])
        }
    } else if e >= 0 {
        // e in 0..p-1: (e+1) integer digits, then any remaining as fraction.
        let ip = (e + 1) as usize;
        if ip == p {
            s
        } else {
            format!("{}.{}", &s[..ip], &s[ip..])
        }
    } else {
        // -6 ≤ e < 0: "0." then (−e−1) zeros then all p digits.
        format!("0.{}{}", "0".repeat((-e - 1) as usize), s)
    };
    if neg {
        format!("-{body}")
    } else {
        body
    }
}

/// `Number.prototype.toString(radix)` for radix 2..=36 (radix 10 goes through
/// `fmt_number`). Faithful port of V8's `DoubleToRadixCString`: the integer part
/// is emitted exact, and fractional digits are produced up to the input double's
/// precision (terminating via a ULP-sized `delta`), with round-half-to-even and
/// carry-over back into already-written digits (and into the integer part).
fn to_radix(n: f64, radix: u32) -> String {
    if !n.is_finite() {
        return host::fmt_number(n);
    }
    let digits = b"0123456789abcdefghijklmnopqrstuvwxyz";
    let rf = radix as f64;
    let neg = n < 0.0;
    let value = n.abs();

    let mut integer = value.floor();
    let mut fraction = value - integer;

    // Fraction digits, most-significant first.
    let mut frac: Vec<u8> = Vec::new();
    // Only compute fractional digits down to the input double's precision.
    let mut delta = 0.5 * (next_up(value) - value);
    delta = delta.max(next_up(0.0));
    if fraction >= delta {
        loop {
            // Shift up by one digit.
            fraction *= rf;
            delta *= rf;
            let digit = fraction as usize;
            frac.push(digits[digit]);
            fraction -= digit as f64;
            // Round to even.
            if (fraction > 0.5 || (fraction == 0.5 && (digit & 1) == 1)) && fraction + delta > 1.0 {
                // Carry-over: back-trace already-written fraction digits.
                loop {
                    match frac.pop() {
                        None => {
                            // Carried past the point into the integer part.
                            integer += 1.0;
                            break;
                        }
                        Some(c) => {
                            let d = if c > b'9' {
                                (c - b'a' + 10) as u32
                            } else {
                                (c - b'0') as u32
                            };
                            if d + 1 < radix {
                                frac.push(digits[(d + 1) as usize]);
                                break;
                            }
                            // digit was radix-1: drop it and keep carrying.
                        }
                    }
                }
                break;
            }
            if fraction < delta {
                break;
            }
        }
    }

    // Integer digits, least-significant first (reversed at the end).
    let mut int_out: Vec<u8> = Vec::new();
    // For magnitudes ≥ 2^53, `fmod` loses low bits: pre-fill trailing zeros.
    while v8_exponent(integer / rf) > 0 {
        integer /= rf;
        int_out.push(b'0');
    }
    loop {
        let remainder = integer % rf;
        int_out.push(digits[remainder as usize]);
        integer = (integer - remainder) / rf;
        if integer <= 0.0 {
            break;
        }
    }
    int_out.reverse();

    let mut out: Vec<u8> = Vec::new();
    if neg {
        out.push(b'-');
    }
    out.extend_from_slice(&int_out);
    if !frac.is_empty() {
        out.push(b'.');
        out.extend_from_slice(&frac);
    }
    String::from_utf8(out).unwrap()
}

/// Next representable f64 above `x` (`x` finite, `x ≥ 0`) — V8's `NextDouble`.
fn next_up(x: f64) -> f64 {
    f64::from_bits(x.to_bits() + 1)
}

/// V8's `Double::Exponent`: the binary exponent of the significand-scaled value
/// (`> 0` iff |x| ≥ 2^53). Used to detect integers past `fmod`'s exact range.
fn v8_exponent(x: f64) -> i32 {
    let biased = ((x.to_bits() >> 52) & 0x7ff) as i32;
    if biased == 0 {
        -1074 // denormal
    } else {
        biased - 1075
    }
}

// ══ Map / Set / Symbol / generator methods ═══════════════════════════════════

/// `Map.prototype.set` step 6 and `Set.prototype.add` step 4: a key of `-0` is
/// STORED as `+0`. `map_key` already treats the two as one key (SameValueZero),
/// but the value kept alongside it is what iteration and `console.log` report,
/// and node shows `0` there — `new Map().set(-0, 1)` renders `Map(1) { 0 => 1 }`.
fn normalize_zero_key(v: Value) -> Value {
    match v {
        Value::Float(f) if f == 0.0 && f.is_sign_negative() => Value::Float(0.0),
        other => other,
    }
}

fn map_method(recv: &Value, name: &str, args: Vec<Value>) -> Result<Value, String> {
    match name {
        "get" => {
            let key = with_host(|h| host::map_key(h, &arg0(&args)));
            Ok(with_host(|h| match h.get(recv) {
                Some(JsObj::Map { entries, .. }) => entries
                    .get(&key)
                    .map(|(_, v)| v.clone())
                    .unwrap_or(Value::Undef),
                _ => Value::Undef,
            }))
        }
        "set" => {
            let kv = normalize_zero_key(arg0(&args));
            let vv = args.get(1).cloned().unwrap_or(Value::Undef);
            reject_non_object_weak_key(recv, &kv, "WeakMap")?;
            let key = with_host(|h| host::map_key(h, &kv));
            with_host(|h| {
                if let Some(JsObj::Map { entries, .. }) = h.get_mut(recv) {
                    entries.insert(key, (kv, vv));
                }
            });
            Ok(recv.clone())
        }
        "has" => {
            let key = with_host(|h| host::map_key(h, &arg0(&args)));
            Ok(Value::Bool(with_host(
                |h| matches!(h.get(recv), Some(JsObj::Map { entries, .. }) if entries.contains_key(&key)),
            )))
        }
        "delete" => {
            let key = with_host(|h| host::map_key(h, &arg0(&args)));
            Ok(Value::Bool(with_host(|h| match h.get_mut(recv) {
                Some(JsObj::Map { entries, .. }) => entries.shift_remove(&key).is_some(),
                _ => false,
            })))
        }
        "clear" => {
            with_host(|h| {
                if let Some(JsObj::Map { entries, .. }) = h.get_mut(recv) {
                    entries.clear();
                }
            });
            Ok(Value::Undef)
        }
        "forEach" => {
            let cb = arg0(&args);
            let pairs: Vec<(Value, Value)> = with_host(|h| match h.get(recv) {
                Some(JsObj::Map { entries, .. }) => entries.values().cloned().collect(),
                _ => Vec::new(),
            });
            for (k, v) in pairs {
                host::invoke(&cb, vec![v, k, recv.clone()], this_arg(&args, 1))?;
            }
            Ok(Value::Undef)
        }
        // LIVE, not a snapshot: an entry added during iteration is visited and
        // one deleted before it is reached is not.
        "keys" | "values" | "entries" | "@@iterator" => Ok(collection_iterator(
            recv,
            if name == "@@iterator" {
                "entries"
            } else {
                name
            },
        )),
        _ => Err(host::type_error(&format!("map.{name} is not a function"))),
    }
}

/// A weak collection can only hold objects (and unregistered symbols) — a
/// primitive key is a `TypeError`, which is how packages probe for weak support.
fn reject_non_object_weak_key(recv: &Value, key: &Value, kind: &str) -> Result<(), String> {
    let weak = with_host(|h| {
        matches!(
            h.get(recv),
            Some(JsObj::Map { weak: true, .. }) | Some(JsObj::Set { weak: true, .. })
        )
    });
    if !weak {
        return Ok(());
    }
    let is_object = with_host(|h| match key {
        Value::Obj(_) => !h.is_null(key) && h.as_str(key).is_none() && h.as_bigint(key).is_none(),
        _ => false,
    });
    if is_object {
        return Ok(());
    }
    Err(host::type_error(if kind == "WeakMap" {
        "Invalid value used as weak map key"
    } else {
        "Invalid value used in weak set"
    }))
}

/// A `Set`-like operand of the ES2025 set methods — 24.2.1.2 `GetSetRecord`.
///
/// The seven set operations do NOT require a real `Set` on the right-hand side:
/// anything with a numeric `size` and callable `has`/`keys` participates, which
/// is what lets a `Map`'s key view or a user-written set stand in. The reads
/// happen in this order (`size`, `has`, `keys`) and each failure has its own
/// diagnostic, so a bad operand reports which field was wrong rather than
/// failing later inside the iteration.
struct SetRecord {
    obj: Value,
    /// `size` truncated toward zero, as the spec's `intSize` is; the fractional
    /// part is dropped BEFORE the negative check, so `size: -0.5` truncates to
    /// `-0` and is accepted while `-1.5` reports `'-1' is an invalid size`.
    size: f64,
    has: Value,
    keys: Value,
}

fn get_set_record(other: &Value, method: &str) -> Result<SetRecord, String> {
    if !with_host(|h| is_object_like(h, other)) {
        return Err(host::type_error(&format!(
            "Set.prototype.{method} argument must be an object"
        )));
    }
    let raw = get_property(other, "size")?;
    let num = host::to_number_value(&raw)?;
    if num.is_nan() {
        return Err(host::type_error("The .size property is NaN"));
    }
    let size = num.trunc();
    if size < 0.0 {
        return Err(host::range_error(&format!("'{size}' is an invalid size")));
    }
    let has = get_property(other, "has")?;
    if !with_host(|h| host::is_callable(h, &has)) {
        return Err(host::type_error("string \"has\" is not a function"));
    }
    let keys = get_property(other, "keys")?;
    if !with_host(|h| host::is_callable(h, &keys)) {
        return Err(host::type_error("string \"keys\" is not a function"));
    }
    Ok(SetRecord {
        obj: other.clone(),
        size,
        has,
        keys,
    })
}

impl SetRecord {
    /// `Call(has, obj, [v])`, coerced to a boolean the way the spec's
    /// `ToBoolean(Call(...))` is — a set-like may answer with anything truthy.
    fn has(&self, v: &Value) -> Result<bool, String> {
        let r = host::invoke(&self.has, vec![v.clone()], Some(self.obj.clone()))?;
        Ok(with_host(|h| h.truthy(&r)))
    }

    /// The operand's elements, drained from the iterator its `keys` method
    /// returns. A non-object result is the spec's `Result of the keys method is
    /// not an object`, reported before anything is iterated.
    fn keys(&self) -> Result<Vec<Value>, String> {
        let it = host::invoke(&self.keys, Vec::new(), Some(self.obj.clone()))?;
        if !with_host(|h| is_object_like(h, &it)) {
            return Err(host::type_error(
                "Result of the keys method is not an object",
            ));
        }
        host::drain_iterator(&it)
    }
}

/// The receiver of a set operation must be a real (non-weak) `Set`: these seven
/// methods read `[[SetData]]` directly, so a look-alike cannot stand in on the
/// LEFT even though it can on the right.
fn require_set_receiver(recv: &Value, method: &str) -> Result<(), String> {
    if with_host(|h| matches!(h.get(recv), Some(JsObj::Set { weak: false, .. }))) {
        return Ok(());
    }
    Err(host::type_error(&format!(
        "Method Set.prototype.{method} called on incompatible receiver {}",
        with_host(|h| object_tag(h, recv))
    )))
}

/// The receiver's elements, READ AT THE POINT THE SPEC READS THEM.
///
/// Every one of these operations copies `[[SetData]]` *after* it has touched
/// the operand — `union` and `symmetricDifference` call the operand's `keys`
/// first — so a `keys` (or a `has`) that mutates the receiver is visible in the
/// result. Snapshotting the receiver up front instead dropped such an element:
/// node's `s.union({ keys(){ s.add(99); … } })` contains `99`.
fn set_values(recv: &Value) -> Vec<Value> {
    with_host(|h| match h.get(recv) {
        Some(JsObj::Set { entries, .. }) => entries.values().cloned().collect(),
        _ => Vec::new(),
    })
}

fn set_size(recv: &Value) -> f64 {
    with_host(|h| match h.get(recv) {
        Some(JsObj::Set { entries, .. }) => entries.len() as f64,
        _ => 0.0,
    })
}

/// A fresh, ordinary `Set`. The set operations are NOT species-aware: on node
/// `class S extends Set {}`, `new S([1]).union(other).constructor` is `Set`.
fn new_set(items: Vec<Value>) -> Result<Value, String> {
    let s = with_host(|h| {
        h.alloc(JsObj::Set {
            entries: IndexMap::new(),
            weak: false,
        })
    });
    for v in items {
        set_method(&s, "add", vec![v])?;
    }
    Ok(s)
}

fn set_contains(s: &Value, v: &Value) -> bool {
    let key = with_host(|h| host::map_key(h, v));
    with_host(
        |h| matches!(h.get(s), Some(JsObj::Set { entries, .. }) if entries.contains_key(&key)),
    )
}

/// The seven ES2025 set operations (24.2.4.3, .8, .5, .16, .10, .12, .7).
///
/// Each one branches on the two sizes and iterates the SMALLER side — not an
/// optimization but observable behaviour: which side is walked decides the
/// result's order and whether the operand's `has` or its `keys` is the method
/// that runs. `intersection` of a 3-element receiver with a 2-element operand
/// yields the operand's order, and its `keys` (never its `has`) is called.
fn set_operation(recv: &Value, name: &str, args: Vec<Value>) -> Result<Value, String> {
    require_set_receiver(recv, name)?;
    let other = get_set_record(&arg0(&args), name)?;
    let my_size = set_size(recv);
    match name {
        "union" => {
            let keys = other.keys()?;
            let mut out = set_values(recv);
            out.extend(keys);
            new_set(out)
        }
        "intersection" => {
            let mut out = Vec::new();
            if my_size <= other.size {
                for v in set_values(recv) {
                    if other.has(&v)? {
                        out.push(v);
                    }
                }
            } else {
                for k in other.keys()? {
                    if set_contains(recv, &k) {
                        out.push(k);
                    }
                }
            }
            new_set(out)
        }
        "difference" => {
            if my_size <= other.size {
                let mut out = Vec::new();
                for v in set_values(recv) {
                    if !other.has(&v)? {
                        out.push(v);
                    }
                }
                return new_set(out);
            }
            let out = new_set(set_values(recv))?;
            for k in other.keys()? {
                set_method(&out, "delete", vec![k])?;
            }
            Ok(out)
        }
        "symmetricDifference" => {
            // The operand is drained FIRST — the spec takes the iterator before
            // it copies `[[SetData]]`, so a `keys` that mutates the receiver is
            // reflected in the result.
            let keys = other.keys()?;
            let out = new_set(set_values(recv))?;
            for k in keys {
                if set_contains(recv, &k) {
                    set_method(&out, "delete", vec![k])?;
                } else {
                    set_method(&out, "add", vec![k])?;
                }
            }
            Ok(out)
        }
        "isSubsetOf" => {
            if my_size > other.size {
                return Ok(Value::Bool(false));
            }
            for v in set_values(recv) {
                if !other.has(&v)? {
                    return Ok(Value::Bool(false));
                }
            }
            Ok(Value::Bool(true))
        }
        "isSupersetOf" => {
            if my_size < other.size {
                return Ok(Value::Bool(false));
            }
            for k in other.keys()? {
                if !set_contains(recv, &k) {
                    return Ok(Value::Bool(false));
                }
            }
            Ok(Value::Bool(true))
        }
        "isDisjointFrom" => {
            if my_size <= other.size {
                for v in set_values(recv) {
                    if other.has(&v)? {
                        return Ok(Value::Bool(false));
                    }
                }
            } else {
                for k in other.keys()? {
                    if set_contains(recv, &k) {
                        return Ok(Value::Bool(false));
                    }
                }
            }
            Ok(Value::Bool(true))
        }
        _ => Err(host::type_error(&format!("set.{name} is not a function"))),
    }
}

fn set_method(recv: &Value, name: &str, args: Vec<Value>) -> Result<Value, String> {
    match name {
        "add" => {
            let vv = normalize_zero_key(arg0(&args));
            reject_non_object_weak_key(recv, &vv, "WeakSet")?;
            let key = with_host(|h| host::map_key(h, &vv));
            with_host(|h| {
                if let Some(JsObj::Set { entries, .. }) = h.get_mut(recv) {
                    entries.insert(key, vv);
                }
            });
            Ok(recv.clone())
        }
        "has" => {
            let key = with_host(|h| host::map_key(h, &arg0(&args)));
            Ok(Value::Bool(with_host(
                |h| matches!(h.get(recv), Some(JsObj::Set { entries, .. }) if entries.contains_key(&key)),
            )))
        }
        "delete" => {
            let key = with_host(|h| host::map_key(h, &arg0(&args)));
            Ok(Value::Bool(with_host(|h| match h.get_mut(recv) {
                Some(JsObj::Set { entries, .. }) => entries.shift_remove(&key).is_some(),
                _ => false,
            })))
        }
        "clear" => {
            with_host(|h| {
                if let Some(JsObj::Set { entries, .. }) = h.get_mut(recv) {
                    entries.clear();
                }
            });
            Ok(Value::Undef)
        }
        "forEach" => {
            let cb = arg0(&args);
            let vals: Vec<Value> = with_host(|h| match h.get(recv) {
                Some(JsObj::Set { entries, .. }) => entries.values().cloned().collect(),
                _ => Vec::new(),
            });
            for v in vals {
                host::invoke(&cb, vec![v.clone(), v, recv.clone()], this_arg(&args, 1))?;
            }
            Ok(Value::Undef)
        }
        "union"
        | "intersection"
        | "difference"
        | "symmetricDifference"
        | "isSubsetOf"
        | "isSupersetOf"
        | "isDisjointFrom" => set_operation(recv, name, args),
        // LIVE, as for `Map`. A Set's `keys` and `values` are the same thing.
        "keys" | "values" | "entries" | "@@iterator" => Ok(collection_iterator(
            recv,
            if name == "entries" {
                "entries"
            } else {
                "values"
            },
        )),
        _ => Err(host::type_error(&format!("set.{name} is not a function"))),
    }
}

fn generator_method(recv: &Value, name: &str, args: Vec<Value>) -> Result<Value, String> {
    // A generator IS its own iterator: both symbol forms return the receiver.
    if matches!(name, "@@iterator" | "@@asyncIterator") {
        return Ok(recv.clone());
    }
    // An `async function*` object's methods return PROMISES of the record, and
    // its body has to be driven through the await-aware stepper (a plain
    // `gen_resume` would surface an internal `await` suspension as a bogus yield).
    if host::is_async_generator(recv) {
        // All three go through `[[AsyncGeneratorQueue]]` (ECMA-262 27.6.3.6):
        // `.return`/`.throw` must wait behind a `.next()` that is still
        // suspended on an internal `await`, or that `.next()` would report
        // `{done: true}` for a value the body had not yet reached. An uncaught
        // `.throw(e)` rejects the returned promise; it does not throw here.
        return match name {
            "next" => Ok(host::async_gen_enqueue(
                recv,
                host::GenReq::Next(arg0(&args)),
            )),
            "return" => Ok(host::async_gen_enqueue(
                recv,
                host::GenReq::Return(arg0(&args)),
            )),
            "throw" => Ok(host::async_gen_enqueue(
                recv,
                host::GenReq::Throw(arg0(&args)),
            )),
            "@@asyncIterator" => Ok(recv.clone()),
            _ => Err(host::type_error(&format!(
                "asyncGenerator.{name} is not a function"
            ))),
        };
    }
    match name {
        "next" => {
            let send = arg0(&args);
            match host::gen_resume(recv, send)? {
                host::GenStep::Yield(v) => Ok(iter_result(v, false)),
                host::GenStep::Done(v) => Ok(iter_result(v, true)),
            }
        }
        "return" => {
            // Resume with an injected return so any pending `finally` runs; the
            // completion may itself be a `finally` yield (not-done) or the value.
            match host::gen_return(recv, arg0(&args))? {
                host::GenStep::Yield(v) => Ok(iter_result(v, false)),
                host::GenStep::Done(v) => Ok(iter_result(v, true)),
            }
        }
        "throw" => {
            // Inject a throw at the suspension point: an enclosing `try/catch` in
            // the body can handle it (and any `finally` runs); otherwise it
            // propagates to the caller.
            match host::gen_throw(recv, arg0(&args))? {
                host::GenStep::Yield(v) => Ok(iter_result(v, false)),
                host::GenStep::Done(v) => Ok(iter_result(v, true)),
            }
        }
        _ => Err(host::type_error(&format!(
            "generator.{name} is not a function"
        ))),
    }
}

/// A `{ value, done }` iterator-result object.
fn iter_result(value: Value, done: bool) -> Value {
    with_host(|h| {
        let mut m: IndexMap<String, Value> = IndexMap::new();
        m.insert("value".into(), value);
        m.insert("done".into(), Value::Bool(done));
        h.new_object(m)
    })
}

/// A live iterator over array `arr` — what `keys()`, `values()`, `entries()`
/// and `Symbol.iterator` return, and what a `for-of` over an array steps.
pub(crate) fn array_iterator(arr: &Value, kind: host::ArrayIterKind) -> Value {
    with_host(|h| {
        h.alloc(JsObj::Iter {
            items: Vec::new(),
            idx: 0,
            array: Some((arr.clone(), kind)),
        })
    })
}

/// One step of a `JsObj::Iter`: `None` when `it` is not one, `Some(None)` once
/// it is exhausted, otherwise the next value.
///
/// An array iterator reads the array at every step (23.1.5.1
/// `%ArrayIteratorPrototype%.next`): the length is re-read, so an element
/// pushed during a `for-of` is visited and one popped is not, and the element
/// is read as `a[i]` reads it — an accessor runs, a hole reads through the
/// prototype. Once it reports done it stays done, even if the array grows.
pub(crate) fn iter_step(it: &Value) -> Option<Option<Value>> {
    use host::ArrayIterKind;
    // One host borrow for the common case — a snapshot, or an array slot that
    // is neither a hole nor an accessor. `Err` carries what only `[[Get]]` can
    // read: the array, the kind and the index.
    let step = with_host(|h| {
        let (arr, kind, i) = match h.get_mut(it) {
            Some(JsObj::Iter {
                items,
                idx,
                array: None,
            }) => {
                let v = items.get(*idx).cloned();
                if v.is_some() {
                    *idx += 1;
                }
                return Some(Ok(v));
            }
            Some(JsObj::Iter {
                idx,
                array: Some((arr, kind)),
                ..
            }) => (arr.clone(), *kind, *idx),
            _ => return None,
        };
        // `usize::MAX` marks an iterator that has already reported done, and
        // it stays done even if the array grows.
        let len = match h.get(&arr) {
            Some(JsObj::Array(items)) => items.len(),
            _ => 0,
        };
        let done = i == usize::MAX || i >= len;
        if let Some(JsObj::Iter { idx, .. }) = h.get_mut(it) {
            *idx = if done { usize::MAX } else { i + 1 };
        }
        if done {
            return Some(Ok(None));
        }
        let key = Value::Float(i as f64);
        let slot = match (kind, h.get(&arr)) {
            (ArrayIterKind::Keys, _) => return Some(Ok(Some(key))),
            (_, Some(JsObj::Array(items)))
                if !h.is_hole(&arr, i) && h.own_accessor_keys(&arr).is_empty() =>
            {
                items[i].clone()
            }
            _ => return Some(Err((arr, kind, i))),
        };
        Some(Ok(Some(match kind {
            ArrayIterKind::Entries => h.new_array(vec![key, slot]),
            _ => slot,
        })))
    })?;
    let (arr, kind, i) = match step {
        Ok(step) => return Some(step),
        Err(slow) => slow,
    };
    let value = get_property(&arr, &i.to_string()).unwrap_or(Value::Undef);
    Some(Some(match kind {
        ArrayIterKind::Entries => with_host(|h| h.new_array(vec![Value::Float(i as f64), value])),
        _ => value,
    }))
}

/// Built-in iterator object (`arr.values()`, `arr[Symbol.iterator]()`): a
/// cursor over a snapshot, or live over an array ([`iter_step`]).
fn iter_method(recv: &Value, name: &str, args: Vec<Value>) -> Result<Value, String> {
    match name {
        "next" => Ok(match iter_step(recv).flatten() {
            Some(v) => iter_result(v, false),
            None => iter_result(Value::Undef, true),
        }),
        "return" => {
            // Exhaust the cursor and report done.
            with_host(|h| {
                if let Some(JsObj::Iter { items, idx, array }) = h.get_mut(recv) {
                    *idx = if array.is_some() {
                        usize::MAX
                    } else {
                        items.len()
                    };
                }
            });
            Ok(iter_result(arg0(&args), true))
        }
        // An iterator is its own iterable.
        "@@iterator" => Ok(recv.clone()),
        _ => Err(host::type_error(&format!(
            "iterator.{name} is not a function"
        ))),
    }
}

fn symbol_method(recv: &Value, name: &str, _args: Vec<Value>) -> Result<Value, String> {
    match name {
        "toString" => Ok(with_host(|h| {
            let s = h.str_of(recv);
            h.new_str(s)
        })),
        // 20.4.3.5: `Symbol.prototype[@@toPrimitive]` returns the symbol
        // itself for EVERY hint — it ignores its argument. That is what makes
        // `sym + ''` a TypeError rather than a concatenation: the conversion
        // succeeds and hands back a symbol, and it is `+` that then rejects it.
        "@@toPrimitive" | "valueOf" => Ok(recv.clone()),
        _ => Err(host::type_error(&format!(
            "symbol.{name} is not a function"
        ))),
    }
}

// ══ Object.* prototype helpers, `in`, deep clone ═════════════════════════════

fn object_create(args: Vec<Value>) -> Result<Value, String> {
    let proto = arg0(&args);
    // 20.1.2.2 step 1: the prototype must be an Object or exactly `null`.
    // `undefined` is NOT accepted — measured on node v26.7.0,
    // `Object.create(undefined)` is
    // `TypeError: Object prototype may only be an Object or null: undefined`,
    // where node-js quietly built a normal object.
    reject_bad_prototype(&proto)?;
    let obj = with_host(|h| h.new_object(IndexMap::new()));
    // `set_proto` records a null proto as an explicit null-prototype object.
    with_host(|h| h.set_proto(&obj, proto));
    // Optional second arg: a property-descriptor map.
    if let Some(descs) = args.get(1).filter(|d| !matches!(d, Value::Undef)) {
        let entries: Vec<(String, Value)> = with_host(|h| match h.get(descs) {
            Some(JsObj::Object(p)) => p.iter().map(|(k, v)| (k.clone(), v.clone())).collect(),
            _ => Vec::new(),
        });
        for (k, d) in entries {
            apply_descriptor(&obj, &k, &d)?;
        }
    }
    Ok(obj)
}

/// The enumerable method names of a builtin `<Ctor>.prototype` namespace that
/// supports being copied via `mixin`/`getOwnPropertyNames`. Currently only
/// `EventEmitter.prototype` (the one express mixes onto its app function).
/// The own property names of `<Ctor>.prototype`, and whether each is
/// enumerable, from the generated [`crate::arity::PROTO_MEMBERS`] table.
///
/// `Object.getOwnPropertyNames(Map.prototype)` answered `[]` for every
/// intrinsic — the members are reachable by NAME through the `@proto:` thunks
/// but were not enumerable, so feature detection that walks a prototype found
/// nothing there. The table is read from the reference engine rather than
/// derived from the arity table because the arity table holds functions only:
/// `Map.prototype.size`, `RegExp.prototype.source` and the twelve
/// `URL.prototype` components are accessors.
fn intrinsic_proto_members(ns: &str) -> Option<&'static [&'static str]> {
    let ctor = ns.strip_suffix(".prototype")?;
    crate::arity::PROTO_MEMBERS
        .binary_search_by(|(k, _)| (*k).cmp(ctor))
        .ok()
        .map(|i| crate::arity::PROTO_MEMBERS[i].1)
}

fn builtin_proto_method_names(ns: &str) -> Option<&'static [&'static str]> {
    match ns {
        "EventEmitter.prototype" => Some(crate::stdlib::events::METHODS),
        _ => None,
    }
}

/// The own SYMBOL-keyed property keys of `v` as symbol values. A Proxy's come
/// from its `ownKeys` trap (the symbol half of the same list the string keys are
/// filtered out of); every other receiver answers from its property map.
fn proxy_or_own_symbol_keys(v: &Value) -> Result<Vec<Value>, String> {
    if let Some(keys) = crate::proxy::own_keys(v)? {
        return Ok(keys
            .iter()
            .filter(|k| host::is_symbol_key(k))
            .map(|k| crate::proxy::key_value(k))
            .collect());
    }
    // An intrinsic prototype's symbol-keyed members come from the generated
    // table, which is the only record of them: they own no map entry, so
    // `Object.getOwnPropertySymbols(Array.prototype)` was `[]` where node
    // reports `Symbol.iterator` and `Symbol.unscopables`.
    if let Some(ns) = intrinsic_proto_of(v).map(|c| format!("{c}.prototype")) {
        if let Some(members) = intrinsic_proto_members(&ns) {
            return Ok(with_host(|h| {
                members
                    .iter()
                    .filter_map(|m| m.strip_prefix('+').unwrap_or(m).strip_prefix("@@"))
                    .map(|name| h.well_known_symbol(name))
                    .collect()
            }));
        }
    }
    Ok(with_host(|h| h.own_symbol_keys(v)))
}

/// `[[DefineOwnProperty]]` reachable from `crate::proxy`'s no-trap forward.
pub fn define_property_pub(obj: &Value, key: Value, desc: Value) -> Result<Value, String> {
    object_define_property(vec![obj.clone(), key, desc])
}

/// `[[GetOwnProperty]]` reachable from `crate::proxy`'s no-trap forward.
pub fn own_descriptor_pub(obj: &Value, key: Value) -> Result<Value, String> {
    object_get_own_descriptor(vec![obj.clone(), key])
}

fn object_define_property(args: Vec<Value>) -> Result<Value, String> {
    let obj = arg0(&args);
    // A Proxy defines through its `defineProperty` trap; the target it forwards
    // to is where the ordinary path below finally runs.
    if with_host(|h| h.kind_of(&obj)) == Some(ObjKind::Proxy) {
        let key = host::to_property_key(&args.get(1).cloned().unwrap_or(Value::Undef))?;
        let desc = args.get(2).cloned().unwrap_or(Value::Undef);
        if !with_host(|h| is_object_like(h, &desc)) {
            return Err(host::type_error(&format!(
                "Property description must be an object: {}",
                with_host(|h| h.str_of(&desc))
            )));
        }
        // `Object.defineProperty` THROWS on a refusing trap — in sloppy code
        // too. `Reflect.defineProperty` is the form that reports `false`.
        if !crate::proxy::define_property(&obj, &key, &desc)? {
            return Err(host::type_error(&format!(
                "'defineProperty' on proxy: trap returned falsish for property '{key}'"
            )));
        }
        return Ok(obj);
    }
    // 20.1.2.4 steps 1-3, both of which node-js skipped entirely: a non-object
    // target and a non-object descriptor each throw before anything is written.
    if !with_host(|h| is_object_like(h, &obj)) {
        return Err(host::type_error(
            "Object.defineProperty called on non-object",
        ));
    }
    let desc = args.get(2).cloned().unwrap_or(Value::Undef);
    if !with_host(|h| is_object_like(h, &desc)) {
        return Err(host::type_error(&format!(
            "Property description must be an object: {}",
            with_host(|h| h.str_of(&desc))
        )));
    }
    let key = host::to_property_key(&args.get(1).cloned().unwrap_or(Value::Undef))?;
    apply_descriptor(&obj, &key, &desc)?;
    Ok(obj)
}

/// Every `Reflect` method requires an OBJECT target and reports a `TypeError`
/// for anything else (28.1). A primitive was being accepted and silently
/// producing nothing.
/// `CreateListFromArrayLike` (7.3.18) — the argument list `Reflect.apply` and
/// `Reflect.construct` take.
///
/// An ARRAY-LIKE counts: `{length: 2, 0: 1, 1: 5}` is a two-element list. The
/// iterator was being used instead, so an array-like produced nothing and a
/// primitive produced nothing rather than the TypeError node raises.
/// Whether `p` is ALREADY `obj`'s prototype — the one case a non-extensible
/// object still accepts, because it changes nothing.
///
/// The observable prototype, not the stored link: an ordinary object has no
/// explicit link and inherits `Object.prototype`, so comparing the raw slot
/// reported "different" for `setPrototypeOf(frozen, Object.prototype)`.
/// Whether making `p` the prototype of `obj` would create a CYCLE — 10.1.2.1
/// step 8 walks up from `p` looking for `obj`.
///
/// Without the check `Object.setPrototypeOf(a, b)` followed by the reverse
/// built a ring. Nothing hung, because every chain walk in this host carries a
/// hop limit, but a lookup then silently gave up instead of finding a property
/// that really was there.
fn would_cycle(obj: &Value, p: &Value) -> bool {
    let mut cur = Some(p.clone());
    for _ in 0..1000 {
        let Some(c) = cur else { return false };
        if with_host(|h| h.strict_eq(&c, obj)) {
            return true;
        }
        // A PROXY's prototype is its handler's business; the spec skips the
        // walk entirely when one is in the chain.
        if with_host(|h| h.kind_of(&c)) == Some(ObjKind::Proxy) {
            return false;
        }
        cur = with_host(|h| h.proto_of(&c));
    }
    false
}

fn same_prototype(obj: &Value, p: &Value) -> bool {
    let cur = prototype_of(obj);
    with_host(|h| h.strict_eq(&cur, p) || (h.is_null(&cur) && h.is_null(p)))
}

fn create_list_from_array_like(v: &Value) -> Result<Vec<Value>, String> {
    if !with_host(|h| is_object_like(h, v)) {
        return Err(host::type_error(
            "CreateListFromArrayLike called on non-object",
        ));
    }
    let len = get_property(v, "length")?;
    let n = with_host(|h| h.to_number(&len));
    let n = if n.is_finite() && n > 0.0 {
        n as usize
    } else {
        0
    };
    (0..n).map(|i| get_property(v, &i.to_string())).collect()
}

fn reflect_require_object(v: &Value, method: &str) -> Result<(), String> {
    if with_host(|h| is_object_like(h, v)) {
        return Ok(());
    }
    Err(host::type_error(&format!(
        "Reflect.{method} called on non-object"
    )))
}

/// Whether `v` is an Object in the language sense — anything `typeof` calls
/// `"object"` (bar `null`) or `"function"`. Used by the argument checks that
/// distinguish "an object" from a primitive.
fn is_object_like(h: &host::JsHost, v: &Value) -> bool {
    matches!(v, Value::Obj(_)) && !h.is_null(v) && !host::is_primitive(h, v)
}

/// `RequireObjectCoercible(v)` — 7.2.1. The check in front of every `ToObject`,
/// which node-js was missing on the whole `Object.keys`/`values`/`entries`/
/// `getOwnPropertyNames`/`getOwnPropertySymbols`/`getOwnPropertyDescriptor`/
/// `assign` family: each returned an empty result for `null` where node v26.7.0
/// throws `TypeError: Cannot convert undefined or null to object`. A PRIMITIVE
/// is coercible and keeps working (`Object.keys(1)` is `[]`).
fn require_object_coercible(v: &Value) -> Result<(), String> {
    if with_host(|h| matches!(v, Value::Undef) || h.is_null(v)) {
        return Err(host::type_error(
            "Cannot convert undefined or null to object",
        ));
    }
    Ok(())
}

/// 10.1.2 / 20.1.2.2 step 1: reject a `[[Prototype]]` that is neither an Object
/// nor `null`, with V8's wording. Measured on node v26.7.0:
/// `Object.create("s")` is
/// `TypeError: Object prototype may only be an Object or null: s`.
fn reject_bad_prototype(proto: &Value) -> Result<(), String> {
    if with_host(|h| h.is_null(proto) || is_object_like(h, proto)) {
        return Ok(());
    }
    Err(host::type_error(&format!(
        "Object prototype may only be an Object or null: {}",
        with_host(|h| h.str_of(proto))
    )))
}

/// Apply a `{ value | get | set }` descriptor object to `obj[key]`.
///
/// Per ECMAScript `ToPropertyDescriptor`, an omitted `writable`/`enumerable`/
/// `configurable` field defaults to **false** — which is why a `defineProperty`
/// data property is invisible to `Object.keys` unless the caller opts in. That
/// asymmetry against plain assignment is the whole reason the attribute table
/// exists.
/// The requested fields of a property descriptor — 10.1.6.2
/// `ToPropertyDescriptor`. Each is `None` when the descriptor omits it, which
/// is the distinction the merge below turns on: an omitted field LEAVES an
/// existing attribute alone rather than resetting it.
struct Requested {
    value: Option<Value>,
    get: Option<Option<Value>>,
    set: Option<Option<Value>>,
    writable: Option<bool>,
    enumerable: Option<bool>,
    configurable: Option<bool>,
}

impl Requested {
    /// Reads through the prototype chain, as `ToPropertyDescriptor`'s
    /// `HasProperty`/`Get` pairs do — a descriptor built with
    /// `Object.create({ value: 1 })` is legal.
    fn read(desc: &Value) -> Self {
        let has = |k: &str| {
            with_host(|h| {
                host::lookup_chain(h, desc, k).is_some()
                    || host::lookup_accessor(h, desc, k).is_some()
            })
        };
        let val = |k: &str| get_property(desc, k).unwrap_or(Value::Undef);
        // Resolve the value BEFORE the borrow: `val` re-enters the host, and
        // doing it inside the `with_host` closure aborts on the double borrow.
        let flag = |k: &str| {
            has(k).then(|| {
                let v = val(k);
                with_host(|h| h.truthy(&v))
            })
        };
        Requested {
            value: has("value").then(|| val("value")),
            get: has("get").then(|| match val("get") {
                Value::Undef => None,
                g => Some(g),
            }),
            set: has("set").then(|| match val("set") {
                Value::Undef => None,
                st => Some(st),
            }),
            writable: flag("writable"),
            enumerable: flag("enumerable"),
            configurable: flag("configurable"),
        }
    }

    fn is_accessor(&self) -> bool {
        self.get.is_some() || self.set.is_some()
    }

    fn is_data(&self) -> bool {
        self.value.is_some() || self.writable.is_some()
    }
}

/// The own property already at `key`, if any, read back through
/// `Object.getOwnPropertyDescriptor` so every object kind (array indices, the
/// fn-prop side table, Buffer bytes) is covered by one code path.
struct Existing {
    accessor: bool,
    value: Value,
    get: Option<Value>,
    set: Option<Value>,
    writable: bool,
    enumerable: bool,
    configurable: bool,
}

fn existing_property(obj: &Value, key: &str) -> Option<Existing> {
    let k = with_host(|h| h.new_str(key.to_string()));
    let d = own_descriptor_pub(obj, k).ok()?;
    if matches!(d, Value::Undef) {
        return None;
    }
    let field = |n: &str| get_property(&d, n).unwrap_or(Value::Undef);
    let truthy = |n: &str| {
        let v = field(n);
        with_host(|h| h.truthy(&v))
    };
    let accessor = with_host(|h| host::lookup_chain(h, &d, "get").is_some());
    Some(Existing {
        accessor,
        value: field("value"),
        get: match field("get") {
            Value::Undef => None,
            g => Some(g),
        },
        set: match field("set") {
            Value::Undef => None,
            st => Some(st),
        },
        writable: truthy("writable"),
        enumerable: truthy("enumerable"),
        configurable: truthy("configurable"),
    })
}

/// SameValue (7.2.11) — `===` except that `NaN` equals itself and `+0` and
/// `-0` are distinct. 10.1.6.3 compares a redefined value against the current
/// one with this, not with strict equality.
pub(crate) fn same_value(a: &Value, b: &Value) -> bool {
    let num = |v: &Value| match v {
        Value::Int(n) => Some(*n as f64),
        Value::Float(f) => Some(*f),
        _ => None,
    };
    match (num(a), num(b)) {
        (Some(x), Some(y)) => {
            if x.is_nan() && y.is_nan() {
                true
            } else if x == 0.0 && y == 0.0 {
                x.is_sign_negative() == y.is_sign_negative()
            } else {
                x == y
            }
        }
        _ => with_host(|h| h.strict_eq(a, b)),
    }
}

/// 10.1.6.3 `ValidateAndApplyPropertyDescriptor`.
///
/// None of the validation existed: every `Object.defineProperty` was applied
/// unconditionally, so redefining a non-configurable property silently
/// succeeded where node throws. Worse in practice, an OMITTED field was read as
/// `false` rather than "leave alone", so the ordinary
/// `Object.defineProperty(o, 'k', { enumerable: false })` also stripped
/// `writable` and `configurable` from a property that had both.
///
/// Converting an accessor to a data property did not take effect at all: the
/// value was written but the accessor stayed in its side table, and accessors
/// win on read, so the getter kept answering.
fn apply_descriptor(obj: &Value, key: &str, desc: &Value) -> Result<(), String> {
    let req = Requested::read(desc);
    let cur = existing_property(obj, key);

    // An array's `length` is the exotic own property whose write resizes the
    // array (10.4.2.1); routing it through the ordinary path stored a shadowing
    // key and left the elements untouched.
    if key == "length" && with_host(|h| h.kind_of(obj)) == Some(ObjKind::Array) {
        if let Some(v) = req.value.clone() {
            return set_property_pub(obj, "length", v);
        }
    }

    // The other exotics whose own properties are SYNTHESIZED rather than stored
    // in a property map: a typed array's elements and a RegExp's `lastIndex`.
    // The ordinary path below writes a shadowing map entry the read never
    // consults, so `Object.defineProperty(u8, '0', {value: 9})` left `u8[0]`
    // unchanged.
    // A builtin namespace/prototype has no property map either, so a data
    // descriptor has to reach the same side table an assignment does.
    // `Object.defineProperty(Array.prototype, 'at', {value: impl})` — how a
    // careful polyfill installs itself, precisely to avoid the enumerable
    // property a bare assignment creates — wrote a map entry nothing read.
    if with_host(|h| h.kind_of(obj)) == Some(ObjKind::Builtin) {
        if let Some(v) = req.value.clone() {
            return set_property_pub(obj, key, v);
        }
    }
    let exotic_own = (crate::stdlib::native_tag(obj).as_deref() == Some("TypedArray")
        && key.parse::<usize>().is_ok())
        || (key == "lastIndex" && with_host(|h| matches!(h.get(obj), Some(JsObj::RegExp(_)))));
    if exotic_own {
        if let Some(v) = req.value.clone() {
            return set_property_pub(obj, key, v);
        }
    }

    // 10.1.6.3 step 2: a NEW property cannot be added to a non-extensible
    // object. Only an existing property's attributes were being validated, so
    // `defineProperty(Object.freeze({}), 'z', …)` silently added one.
    if cur.is_none() && !with_host(|h| h.is_extensible(obj)) {
        return Err(host::type_error(&format!(
            "Cannot define property {key}, object is not extensible"
        )));
    }
    if let Some(c) = &cur {
        if !c.configurable {
            let rejected = req.configurable == Some(true)
                || req.enumerable.is_some_and(|e| e != c.enumerable)
                || (req.is_accessor() && !c.accessor)
                || (req.is_data() && c.accessor)
                || (c.accessor
                    && ((req.get.is_some() && req.get.clone().flatten() != c.get)
                        || (req.set.is_some() && req.set.clone().flatten() != c.set)))
                || (!c.accessor
                    && !c.writable
                    && (req.writable == Some(true)
                        || req.value.as_ref().is_some_and(|v| !same_value(v, &c.value))));
            if rejected {
                return Err(host::type_error(&format!(
                    "Cannot redefine property: {key}"
                )));
            }
        }
    }

    // An omitted field keeps what the property already had; a brand-new
    // property defaults every one of them to false.
    let attrs = host::PropAttrs {
        writable: req
            .writable
            .unwrap_or(cur.as_ref().is_some_and(|c| c.writable)),
        enumerable: req
            .enumerable
            .unwrap_or(cur.as_ref().is_some_and(|c| c.enumerable)),
        configurable: req
            .configurable
            .unwrap_or(cur.as_ref().is_some_and(|c| c.configurable)),
    };
    with_host(|h| h.set_prop_attrs(obj, key, attrs));

    if req.is_accessor() {
        let get = req
            .get
            .clone()
            .unwrap_or_else(|| cur.as_ref().and_then(|c| c.get.clone()));
        let set = req
            .set
            .clone()
            .unwrap_or_else(|| cur.as_ref().and_then(|c| c.set.clone()));
        // An ACCESSOR at an index past the end still extends the array
        // (10.4.2.1): `Object.defineProperty([1], '4', {get})` gives
        // `length === 5` with holes between. Only the DATA path grew it, so
        // the accessor landed in the side table while `length` stayed put —
        // and with it out of range, `Object.keys` and `JSON.stringify` never
        // saw the index at all.
        if let (Some(ObjKind::Array), Ok(i)) = (with_host(|h| h.kind_of(obj)), key.parse::<usize>())
        {
            with_host(|h| {
                let old_len = match h.get(obj) {
                    Some(JsObj::Array(items)) => items.len(),
                    _ => 0,
                };
                if i >= old_len {
                    if let Some(JsObj::Array(items)) = h.get_mut(obj) {
                        items.resize(i + 1, Value::Undef);
                    }
                    h.mark_hole_range(obj, old_len..i + 1);
                }
            });
        }
        with_host(|h| h.set_accessor(obj, key, get, set));
        return Ok(());
    }

    if let Some(c) = &cur {
        if c.accessor {
            if !req.is_data() {
                // A generic descriptor — flags only — leaves an accessor an
                // accessor. They were already applied above.
                return Ok(());
            }
            let v = req.value.clone().unwrap_or(Value::Undef);
            with_host(|h| h.accessor_to_data(obj, key, v));
            return Ok(());
        }
    }

    let Some(v) = req.value else {
        // Nothing to write: a flags-only redefinition of a data property.
        return Ok(());
    };
    write_data_slot(obj, key, v);
    Ok(())
}

/// Store `v` as an own data property, in whichever slot the object kind keeps
/// its own properties.
fn write_data_slot(obj: &Value, key: &str, v: Value) {
    // A function/class receiver stores its own props in the fn-prop side table
    // (express `mixin(app, proto)` defines methods onto the `app` *function*).
    if matches!(
        with_host(|h| h.get(obj).cloned()),
        Some(JsObj::Func(_)) | Some(JsObj::Class(_))
    ) || uses_side_table(obj)
    {
        with_host(|h| h.set_fn_prop(obj, key, v));
        return;
    }
    if let (Some(ObjKind::Array), Ok(i)) = (with_host(|h| h.kind_of(obj)), key.parse::<usize>()) {
        // An array's index keys ARE its elements, and defining one past the end
        // grows the array with holes in between (10.4.2.1). This whole branch
        // used to be missing: `Object.defineProperty(arr, 1, {value})` wrote
        // into the ordinary property map an array does not have, so it was a
        // silent no-op.
        with_host(|h| {
            let old = match h.get(obj) {
                Some(JsObj::Array(items)) => items.len(),
                _ => 0,
            };
            if let Some(JsObj::Array(items)) = h.get_mut(obj) {
                if i >= old {
                    items.resize(i + 1, Value::Undef);
                }
                items[i] = v;
            }
            if i > old {
                h.mark_hole_range(obj, old..i);
            }
            h.clear_hole(obj, i);
        });
        return;
    }
    with_host(|h| {
        if let Some(JsObj::Object(p)) = h.get_mut(obj) {
            p.insert(key.to_string(), v);
            host::canonicalize_own_keys(p);
        }
    });
}

/// `Object.defineProperties(obj, descriptorMap)`.
fn object_define_properties(args: Vec<Value>) -> Result<Value, String> {
    let obj = arg0(&args);
    let descs = args.get(1).cloned().unwrap_or(Value::Undef);
    let entries: Vec<(String, Value)> = with_host(|h| match h.get(&descs) {
        Some(JsObj::Object(p)) => p.iter().map(|(k, v)| (k.clone(), v.clone())).collect(),
        _ => Vec::new(),
    });
    for (k, d) in entries {
        apply_descriptor(&obj, &k, &d)?;
    }
    Ok(obj)
}

/// The descriptor of an own property a function, a typed array or a RegExp
/// SYNTHESIZES rather than keeping in a property map.
///
/// These read back through the ordinary path but owned no descriptor and did
/// not appear under `hasOwnProperty` or `getOwnPropertyNames`, so the five
/// views of "does this property exist" disagreed — a read said yes while
/// `Object.getOwnPropertyDescriptor(f, 'name')` said no such property, which is
/// what a shim checks before patching.
fn synthesized_own_descriptor(obj: &Value, key: &str) -> Option<(Value, host::PropAttrs)> {
    let ro_configurable = host::PropAttrs {
        writable: false,
        enumerable: false,
        configurable: true,
    };
    // A callable's `length`/`name` are read-only but configurable; its
    // `prototype` is writable and NOT configurable, and a class's is neither.
    // An arrow, a method and a bound function own no `prototype` at all.
    if with_host(|h| host::is_callable(h, obj)) && !matches!(key, "length" | "name" | "prototype") {
        return None;
    }
    if with_host(|h| host::is_callable(h, obj)) {
        if key == "prototype" {
            let p = get_property(obj, "prototype").ok()?;
            if matches!(p, Value::Undef) {
                return None;
            }
            return Some((
                p,
                host::PropAttrs {
                    writable: with_host(|h| h.kind_of(obj)) != Some(ObjKind::Class),
                    enumerable: false,
                    configurable: false,
                },
            ));
        }
        return Some((get_property(obj, key).ok()?, ro_configurable));
    }
    // A typed array's elements are own, enumerable, writable, configurable
    // properties; an index past the end owns nothing.
    if crate::stdlib::native_tag(obj).as_deref() == Some("TypedArray") {
        let v = crate::stdlib::typedarray::elem_get(obj, key)?;
        return Some((
            v,
            host::PropAttrs {
                writable: true,
                enumerable: true,
                configurable: true,
            },
        ));
    }
    // A RegExp's `lastIndex` is its own, writable, non-configurable cursor.
    if with_host(|h| matches!(h.get(obj), Some(JsObj::RegExp(_)))) && key == "lastIndex" {
        return Some((
            get_property(obj, "lastIndex").ok()?,
            host::PropAttrs {
                writable: true,
                enumerable: false,
                configurable: false,
            },
        ));
    }
    None
}

fn object_get_own_descriptor(args: Vec<Value>) -> Result<Value, String> {
    let obj = arg0(&args);
    require_object_coercible(&obj)?;
    let key = host::to_property_key(&args.get(1).cloned().unwrap_or(Value::Undef))?;
    // A string primitive's boxed own properties: each code-unit index is an
    // enumerable, non-writable, non-configurable data property, and `length` is
    // the same minus enumerable.
    if let Some(units) = string_primitive_units(&obj) {
        let entry = match key.parse::<usize>() {
            Ok(i) => units
                .get(i)
                .map(|c| (with_host(|h| h.new_str(c.clone())), true)),
            Err(_) if key == "length" => Some((Value::Float(units.len() as f64), false)),
            Err(_) => None,
        };
        return Ok(match entry {
            Some((value, enumerable)) => with_host(|h| {
                let mut m: IndexMap<String, Value> = IndexMap::new();
                m.insert("value".into(), value);
                m.insert("writable".into(), Value::Bool(false));
                m.insert("enumerable".into(), Value::Bool(enumerable));
                m.insert("configurable".into(), Value::Bool(false));
                h.new_object(m)
            }),
            None => Value::Undef,
        });
    }
    if with_host(|h| h.kind_of(&obj)) == Some(ObjKind::Proxy) {
        return Ok(crate::proxy::get_own_descriptor(&obj, &key)?.unwrap_or(Value::Undef));
    }
    // A method read off an enumerable builtin prototype (`EventEmitter.prototype`)
    // yields a `{ value: <method thunk> }` data descriptor so `mixin` can copy it.
    if let Some(JsObj::Builtin(ns)) = with_host(|h| h.get(&obj).cloned()) {
        if let Some(names) = builtin_proto_method_names(&ns) {
            if names.contains(&key.as_str()) {
                return Ok(with_host(|h| {
                    let thunk = h.alloc(JsObj::Builtin(format!(
                        "@proto:{}:{key}",
                        ns.trim_end_matches(".prototype")
                    )));
                    let mut m: IndexMap<String, Value> = IndexMap::new();
                    m.insert("value".into(), thunk);
                    m.insert("writable".into(), Value::Bool(true));
                    m.insert("enumerable".into(), Value::Bool(true));
                    m.insert("configurable".into(), Value::Bool(true));
                    h.new_object(m)
                }));
            }
        }
    }
    // A global the object does not own outright is still an own property of the
    // global object — the same lazy binding the bare identifier resolves to.
    // Every one of them reported `undefined`, so a feature probe written as
    // `getOwnPropertyDescriptor(globalThis, 'structuredClone')` concluded the
    // global was absent. The immutable trio (11.1.1 / 19.1.1-3) is frozen; the
    // rest are ordinary writable, non-enumerable, configurable bindings.
    if with_host(|h| h.is_global_object(&obj)) {
        let owned = with_host(|h| match h.get(&obj) {
            Some(JsObj::Object(p)) => p.contains_key(&key),
            _ => false,
        });
        if !owned && !CJS_WRAPPER_LOCALS.contains(&key.as_str()) {
            // A global a SCRIPT created — `x = 1` with no declaration — is an
            // ordinary enumerable property, unlike the builtins.
            let script_made = with_host(|h| h.read_global(&key).is_some());
            if let Some(v) = global_object_binding(&key) {
                let frozen = matches!(key.as_str(), "undefined" | "NaN" | "Infinity");
                return Ok(with_host(|h| {
                    let mut m: IndexMap<String, Value> = IndexMap::new();
                    m.insert("value".into(), v);
                    m.insert("writable".into(), Value::Bool(!frozen));
                    m.insert(
                        "enumerable".into(),
                        Value::Bool(script_made || ENUMERABLE_GLOBALS.contains(&key.as_str())),
                    );
                    m.insert("configurable".into(), Value::Bool(!frozen));
                    h.new_object(m)
                }));
            }
        }
    }
    // Any other member of a builtin namespace (`Math.PI`, `Math.floor`,
    // `Array.prototype.slice`, a builtin function's own `name`/`length`). Every
    // one of these reads back a value, but none owned a DESCRIPTOR:
    // `Object.getOwnPropertyDescriptor(Math, 'PI')` was `undefined`, which reads
    // as "no such property" to the shim/polyfill family that probes a namespace
    // before patching it.
    // An ACCESSOR member describes itself with a `get`, never a `value` — and
    // it must do so without READING the property, since running the getter
    // against the prototype is exactly what throws. Both prototype
    // representations are covered, so `Symbol.prototype.description` and
    // `Map.prototype.size` answer alike; both were `undefined`, which reads as
    // "no such property" to anything that probes before patching.
    if let Some(ctor) = intrinsic_proto_of(&obj) {
        if is_proto_accessor(&ctor, &key) {
            let getter = proto_getter(&ctor, &key);
            // The poison pair is the only ECMAScript accessor here with a
            // SETTER, but a WebIDL class has plenty: `URL.prototype.href`,
            // `hostname` and the rest are all writable, and reporting them as
            // read-only made `Object.getOwnPropertyDescriptor(URL.prototype,
            // 'href').set` read `undefined` for a setter that runs.
            let writable = (ctor == "Function" && matches!(key.as_str(), "arguments" | "caller"))
                || crate::stdlib::instance_accessors(&ctor)
                    .0
                    .iter()
                    .any(|(k, settable)| *k == key && *settable);
            let setter = writable
                .then(|| with_host(|h| h.alloc(JsObj::Builtin(format!("@protoset:{ctor}:{key}")))));
            return Ok(with_host(|h| {
                let mut m: IndexMap<String, Value> = IndexMap::new();
                m.insert("get".into(), getter);
                // `undefined`, not null: a read-only accessor has no setter at
                // all, and `JSON.stringify` of the descriptor must drop the key
                // rather than report `"set": null`.
                m.insert("set".into(), setter.unwrap_or(Value::Undef));
                m.insert("enumerable".into(), Value::Bool(is_webidl_proto(&ctor)));
                m.insert("configurable".into(), Value::Bool(true));
                h.new_object(m)
            }));
        }
    }
    if let Some(ns) = with_host(|h| match h.get(&obj) {
        Some(JsObj::Builtin(ns)) => Some(ns.clone()),
        _ => None,
    }) {
        let value = namespace_property(&ns, &key);
        if !matches!(value, Value::Undef) {
            return Ok(builtin_member_descriptor(&ns, &key, value));
        }
    }
    if let Some((value, attrs)) = synthesized_own_descriptor(&obj, &key) {
        return Ok(with_host(|h| {
            let mut m: IndexMap<String, Value> = IndexMap::new();
            m.insert("value".into(), value);
            m.insert("writable".into(), Value::Bool(attrs.writable));
            m.insert("enumerable".into(), Value::Bool(attrs.enumerable));
            m.insert("configurable".into(), Value::Bool(attrs.configurable));
            h.new_object(m)
        }));
    }
    // Accessor descriptor?
    if let Some((get, set)) = with_host(|h| h.own_accessor(&obj, &key)) {
        return Ok(with_host(|h| {
            let a = h.prop_attrs(&obj, &key);
            let mut m: IndexMap<String, Value> = IndexMap::new();
            m.insert("get".into(), get.unwrap_or(Value::Undef));
            m.insert("set".into(), set.unwrap_or(Value::Undef));
            m.insert("enumerable".into(), Value::Bool(a.enumerable));
            m.insert("configurable".into(), Value::Bool(a.configurable));
            h.new_object(m)
        }));
    }
    let val = with_host(|h| match h.get(&obj) {
        // A Buffer's own properties are exactly its byte indices, read out of the
        // hidden `@@bytes` slot; `length`/`byteLength` are internal bookkeeping
        // that V8 keeps on the prototype, so they own no descriptor.
        Some(JsObj::Object(p))
            if p.get("@@native").map(|t| h.str_of(t)).as_deref() == Some("Buffer") =>
        {
            match (
                p.get("@@bytes").and_then(|b| h.get(b)),
                key.parse::<usize>(),
            ) {
                (Some(JsObj::Array(items)), Ok(i)) => items.get(i).cloned(),
                _ => None,
            }
        }
        Some(JsObj::Object(p)) => p.get(&key).cloned(),
        // An array's index keys read the elements; `length` is the exotic own
        // property; anything else is an ordinary own key in the side table.
        Some(JsObj::Array(items)) => match key.parse::<usize>() {
            // An ELIDED index owns no property at all, so it has no descriptor.
            Ok(i) if h.is_hole(&obj, i) => None,
            Ok(i) => items.get(i).cloned(),
            Err(_) if key == "length" => Some(Value::Float(items.len() as f64)),
            Err(_) => h.fn_prop(&obj, &key),
        },
        // A function/class own prop lives in the fn-prop side table.
        Some(JsObj::Func(_)) | Some(JsObj::Class(_)) => h.fn_prop(&obj, &key),
        _ => None,
    });
    match val {
        Some(v) => Ok(with_host(|h| {
            let a = h.prop_attrs(&obj, &key);
            let mut m: IndexMap<String, Value> = IndexMap::new();
            m.insert("value".into(), v);
            m.insert("writable".into(), Value::Bool(a.writable));
            m.insert("enumerable".into(), Value::Bool(a.enumerable));
            m.insert("configurable".into(), Value::Bool(a.configurable));
            h.new_object(m)
        })),
        None => Ok(Value::Undef),
    }
}

/// `Object.getOwnPropertyDescriptors(obj)` — the descriptor of every own string
/// key, keyed by name. `Object.create(proto, getOwnPropertyDescriptors(src))` is
/// the standard "clone with accessors intact" idiom, so this must agree
/// key-for-key with `getOwnPropertyNames`.
fn object_get_own_descriptors(args: Vec<Value>) -> Result<Value, String> {
    let obj = arg0(&args);
    let names = object_keys(vec![obj.clone()], 3)?;
    let keys: Vec<String> = with_host(|h| match h.get(&names) {
        Some(JsObj::Array(items)) => items.iter().map(|k| h.str_of(k)).collect(),
        _ => Vec::new(),
    });
    let mut out: IndexMap<String, Value> = IndexMap::new();
    for k in keys {
        let ks = with_host(|h| h.new_str(k.clone()));
        let d = object_get_own_descriptor(vec![obj.clone(), ks])?;
        if !matches!(d, Value::Undef) {
            out.insert(k, d);
        }
    }
    Ok(with_host(|h| h.new_object(out)))
}

/// `key in obj` respecting the prototype chain. Reports a `Result` because a
/// Proxy's `has` trap is user code and may throw.
pub fn has_property(obj: &Value, key: &str) -> Result<bool, String> {
    if let Some(b) = crate::proxy::has(obj, key)? {
        return Ok(b);
    }
    Ok(has_property_ordinary(obj, key))
}

/// `[[HasProperty]]` for every non-Proxy receiver.
fn has_property_ordinary(obj: &Value, key: &str) -> bool {
    // `key in globalThis`: membership matches what the READ answers, which for
    // the global object includes every lazily-bound builtin and every global a
    // script created. `'Math' in globalThis` and `'x' in globalThis` after
    // `x = 1` both answered FALSE while `globalThis.Math` and `globalThis.x`
    // read back fine.
    if with_host(|h| h.is_global_object(obj))
        && !CJS_WRAPPER_LOCALS.contains(&key)
        && global_object_binding(key).is_some()
    {
        return true;
    }
    // `key in <builtin namespace/prototype>`: membership matches what a property
    // read would yield. `String.prototype.indexOf` (and the rest of the builtin
    // prototype methods) resolve as callable thunks via `namespace_property`, so
    // `'indexOf' in String.prototype` must report true (get-intrinsic probes this
    // with the `in` operator before reading the intrinsic).
    if let Some(JsObj::Builtin(ns)) = with_host(|h| h.get(obj).cloned()) {
        return !matches!(namespace_property(&ns, key), Value::Undef);
    }
    // An integer index of a typed array / Buffer is an own property, and lives
    // in the hidden element array rather than the property map — the same
    // question `hasOwnProperty` answers, through the same helper. Only a hit
    // short-circuits: a non-index key like `'length'` must still fall through
    // to the ordinary chain lookup below.
    if crate::stdlib::typedarray::has_index(obj, key) == Some(true) {
        return true;
    }
    if with_host(|h| host::lookup_chain(h, obj, key)).is_some() {
        return true;
    }
    if with_host(|h| host::lookup_accessor(h, obj, key)).is_some() {
        return true;
    }
    // A member patched onto the receiver's intrinsic prototype. The READ
    // resolves it, so without this `Array.prototype.at = f` made `[].at` a
    // function while `'at' in []` stayed false.
    if !key.starts_with('#') && inherited_builtin_static(obj, key).is_some() {
        return true;
    }
    if with_host(|h| match h.get(obj) {
        Some(JsObj::Object(p)) => p.contains_key(key),
        Some(JsObj::Array(items)) => {
            key == "length"
                || key
                    .parse::<usize>()
                    .map(|i| i < items.len() && !h.is_hole(obj, i))
                    .unwrap_or(false)
                // A non-index own property (`arr.foo`, `arr[sym]`) lives in the
                // side table, and `in` must see it.
                || h.fn_prop(obj, key).is_some()
        }
        Some(JsObj::Func(_)) | Some(JsObj::Class(_)) => h.fn_prop(obj, key).is_some(),
        // A RegExp's `lastIndex` is an OWN property in node. Here it lives in
        // the `RegExpObj` struct rather than a property map, so nothing above
        // can see it.
        Some(JsObj::RegExp(_)) => key == "lastIndex" || h.fn_prop(obj, key).is_some(),
        _ => false,
    }) {
        return true;
    }
    // An INHERITED builtin prototype method. These are not objects on the
    // prototype chain — they are synthesized by the read path from the
    // intrinsic table — so neither `lookup_chain` nor the property map above
    // can see them, and `'toString' in {}`, `'push' in []` and `'then' in
    // Promise.resolve()` all answered false. That last one is the standard
    // thenable test, so the `in` operator disagreed with what a read gives for
    // every builtin method of every builtin kind.
    inherited_builtin_method(obj, key)
}

/// Whether a READ of `key` on `obj` would resolve to an inherited builtin
/// prototype method. Asked by `in` and `hasOwnProperty`'s negative case; it
/// performs no read, so a getter cannot fire.
/// A property a script MONKEY-PATCHED onto the intrinsic prototype `obj`
/// inherits from (`Array.prototype.at = impl`, `Object.prototype.foo = 1`), or
/// `None`.
///
/// The intrinsic prototypes are namespace handles rather than real objects on
/// the chain, so an assignment onto one lands in `builtin_statics` and no
/// ordinary chain walk can see it. This is the read side: the receiver's own
/// constructor's prototype first, then `Object.prototype`, mirroring
/// `inherited_method_owner`'s two-step.
pub(crate) fn inherited_builtin_static(obj: &Value, key: &str) -> Option<Value> {
    if with_host(|h| h.has_null_proto(obj)) {
        return None;
    }
    let ctor = match wrapped_primitive(obj).as_ref().and_then(wrapper_ctor_of) {
        Some(c) => Some(c),
        None if is_arguments(obj) => Some("Object"),
        None => with_host(|h| default_ctor_name(h, obj)),
    };
    // Only the side table is consulted, never the real prototype OBJECT's map:
    // `String.prototype` and friends are materialized with their intrinsic
    // members present, so reading their maps here would re-route every ordinary
    // `"a".toString()` through this path — which recursed until the stack blew.
    // `set_property` mirrors a write onto a real intrinsic prototype INTO this
    // table precisely so the read side can stay this narrow.
    let on = |c: &str| with_host(|h| h.builtin_static(&format!("{c}.prototype"), key));
    let found = ctor.and_then(on).or_else(|| on("Object"))?;
    // Restoring a saved intrinsic (`const orig = Array.prototype.join; …;
    // Array.prototype.join = orig`) stores the SYNTHESIZED thunk for this very
    // name back into the table. Dispatching to it would re-enter this lookup
    // and recurse until the stack blew, so a thunk that is already this key's
    // own intrinsic reports nothing and the ordinary builtin path answers.
    let self_thunk = with_host(
        |h| matches!(h.get(&found), Some(JsObj::Builtin(s)) if s.starts_with("@proto:") && s.ends_with(&format!(":{key}"))),
    );
    (!self_thunk).then_some(found)
}

/// Whether `recv` carries `key` as an OWN property — the guard on
/// [`inherited_builtin_static`], since an own property shadows anything
/// patched onto a prototype.
fn has_own_for_shadow(recv: &Value, key: &str) -> bool {
    with_host(|h| {
        if h.fn_prop(recv, key).is_some() || h.own_accessor(recv, key).is_some() {
            return true;
        }
        match h.get(recv) {
            Some(JsObj::Object(p)) => p.contains_key(key),
            // An ELIDED index owns nothing — the whole point of a hole is that
            // the lookup continues up the chain — so it must not count as a
            // shadow here or an inherited value at that index stays invisible.
            Some(JsObj::Array(items)) => {
                key == "length" || {
                    key.parse::<usize>()
                        .is_ok_and(|i| i < items.len() && !h.is_hole(recv, i))
                }
            }
            _ => false,
        }
    })
}

fn inherited_builtin_method(obj: &Value, key: &str) -> bool {
    if with_host(|h| h.has_null_proto(obj)) {
        return false;
    }
    if let Some(tag) = crate::stdlib::native_tag(obj) {
        if crate::stdlib::instance_has_method(&tag, key) {
            return true;
        }
    }
    inherited_method_owner(obj, key).is_some()
}

/// Whether `recv`'s intrinsic prototype is still on its chain — that is,
/// whether `Array.prototype`'s methods are still reachable from an array.
///
/// A builtin's methods are synthesized from the receiver's KIND rather than
/// found on a chain, so replacing the prototype could not take them away:
/// `Object.setPrototypeOf(a, {})` left `a.join` a function where node reports
/// `undefined`, and `Object.setPrototypeOf(a, null)` did too. The exotic
/// storage is unaffected either way — `Array.isArray`, `a.length` and `a[0]`
/// all still answer, as they do in node.
///
/// The overwhelmingly common case is the DEFAULT link, which is recorded as no
/// link at all, so this answers true after one map probe and allocates nothing.
pub(crate) fn own_intrinsic_reachable_pub(recv: &Value) -> bool {
    own_intrinsic_reachable(recv)
}

fn own_intrinsic_reachable(recv: &Value) -> bool {
    // A BOXED primitive needs no special case here: its methods resolve through
    // `inherited_method_owner`, which applies the wrapper rule itself.
    with_host(|h| default_ctor_name(h, recv)).map_or(true, |c| intrinsic_reachable(recv, c))
}

/// Whether the intrinsic prototype for `ctor` is still on `recv`'s chain.
fn intrinsic_reachable(recv: &Value, ctor: &str) -> bool {
    let own = Some(ctor);
    let mut cur = recv.clone();
    for _ in 0..100 {
        let explicit = with_host(|h| h.proto_of(&cur));
        let Some(p) = explicit else {
            // No explicit link: the implicit prototype is this object's own
            // kind's, which is what `recv` is asking about only while `cur` is
            // still `recv` itself.
            if with_host(|h| h.has_null_proto(&cur)) {
                return false;
            }
            let implicit = with_host(|h| default_ctor_name(h, &cur));
            // Every implicit prototype chain ends at `Object.prototype`, so a
            // question about `Object` is answered yes by any of them.
            return implicit == own || ctor == "Object";
        };
        if with_host(|h| h.is_null(&p)) {
            return false;
        }
        let hit = with_host(|h| {
            own.is_some_and(|c| {
                matches!(h.get(&p), Some(JsObj::Builtin(ns)) if *ns == format!("{c}.prototype"))
                    || h.intrinsic_proto_ctor(&p) == Some(c)
                    || (c == "Object" && h.object_proto() == p)
            })
        });
        if hit {
            return true;
        }
        // A CLASS prototype object is not linked to the builtin its class
        // extends — the `extends` relationship is recorded on the class value —
        // so the walk has to cross over there or `class D extends Array {}` ends
        // it, and every inherited method of every subclass instance vanishes.
        if let Some(builtin) = with_host(|h| {
            h.class_owning_proto(&p)
                .and_then(|c| h.class_builtin_ancestor(&c))
                .map(|b| h.callable_name(&b))
        }) {
            if own == Some(builtin.as_str()) || ctor == "Object" {
                return true;
            }
        }
        cur = p;
    }
    false
}

/// The intrinsic prototypes actually ON `recv`'s explicit chain, nearest first
/// — the complement of [`intrinsic_reachable`], which asks about one known
/// constructor.
///
/// `Object.create(Array.prototype)` is an ordinary object whose chain reaches
/// `Array.prototype`, and node resolves the whole of `Array.prototype` through
/// it: `o.push(1)` works, because those methods are generic over their receiver
/// (which is also why `Array.prototype.push.call({length: 0}, 1)` already
/// worked here). Deciding the owner from the receiver's KIND alone made every
/// one of them `undefined` — the same "methods come from the kind, not the
/// chain" mistake as the detachment case, in the opposite direction.
pub(crate) fn chain_intrinsic_ctors_pub(recv: &Value) -> Vec<&'static str> {
    chain_intrinsic_ctors(recv)
}

fn chain_intrinsic_ctors(recv: &Value) -> Vec<&'static str> {
    with_host(|h| chain_intrinsic_ctors_h(h, recv))
}

/// [`chain_intrinsic_ctors`] against an already-held host borrow, for the
/// callers that are inside one — `can_write_prop` takes `&JsHost`, so going
/// back through `with_host` there aborts the process on a double borrow.
pub(crate) fn chain_intrinsic_ctors_h(h: &host::JsHost, recv: &Value) -> Vec<&'static str> {
    let mut out: Vec<&'static str> = Vec::new();
    let mut cur = recv.clone();
    for _ in 0..100 {
        let Some(p) = h.proto_of(&cur) else {
            break;
        };
        if h.is_null(&p) {
            break;
        }
        let name = match h.get(&p) {
            Some(JsObj::Builtin(ns)) => ns.strip_suffix(".prototype").map(str::to_string),
            _ => h.intrinsic_proto_ctor(&p).map(str::to_string),
        };
        if let Some(n) = name {
            if let Some(c) = crate::arity::PROTO_MEMBERS
                .iter()
                .map(|(k, _)| *k)
                .find(|k| *k == n)
            {
                if !out.contains(&c) {
                    out.push(c);
                }
            }
        }
        cur = p;
    }
    out
}

/// The constructor whose prototype defines `key` for `obj` — its own if that
/// prototype has it, otherwise `Object` — or `None` when neither does.
///
/// Used both by `in` and by the READ, so the two cannot disagree about which
/// prototype a name comes from. `new Map().toString` is `Map.prototype`'s and
/// `new Map().hasOwnProperty` is `Object.prototype`'s.
pub(crate) fn inherited_method_owner_pub(obj: &Value, key: &str) -> Option<&'static str> {
    inherited_method_owner(obj, key)
}

fn inherited_method_owner(obj: &Value, key: &str) -> Option<&'static str> {
    if with_host(|h| h.has_null_proto(obj)) {
        return None;
    }
    // The generated prototype-member table, which unlike the arity table knows
    // about the ACCESSORS — `size` on a Map, `source` on a RegExp, `description`
    // on a Symbol are members but not functions — and about `constructor`.
    // A BOXED primitive reports its wrapper's constructor, not `Object` —
    // `'description' in Object(Symbol())` is true. The box is an ordinary
    // object carrying the primitive in a slot, so the ctor comes from what it
    // holds rather than from the box itself.
    let ctor = match wrapped_primitive(obj).as_ref().and_then(wrapper_ctor_of) {
        Some(c) => Some(c),
        // An `arguments` object is ARRAY-BACKED here so that indices, `length`,
        // spread and `for-of` work, but node's is an exotic that inherits from
        // `Object.prototype` — `typeof arguments.map` is `undefined`. Reporting
        // its backing kind would hand it the whole `Array.prototype`.
        None if is_arguments(obj) => Some("Object"),
        None => with_host(|h| default_ctor_name(h, obj)),
    };
    let on_proto = |c: &str| {
        crate::arity::PROTO_MEMBERS
            .binary_search_by(|(k, _)| (*k).cmp(c))
            .ok()
            .is_some_and(|i| {
                crate::arity::PROTO_MEMBERS[i]
                    .1
                    .iter()
                    .any(|m| m.strip_prefix('+').unwrap_or(m) == key)
            })
    };
    // `PROTO_MEMBERS` is generated from the prototypes' STRING keys, so a
    // well-known symbol member is absent from it. For an object whose CHAIN
    // reaches an intrinsic prototype the intrinsic table has to be consulted as
    // well, or `[...Object.create(Array.prototype)]` finds no `Symbol.iterator`
    // at all. It is deliberately NOT consulted for the receiver's own kind:
    // there a thunk would be minted for every `@@` member the table names,
    // including ones whose dispatch has no implementation for that receiver,
    // and `[...buffer]` then failed with `@@iterator is not a function`.
    //
    // It is narrowed further to an ORDINARY object: a natively-tagged receiver
    // (a typed array, a Buffer) is linked to a real intrinsic prototype too,
    // and minting a thunk there produced `@@iterator is not a function` for
    // `[...new Uint8Array(ab)]` — those kinds reach their iterator by their own
    // fast path, which the table entry would shadow.
    let plain = with_host(|h| h.kind_of(obj)) == Some(ObjKind::Object)
        && crate::stdlib::native_tag(obj).is_none();
    let on_proto_or_symbol =
        |c: &str| on_proto(c) || (plain && builtin_meta(&format!("@proto:{c}:{key}")).is_some());
    // Each candidate is only an answer while ITS prototype is still on the
    // receiver's chain. The two are asked separately: replacing an array's
    // prototype with a plain object takes `Array.prototype`'s methods away and
    // leaves `Object.prototype`'s, since the replacement inherits from it.
    if let Some(c) = ctor.filter(|c| on_proto(c) && intrinsic_reachable(obj, c)) {
        return Some(c);
    }
    // An intrinsic prototype the receiver's chain passes THROUGH, which its own
    // kind does not account for.
    if let Some(c) = chain_intrinsic_ctors(obj)
        .into_iter()
        .find(|c| on_proto_or_symbol(c))
    {
        return Some(c);
    }
    // Everything else inherits `Object.prototype`'s.
    if on_proto("Object") && intrinsic_reachable(obj, "Object") {
        return Some("Object");
    }
    None
}

/// `structuredClone` — a deep copy of plain data (objects/arrays/primitives).
/// `structuredClone` — the HTML structured-clone algorithm's shape: a deep copy
/// that preserves the *reference graph*. Two properties pointing at the same
/// object clone to two properties pointing at the same clone, and a cycle clones
/// to a cycle instead of recursing forever. `seen` maps each source heap index
/// to its clone, which is what buys both.
/// The rendering node puts in a `DataCloneError` for a value the structured
/// clone algorithm refuses, or `None` when the value IS cloneable.
///
/// Refusing at all is the point: these used to be copied through by reference,
/// so `structuredClone({f: () => 1})` handed back an object sharing the
/// original's function and `structuredClone(new WeakMap())` returned the very
/// same WeakMap. Node throws on every one of them.
fn clone_refusal(v: &Value) -> Option<String> {
    let kind = with_host(|h| h.kind_of(v))?;
    let render = |ctor: &str| Some(format!("#<{ctor}>"));
    match kind {
        // A function renders as its SOURCE TEXT here, which each FuncDef
        // keeps as a span into its script (`JsHost::func_source`).
        ObjKind::Func | ObjKind::Class | ObjKind::BoundFunc | ObjKind::BoundMethod => {
            Some(with_host(|h| h.str_of(v)))
        }
        ObjKind::Builtin if with_host(|h| host::is_callable(h, v)) => {
            Some(with_host(|h| h.str_of(v)))
        }
        ObjKind::Symbol => Some(with_host(|h| h.str_of(v))),
        ObjKind::Promise => render("Promise"),
        ObjKind::Generator => Some("[object Generator]".to_string()),
        // A proxy is refused by its TARGET's shape: a callable one renders like
        // the function it wraps, everything else as a plain object.
        ObjKind::Proxy => Some(if with_host(|h| host::is_callable(h, v)) {
            with_host(|h| h.str_of(v))
        } else {
            "#<Object>".to_string()
        }),
        ObjKind::Map if with_host(|h| matches!(h.get(v), Some(JsObj::Map { weak: true, .. }))) => {
            render("WeakMap")
        }
        ObjKind::Set if with_host(|h| matches!(h.get(v), Some(JsObj::Set { weak: true, .. }))) => {
            render("WeakSet")
        }
        _ => match crate::stdlib::native_tag(v).as_deref() {
            Some(t @ ("WeakRef" | "FinalizationRegistry")) => render(t),
            _ => None,
        },
    }
}

/// `structuredClone(value[, { transfer }])`.
///
/// Everything in `transfer` must be an `ArrayBuffer`, and each one is DETACHED
/// after the clone — its bytes belong to the copy. The option used to be
/// ignored entirely, so the source buffer stayed usable where node leaves it
/// with zero length.
fn structured_clone(args: Vec<Value>) -> Result<Value, String> {
    let list: Vec<Value> = match args.get(1).filter(|v| !matches!(v, Value::Undef)) {
        Some(opts) => {
            let t = get_property(opts, "transfer")?;
            if matches!(t, Value::Undef) {
                Vec::new()
            } else {
                host::iter_all(&t)?
            }
        }
        None => Vec::new(),
    };
    for item in &list {
        if crate::stdlib::native_tag(item).as_deref() != Some("ArrayBuffer") {
            return Err(host::dom_error(
                "DataCloneError",
                "Found invalid value in transferList.",
            ));
        }
    }
    let out = deep_clone(&arg0(&args))?;
    for item in &list {
        crate::stdlib::typedarray::detach_buffer(item);
    }
    Ok(out)
}

pub(crate) fn deep_clone(v: &Value) -> Result<Value, String> {
    deep_clone_seen(v, &mut std::collections::HashMap::new())
}

fn deep_clone_seen(
    v: &Value,
    seen: &mut std::collections::HashMap<u32, Value>,
) -> Result<Value, String> {
    let idx = match v {
        Value::Obj(i) => *i,
        _ => return Ok(v.clone()),
    };
    if let Some(done) = seen.get(&idx) {
        return Ok(done.clone());
    }
    if crate::stdlib::typedarray::is_detached(v) {
        return Err(host::dom_error(
            "DataCloneError",
            "An ArrayBuffer is detached and could not be cloned.",
        ));
    }
    if let Some(render) = clone_refusal(v) {
        return Err(host::dom_error(
            "DataCloneError",
            &format!("{render} could not be cloned."),
        ));
    }
    // A REGEXP is cloned, not shared: it carries a mutable `lastIndex`, so
    // handing back the same object let a write through the clone move the
    // original's match cursor.
    if let Some((src, flags)) = with_host(|h| match h.get(v) {
        Some(JsObj::RegExp(r)) => Some((r.source.clone(), r.flags.clone())),
        _ => None,
    }) {
        let args = with_host(|h| vec![h.new_str(src), h.new_str(flags)]);
        let out = regexp_ctor(&args)?;
        seen.insert(idx, out.clone());
        return Ok(out);
    }
    Ok(match with_host(|h| h.get(v).cloned()) {
        Some(JsObj::Array(items)) => {
            // Register the (empty) clone BEFORE recursing so a self-reference
            // resolves to it.
            let out = with_host(|h| h.new_array(Vec::new()));
            seen.insert(idx, out.clone());
            let mut cloned: Vec<Value> = Vec::with_capacity(items.len());
            for x in &items {
                cloned.push(deep_clone_seen(x, seen)?);
            }
            with_host(|h| {
                if let Some(JsObj::Array(a)) = h.get_mut(&out) {
                    *a = cloned;
                }
                // A sparse source clones to an equally sparse array: the clone
                // walks own properties, so a hole is nothing to copy.
                h.copy_holes(v, &out, Some);
            });
            out
        }
        Some(JsObj::Object(_)) => {
            let out = with_host(|h| h.new_object(IndexMap::new()));
            seen.insert(idx, out.clone());
            // Own ENUMERABLE string keys, read THROUGH any accessor: the clone
            // walked the property map, where an accessor stores nothing, so
            // `structuredClone({get p(){return 1}})` silently lost `p`. A symbol
            // key and a non-enumerable one are dropped, as node drops them.
            let is_error = with_host(|h| h.error_to_string(v)).is_some();
            let proto = clone_proto(v);
            let keeps_proto = !matches!(proto, CloneProto::Plain);
            // An ERROR clones its name, message and stack and NOTHING else —
            // node drops any other own property, even an enumerable one.
            let keys: Vec<String> = if is_error {
                // An ERROR clones its name, message and stack and NOTHING else —
                // node drops any other own property, even an enumerable one.
                ["name", "message", "stack"]
                    .iter()
                    .filter(|k| has_property(v, k).unwrap_or(false))
                    .map(|k| (*k).to_string())
                    .collect()
            } else if keeps_proto {
                // A preserved exotic keeps EVERY own property, including the
                // non-enumerable ones and the internal slots — a Date's time
                // value, an ArrayBuffer's `byteLength` and byte store, a typed
                // array's view. The enumerable-only walk dropped all of those,
                // so a cloned Date read `Invalid Date` and a cloned
                // ArrayBuffer had no `byteLength`.
                with_host(|h| match h.get(v) {
                    Some(JsObj::Object(p)) => p.keys().cloned().collect(),
                    _ => Vec::new(),
                })
            } else {
                with_host(|h| h.own_enum_key_names(v))
            };
            let mut cloned: IndexMap<String, Value> = IndexMap::new();
            for k in keys {
                // An internal slot is read straight out of the map: it is not a
                // property, so a `[[Get]]` would not find it.
                let val = if k.starts_with("@@") {
                    match with_host(|h| match h.get(v) {
                        Some(JsObj::Object(p)) => p.get(&k).cloned(),
                        _ => None,
                    }) {
                        Some(val) => val,
                        None => continue,
                    }
                } else {
                    get_property(v, &k)?
                };
                cloned.insert(k, deep_clone_seen(&val, seen)?);
            }
            // The prototype survives only for the exotics the algorithm knows —
            // a Date, an Error, a typed array, a boxed primitive. A USER class
            // instance becomes a plain object, which is what node produces;
            // keeping every prototype made `structuredClone(new K())
            // instanceof K` true.
            with_host(|h| {
                if let Some(JsObj::Object(p)) = h.get_mut(&out) {
                    *p = cloned;
                }
                match &proto {
                    CloneProto::Same => {
                        if let Some(p) = h.proto_of(v) {
                            h.set_proto(&out, p);
                        }
                    }
                    CloneProto::Ctor(c) => {
                        h.ensure_error_protos();
                        let p = h.error_proto(c).or_else(|| h.ensure_ctor_proto(c));
                        if let Some(p) = p {
                            h.set_proto(&out, p);
                        }
                        // A Buffer clones to a plain `Uint8Array`, so the native
                        // tag has to change with the prototype — left alone,
                        // `Buffer.isBuffer` still answered true for the clone.
                        // A Buffer clones to a plain `Uint8Array`, so the native
                        // tag has to change with the prototype — left alone,
                        // `Buffer.isBuffer` answered true for the clone and the
                        // brand stayed `[object Object]`. A typed array is
                        // tagged `TypedArray` and names its element type in
                        // `@@kind`; `@@native = "Uint8Array"` matches no arm.
                        if c == "Uint8Array" {
                            let tag = h.new_str("TypedArray");
                            let kind = h.new_str("Uint8Array");
                            if let Some(JsObj::Object(p)) = h.get_mut(&out) {
                                p.insert("@@native".into(), tag);
                                p.insert("@@kind".into(), kind);
                            }
                        }
                    }
                    CloneProto::Plain => {}
                }
                h.copy_prop_attrs(v, &out);
            });
            out
        }
        // Map/Set are structured types: clone the entries, keep the kind.
        Some(JsObj::Map { entries, weak }) => {
            let out = with_host(|h| {
                h.alloc(JsObj::Map {
                    entries: IndexMap::new(),
                    weak,
                })
            });
            seen.insert(idx, out.clone());
            let pairs: Vec<(Value, Value)> = entries.values().cloned().collect();
            for (k, val) in pairs {
                let ck = deep_clone_seen(&k, seen)?;
                let cv = deep_clone_seen(&val, seen)?;
                let _ = map_method(&out, "set", vec![ck, cv]);
            }
            out
        }
        Some(JsObj::Set { entries, weak }) => {
            let out = with_host(|h| {
                h.alloc(JsObj::Set {
                    entries: IndexMap::new(),
                    weak,
                })
            });
            seen.insert(idx, out.clone());
            let vals: Vec<Value> = entries.values().cloned().collect();
            for x in vals {
                let cx = deep_clone_seen(&x, seen)?;
                let _ = set_method(&out, "add", vec![cx]);
            }
            out
        }
        // A string, a BigInt and a boxed primitive are immutable enough to
        // share; anything left is a value type.
        _ => v.clone(),
    })
}

/// Whether a cloned object keeps the source's prototype.
///
/// The structured clone algorithm reproduces the exotics it knows and turns
/// everything else into a plain object — so a `Date` clones to a `Date` and a
/// user class instance clones to an `Object`.
fn clone_proto(v: &Value) -> CloneProto {
    // An ERROR clones to the BUILT-IN class its `name` selects, so a subclass
    // flattens: `structuredClone(new (class E extends Error{})('m'))` reports
    // `Error`, not `E`.
    if with_host(|h| h.error_to_string(v)).is_some() {
        let name = get_property(v, "name")
            .map(|n| with_host(|h| h.str_of(&n)))
            .unwrap_or_else(|_| "Error".into());
        let class = if host::ERROR_NAMES.contains(&name.as_str()) {
            name
        } else {
            "Error".to_string()
        };
        return CloneProto::Ctor(class);
    }
    match crate::stdlib::native_tag(v).as_deref() {
        // A Buffer is not reproduced as a Buffer: node hands back a plain
        // `Uint8Array` over the same bytes.
        Some("Buffer") => CloneProto::Ctor("Uint8Array".into()),
        Some(_) => CloneProto::Same,
        // A boxed primitive keeps its wrapper; anything else — a user class
        // instance included — becomes a plain object.
        None if wrapped_primitive(v).is_some() => CloneProto::Same,
        None => CloneProto::Plain,
    }
}

/// Which prototype a clone gets: the source's, a named builtin's, or none.
enum CloneProto {
    Same,
    Ctor(String),
    Plain,
}

// ══ Promises, timers, microtasks (event-loop-driven) ═════════════════════════

/// A short `Name: message` string for an error value (used when an await
/// rejection unwinds as a thrown error).
pub fn error_string(h: &host::JsHost, v: &Value) -> String {
    if let Some(JsObj::Object(props)) = h.get(v) {
        let name = props
            .get("name")
            .map(|x| h.str_of(x))
            .or_else(|| host::lookup_chain(h, v, "name").map(|x| h.str_of(&x)))
            .unwrap_or_else(|| "Error".into());
        if let Some(m) = props.get("message") {
            return format!("{name}: {}", h.str_of(m));
        }
        return name;
    }
    h.str_of(v)
}

/// 27.2.5.3 `thenFinally`/`catchFinally`: `PromiseResolve(result).then(() =>
/// value)`, or `() => { throw reason }` on the reject path.
///
/// Returning the carried value directly — what this used to do — skipped both
/// halves. A promise returned by the callback was never awaited, so the
/// ordinary async-cleanup shape
///
/// ```text
/// work().finally(() => closeConnection()).then(next)
/// ```
///
/// ran `next` before the connection had closed. And the chain settled three
/// microtask ticks early, which is observable in ordering against any other
/// chain, not just against a timer.
///
/// A rejection from the callback's own promise wins over the carried value, so
/// no reject handler is attached here: it propagates on its own.
fn finally_chain(result: Value, carried: Value, rethrow: bool) -> Value {
    // PromiseResolve (27.2.4.7) returns an argument that is already a promise
    // UNCHANGED. Wrapping it anyway costs the extra tick that resolving with a
    // thenable takes to adopt it, which showed up as a callback returning a
    // rejected promise settling one tick late against every other chain.
    let p = match with_host(|h| h.promise_id(&result)) {
        Some(_) => result,
        None => {
            let fresh = with_host(|h| h.new_promise());
            if let Some(pid) = with_host(|h| h.promise_id(&fresh)) {
                host::resolve_promise_val(pid, result);
            }
            fresh
        }
    };
    let cell = with_host(|h| h.new_array(vec![carried]));
    let idx = match cell {
        Value::Obj(i) => i,
        _ => 0,
    };
    let tag = if rethrow { "finrethrow" } else { "finret" };
    let thunk = make_builtin(format!("@@{tag}:{idx}"));
    host::promise_then(&p, thunk, Value::Undef)
}

fn make_builtin(name: String) -> Value {
    with_host(|h| h.alloc(JsObj::Builtin(name)))
}

/// `[[GetPrototypeOf]]` (10.1.1) — the answer `Object.getPrototypeOf`,
/// `Reflect.getPrototypeOf` and a `__proto__` READ all have to agree on.
///
/// `__proto__` used to answer from `JsHost::proto_of` alone, which records only
/// an EXPLICIT link, so an object on the default prototype reported `null`:
/// `({}).__proto__ === Object.prototype` was false while
/// `Object.getPrototypeOf({}) === Object.prototype` was true. One function, so
/// the three cannot drift apart again.
pub fn prototype_of(v: &Value) -> Value {
    // Constructor-side inheritance: `Buffer extends Uint8Array`, so
    // `Object.getPrototypeOf(Buffer)` is the `Uint8Array` constructor itself,
    // not `Function.prototype`. This is the class-side half of the subclass
    // link — the instance-side half is `Buffer.prototype`'s `[[Prototype]]`.
    if matches!(with_host(|h| h.get(v).cloned()), Some(JsObj::Builtin(ref n)) if n == "Buffer") {
        return with_host(|h| h.alloc(JsObj::Builtin("Uint8Array".into())));
    }
    // Constructor-side inheritance for a `class B extends A` (ClassDefinition
    // 15.7.14 step 6.d: the constructor's `[[Prototype]]` is the parent
    // CONSTRUCTOR, not `Function.prototype`). Statics already resolved through
    // `ClassVal.parent`, but the link itself was invisible, so
    // `Object.getPrototypeOf(B) === A` read false and any library walking the
    // constructor chain — rather than calling a static — saw a base class.
    // A base class keeps the default answer below (`Function.prototype`).
    if let Some(JsObj::Class(c)) = with_host(|h| h.get(v).cloned()) {
        if let Some(parent) = c.parent {
            return parent;
        }
    }
    // `Object.create(null)` and friends really do have a null prototype.
    if with_host(|h| h.has_null_proto(v)) {
        return with_host(|h| h.null());
    }
    // `Object.prototype` is the CHAIN ROOT, so its own prototype is `null`. It
    // reported itself, because the fallback below answers by constructor name
    // and a plain object's is `Object` — an infinite chain to anything walking
    // it.
    if with_host(|h| h.strict_eq(v, &h.object_proto())) {
        return with_host(|h| h.null());
    }
    // Every OTHER builtin prototype namespace (`Array.prototype`,
    // `Function.prototype`, …) inherits from `Object.prototype`; the fallback
    // would send it back to a namespace handle for its own constructor.
    if matches!(
        with_host(|h| h.get(v).cloned()),
        Some(JsObj::Builtin(ref n)) if n.ends_with(".prototype")
    ) {
        return with_host(|h| h.object_proto());
    }
    if let Some(p) = with_host(|h| h.proto_of(v)) {
        return p;
    }
    // A builtin exotic with no explicit `[[Prototype]]` link reports its
    // constructor's prototype namespace (`Object.getPrototypeOf([]) ===
    // Array.prototype`), which `strict_eq` compares by name. A plain object
    // reports the one real `Object.prototype` object.
    with_host(|h| {
        h.ensure_native_protos();
        match default_ctor_name(h, v) {
            Some("Object") => h.object_proto(),
            // `String`/`Number`/`Boolean` own REAL prototype objects, so a
            // primitive must report that object and not a fresh namespace
            // thunk — otherwise `Object.getPrototypeOf(1) === Number.prototype`
            // compares a thunk against the real object and reads false.
            Some(c) => h
                .native_proto(c)
                .unwrap_or_else(|| h.alloc(JsObj::Builtin(format!("{c}.prototype")))),
            None => h.null(),
        }
    })
}

/// `new Promise((resolve, reject) => …)` — run the executor synchronously with
/// internal resolve/reject functions.
/// A fresh promise built through the SPECIES constructor, when a `Promise`
/// static was reached through a subclass.
///
/// `class P extends Promise {}` makes `P.resolve(1)` a `P`, because every
/// combinator builds its result with `this` (27.2.4.x). They all allocated a
/// plain promise, so nothing a subclass produced was an instance of it. The
/// executor is a no-op: the result is settled through its promise id, which is
/// what the ordinary path does too.
fn promise_species_create() -> Result<Option<Value>, String> {
    let Some(ctor) = host::current_static_this() else {
        return Ok(None);
    };
    if !matches!(
        with_host(|h| h.kind_of(&ctor)),
        Some(ObjKind::Class) | Some(ObjKind::Func)
    ) {
        return Ok(None);
    }
    let species = match get_property(&ctor, "@@species") {
        Ok(Value::Undef) => ctor,
        Ok(s) if with_host(|h| h.is_null(&s)) => return Ok(None),
        Ok(s) => s,
        Err(_) => ctor,
    };
    if !matches!(
        with_host(|h| h.kind_of(&species)),
        Some(ObjKind::Class) | Some(ObjKind::Func)
    ) {
        return Ok(None);
    }
    let noop = make_builtin("@@pnoop".to_string());
    let p = host::construct(&species, vec![noop])?;
    // Only usable if the subclass really produced a promise; a constructor that
    // returned something else has no id to settle.
    Ok(with_host(|h| h.promise_id(&p)).map(|_| p))
}

/// The species constructor of a promise RECEIVER — what `then`/`catch`/`finally`
/// build their result with (`SpeciesConstructor(p, %Promise%)`, 27.2.5.4 step 3).
///
/// Distinct from `promise_species_create`, which answers for a STATIC reached
/// through a subclass. Here the subclass comes from the receiver itself, so
/// `P.resolve(1).then(f)` is also a `P`.
pub fn promise_species_from(recv: &Value) -> Result<Option<Value>, String> {
    // A chain lookup: a Promise receiver resolves through the stdlib funnel,
    // which has no `constructor` entry of its own.
    let ctor = with_host(|h| host::lookup_chain(h, recv, "constructor")).unwrap_or(Value::Undef);
    if !matches!(
        with_host(|h| h.kind_of(&ctor)),
        Some(ObjKind::Class) | Some(ObjKind::Func)
    ) {
        return Ok(None);
    }
    let species = match get_property(&ctor, "@@species") {
        Ok(Value::Undef) => ctor,
        Ok(s) if with_host(|h| h.is_null(&s)) => return Ok(None),
        Ok(s) => s,
        Err(_) => ctor,
    };
    if !matches!(
        with_host(|h| h.kind_of(&species)),
        Some(ObjKind::Class) | Some(ObjKind::Func)
    ) {
        return Ok(None);
    }
    let noop = make_builtin("@@pnoop".to_string());
    let p = host::construct(&species, vec![noop])?;
    Ok(with_host(|h| h.promise_id(&p)).map(|_| p))
}

fn new_promise(executor: Value) -> Result<Value, String> {
    let p = with_host(|h| h.new_promise());
    let id = with_host(|h| h.promise_id(&p).unwrap());
    let res = make_builtin(format!("@@presolve:{id}"));
    let rej = make_builtin(format!("@@preject:{id}"));
    if let Err(e) = host::invoke(&executor, vec![res, rej], None) {
        // A throw in the executor rejects the promise.
        let ev = host::take_exc_or_error(&e);
        host::reject_promise_val(id, ev);
    }
    Ok(p)
}

/// `Promise.resolve(v)` for stdlib callers that need to hand back an
/// already-settled promise.
pub fn promise_resolve_pub(v: Value) -> Result<Value, String> {
    promise_resolve(v)
}

fn promise_resolve(v: Value) -> Result<Value, String> {
    if let Some(p) = promise_species_create()? {
        let id = with_host(|h| h.promise_id(&p).unwrap());
        host::resolve_promise_val(id, v);
        return Ok(p);
    }
    Ok(host::promise_of(&v))
}
fn promise_reject(v: Value) -> Result<Value, String> {
    let p = match promise_species_create()? {
        Some(p) => p,
        None => with_host(|h| h.new_promise()),
    };
    let id = with_host(|h| h.promise_id(&p).unwrap());
    host::reject_promise_val(id, v);
    Ok(p)
}

/// `Promise.withResolvers()` — a fresh pending promise paired with its own
/// resolve/reject continuations (the same `@@presolve`/`@@preject` thunks the
/// executor receives), returned as a plain `{ promise, resolve, reject }` object.
/// A fresh pending promise paired with the thunk that resolves it, for stdlib
/// callers that hand the resolver to an event listener.
pub fn pending_promise_with_resolver() -> (Value, Value) {
    let p = with_host(|h| h.new_promise());
    let id = with_host(|h| h.promise_id(&p).unwrap());
    let resolve = make_builtin(format!("@@presolve:{id}"));
    (p, resolve)
}

/// `RegExp.escape(s)` (22.2.4.2) — a string that matches `s` literally.
///
/// The rule is not "backslash the syntax characters": it also escapes a LEADING
/// ASCII alphanumeric, so the result can be concatenated after a `\` or a `{`
/// without the two running together, and it escapes the punctuation that is
/// meaningful inside a character class or a group name.
fn regexp_escape(args: Vec<Value>) -> Result<Value, String> {
    let v = arg0(&args);
    if !matches!(v, Value::Str(_)) && !with_host(|h| matches!(h.get(&v), Some(JsObj::Str(_)))) {
        return Err(host::type_error("input argument must be a string"));
    }
    let s = with_host(|h| h.str_of(&v));
    // Punctuation that is escaped by CODE POINT rather than with a backslash.
    // Measured against node over the whole ASCII range, not taken from a list:
    // `-` and `=` are here, `$` and `*` are syntax characters and are not.
    const OTHER_PUNCTUATORS: &str = " !\"#%&',-:;<=>@`~";
    const SYNTAX: &str = "^$\\.*+?()[]{}|/";
    let mut out = String::with_capacity(s.len());
    for (i, c) in s.chars().enumerate() {
        // A leading ASCII alphanumeric, and only a leading one.
        if i == 0 && c.is_ascii_alphanumeric() {
            out.push_str(&format!("\\x{:02x}", c as u32));
            continue;
        }
        if SYNTAX.contains(c) {
            out.push('\\');
            out.push(c);
            continue;
        }
        match c {
            '\t' => out.push_str("\\t"),
            '\n' => out.push_str("\\n"),
            '\u{b}' => out.push_str("\\v"),
            '\u{c}' => out.push_str("\\f"),
            '\r' => out.push_str("\\r"),
            _ if OTHER_PUNCTUATORS.contains(c) || is_regex_escape_space(c) => {
                let n = c as u32;
                if n <= 0xff {
                    out.push_str(&format!("\\x{n:02x}"));
                } else {
                    out.push_str(&format!("\\u{n:04x}"));
                }
            }
            _ => out.push(c),
        }
    }
    Ok(with_host(|h| h.new_str(out)))
}

/// The WhiteSpace and LineTerminator code points `RegExp.escape` spells out.
/// Deliberately NOT `char::is_whitespace`: U+180E and U+200B are whitespace to
/// Unicode but not to ECMAScript, and node leaves both alone.
fn is_regex_escape_space(c: char) -> bool {
    matches!(
        c,
        '\u{a0}' | '\u{1680}' | '\u{2000}'
            ..='\u{200a}'
                | '\u{2028}'
                | '\u{2029}'
                | '\u{202f}'
                | '\u{205f}'
                | '\u{3000}'
                | '\u{feff}'
    )
}

/// `Error.isError(v)` (20.5.2.1) — a brand check for `[[ErrorData]]`, so an
/// object that merely INHERITS from `Error.prototype` is not one.
fn error_is_error(args: Vec<Value>) -> Result<Value, String> {
    let v = arg0(&args);
    Ok(Value::Bool(with_host(|h| has_error_data(h, &v))))
}

/// Whether `v` carries `[[ErrorData]]` — the slot `Error.isError` (20.5.2.1)
/// and `Object.prototype.toString`'s step 9 both test.
///
/// The brand is the OWN `stack` an error is built with (a `DOMException`
/// carries `@@domName` instead); a plain `Object.create(Error.prototype)` has
/// neither, which is why inheriting from an error prototype does not make a
/// value an error. Shared so the two cannot disagree — branding by a chain
/// lookup for `name`/`message` made `Object.create(Error.prototype)` report
/// `[object Error]` where node says `[object Object]`, while `Error.isError`
/// on the same value already said false.
pub(crate) fn has_error_data(h: &host::JsHost, v: &Value) -> bool {
    match h.get(v) {
        Some(JsObj::Object(p)) => {
            p.contains_key("stack") || p.contains_key("@@stackRaw") || p.contains_key("@@domName")
        }
        _ => false,
    }
}

/// `Promise.try(fn, ...args)` (27.2.4.6) — call `fn` and settle the promise with
/// what it does, so a SYNCHRONOUS throw becomes a rejection instead of
/// propagating. `Promise.resolve().then(fn)` is the shape it replaces, and it
/// costs a tick that this does not.
fn promise_try(args: Vec<Value>) -> Result<Value, String> {
    let f = arg0(&args);
    // A non-callable argument REJECTS, it does not throw: `Promise.try(5)`
    // returns a rejected promise, so the surrounding `try` never sees it.
    if !with_host(|h| host::is_callable(h, &f)) {
        // Node names the TYPE alongside the value — `number 5 is not a
        // function` — which the ordinary call-site message does not. A plain
        // object and a symbol name only the type; `null` names both.
        let shown = with_host(|h| {
            let kind = h.type_of(&f);
            match kind {
                "undefined" => "undefined".to_string(),
                "symbol" | "bigint" => kind.to_string(),
                "object" if h.is_null(&f) => "object null".to_string(),
                "object" => "object".to_string(),
                "string" => format!("string \"{}\"", h.str_of(&f)),
                _ => format!("{kind} {}", h.str_of(&f)),
            }
        });
        let p = with_host(|h| h.new_promise());
        let id = with_host(|h| h.promise_id(&p).unwrap());
        let reject = make_builtin(format!("@@preject:{id}"));
        let err =
            with_host(|h| synth_error(h, &host::type_error(&format!("{shown} is not a function"))));
        host::invoke(&reject, vec![err], None)?;
        return Ok(p);
    }
    let rest: Vec<Value> = args.iter().skip(1).cloned().collect();
    let p = with_host(|h| h.new_promise());
    let id = with_host(|h| h.promise_id(&p).unwrap());
    let resolve = make_builtin(format!("@@presolve:{id}"));
    let reject = make_builtin(format!("@@preject:{id}"));
    let promise = p;
    match host::invoke(&f, rest, None) {
        Ok(v) => {
            host::invoke(&resolve, vec![v], None)?;
        }
        Err(e) => {
            // The thrown VALUE, not a re-synthesis of its rendering: a callback
            // that throws a `TypeError` must reject with that object, and
            // rebuilding it from the message string flattened it to a plain
            // `Error` whose message was the rendered `Uncaught TypeError: t`.
            let err =
                with_host(|h| h.exc.clone()).unwrap_or_else(|| with_host(|h| synth_error(h, &e)));
            with_host(|h| {
                h.error = None;
                h.exc = None;
            });
            host::invoke(&reject, vec![err], None)?;
        }
    }
    Ok(promise)
}

fn promise_with_resolvers() -> Result<Value, String> {
    let p = with_host(|h| h.new_promise());
    let id = with_host(|h| h.promise_id(&p).unwrap());
    let resolve = make_builtin(format!("@@presolve:{id}"));
    let reject = make_builtin(format!("@@preject:{id}"));
    let mut props: IndexMap<String, Value> = IndexMap::new();
    props.insert("promise".into(), p);
    props.insert("resolve".into(), resolve);
    props.insert("reject".into(), reject);
    Ok(with_host(|h| h.new_object(props)))
}

/// A promise already rejected with `e` — what every combinator hands back when
/// the ITERABLE misbehaves.
///
/// 27.2.4.1 step 4 catches an abrupt completion from the iteration and rejects
/// rather than letting it propagate, so `Promise.all(badIterable)` returns a
/// rejected promise. Throwing synchronously meant a `.catch()` never attached
/// and the caller saw the error at the call site instead.
fn rejected_promise(e: String) -> Value {
    let p = with_host(|h| h.new_promise());
    let id = with_host(|h| h.promise_id(&p).unwrap());
    let reject = make_builtin(format!("@@preject:{id}"));
    let err = with_host(|h| h.exc.clone()).unwrap_or_else(|| with_host(|h| synth_error(h, &e)));
    with_host(|h| {
        h.error = None;
        h.exc = None;
    });
    let _ = host::invoke(&reject, vec![err], None);
    p
}

#[derive(Clone, Copy)]
enum AllMode {
    All,
    AllSettled,
}

/// `Promise.all` / `Promise.allSettled`.
fn promise_all(args: Vec<Value>, mode: AllMode) -> Result<Value, String> {
    let items = match host::iter_all(&arg0(&args)) {
        Ok(v) => v,
        Err(e) => return Ok(rejected_promise(e)),
    };
    // 27.2.4.1 step 3: the combinator builds its result with `this`, so on a
    // subclass the promise it hands back is an instance of that subclass.
    let result = match promise_species_create()? {
        Some(p) => p,
        None => with_host(|h| h.new_promise()),
    };
    let rid = with_host(|h| h.promise_id(&result).unwrap());
    let n = items.len();
    if n == 0 {
        let empty = with_host(|h| h.new_array(Vec::new()));
        host::resolve_promise_val(rid, empty);
        return Ok(result);
    }
    // Shared mutable accumulator via Rc<RefCell<…>>.
    let slots = std::rc::Rc::new(std::cell::RefCell::new(vec![Value::Undef; n]));
    let remaining = std::rc::Rc::new(std::cell::RefCell::new(n));
    for (i, it) in items.into_iter().enumerate() {
        let ap = host::promise_of(&it);
        let aid = with_host(|h| h.promise_id(&ap).unwrap());
        let slots = slots.clone();
        let remaining = remaining.clone();
        host::subscribe_native(
            aid,
            Box::new(move |state, val| {
                let settled = match mode {
                    AllMode::All => {
                        if state == host::PromiseState::Rejected {
                            host::reject_promise_val(rid, val);
                            return Ok(());
                        }
                        val
                    }
                    AllMode::AllSettled => with_host(|h| {
                        let mut m: IndexMap<String, Value> = IndexMap::new();
                        if state == host::PromiseState::Rejected {
                            m.insert("status".into(), h.new_str("rejected"));
                            m.insert("reason".into(), val);
                        } else {
                            m.insert("status".into(), h.new_str("fulfilled"));
                            m.insert("value".into(), val);
                        }
                        h.new_object(m)
                    }),
                };
                slots.borrow_mut()[i] = settled;
                let mut r = remaining.borrow_mut();
                *r -= 1;
                if *r == 0 {
                    let arr = with_host(|h| h.new_array(slots.borrow().clone()));
                    host::resolve_promise_val(rid, arr);
                }
                Ok(())
            }),
        );
    }
    Ok(result)
}

/// `Promise.race` (first to settle wins) / `Promise.any` (first to fulfill wins).
fn promise_race(args: Vec<Value>, any: bool) -> Result<Value, String> {
    let items = match host::iter_all(&arg0(&args)) {
        Ok(v) => v,
        Err(e) => return Ok(rejected_promise(e)),
    };
    // Built with `this`, as every combinator is (27.2.4.5 / 27.2.4.3).
    let result = match promise_species_create()? {
        Some(p) => p,
        None => with_host(|h| h.new_promise()),
    };
    let rid = with_host(|h| h.promise_id(&result).unwrap());
    let n = items.len();
    let errors = std::rc::Rc::new(std::cell::RefCell::new(vec![Value::Undef; n]));
    let remaining = std::rc::Rc::new(std::cell::RefCell::new(n));
    for (i, it) in items.into_iter().enumerate() {
        let ap = host::promise_of(&it);
        let aid = with_host(|h| h.promise_id(&ap).unwrap());
        let errors = errors.clone();
        let remaining = remaining.clone();
        host::subscribe_native(
            aid,
            Box::new(move |state, val| {
                if any {
                    if state == host::PromiseState::Fulfilled {
                        host::resolve_promise_val(rid, val);
                    } else {
                        errors.borrow_mut()[i] = val;
                        let mut r = remaining.borrow_mut();
                        *r -= 1;
                        if *r == 0 {
                            // All rejected → AggregateError carrying every reason.
                            let reasons = with_host(|h| h.new_array(errors.borrow().clone()));
                            let msg = with_host(|h| h.new_str("All promises were rejected"));
                            let agg = make_error_inner("AggregateError", &[reasons, msg]);
                            host::reject_promise_val(rid, agg);
                        }
                    }
                } else if state == host::PromiseState::Rejected {
                    host::reject_promise_val(rid, val);
                } else {
                    host::resolve_promise_val(rid, val);
                }
                Ok(())
            }),
        );
    }
    Ok(result)
}

/// `.then` / `.catch` / `.finally` on a promise.
fn promise_method(recv: &Value, name: &str, args: Vec<Value>) -> Result<Value, String> {
    match name {
        "then" => Ok(host::promise_then(
            recv,
            args.first().cloned().unwrap_or(Value::Undef),
            args.get(1).cloned().unwrap_or(Value::Undef),
        )),
        "catch" => Ok(host::promise_then(
            recv,
            Value::Undef,
            args.first().cloned().unwrap_or(Value::Undef),
        )),
        "finally" => {
            let cb = arg0(&args);
            // 27.2.5.3 step 3: a non-callable `onFinally` is handed to `then`
            // as BOTH handlers, and `then` ignores a non-callable one — so the
            // value or reason simply passes through. Building the thunks
            // regardless meant `p.finally(null)` tried to call `null`.
            if !with_host(|h| host::is_callable(h, &cb)) {
                return Ok(host::promise_then(recv, cb.clone(), cb));
            }
            let i = match cb {
                Value::Obj(i) => i,
                _ => 0,
            };
            let pass = make_builtin(format!("@@finpass:{i}"));
            let throw = make_builtin(format!("@@finthrow:{i}"));
            Ok(host::promise_then(recv, pass, throw))
        }
        _ => Err(host::type_error(&format!(
            "promise.{name} is not a function"
        ))),
    }
}

fn enqueue_microtask(next_tick: bool, cb: Value, args: Vec<Value>) {
    with_host(|h| {
        if next_tick {
            h.queue_nexttick(cb, args);
        } else {
            h.queue_micro(cb, args);
        }
    });
}

/// `setTimeout`/`setInterval`/`setImmediate` — register a macrotask and return
/// the handle object Node returns (`Timeout` for the first two, `Immediate` for
/// the third), carrying `ref`/`unref`/`hasRef`/`refresh`.
///
/// `setInterval` schedules a *repeating* timer: the loop re-arms it each time it
/// fires, so it runs until cleared and — being referenced — holds the process
/// open exactly as in Node.
fn schedule_timer(name: &str, args: Vec<Value>) -> Value {
    let cb = arg0(&args);
    let delay = if name == "setImmediate" {
        -1.0 // before any 0ms timeout
    } else {
        args.get(1)
            .map(|d| with_host(|h| h.to_number(d)))
            .unwrap_or(0.0)
            .max(0.0)
    };
    let extra = if name == "setImmediate" {
        args.get(1..).map(|s| s.to_vec()).unwrap_or_default()
    } else {
        args.get(2..).map(|s| s.to_vec()).unwrap_or_default()
    };
    // Node clamps a sub-1ms interval to 1ms, so `setInterval(fn, 0)` yields a
    // ~1000Hz timer rather than a busy loop that starves the rest of the queue.
    let interval = (name == "setInterval").then(|| delay.max(1.0));
    let id = with_host(|h| h.add_timer(delay, cb, extra, interval));
    let tag = if name == "setImmediate" {
        "Immediate"
    } else {
        "Timeout"
    };
    crate::stdlib::timers::new_handle(id, tag)
}

/// `clearTimeout`/`clearInterval`/`clearImmediate` — cancel by handle object or
/// by the bare id it coerces to (code that stored `+timer` still works).
fn clear_timer(v: &Value) {
    let id =
        crate::stdlib::timers::handle_id(v).unwrap_or_else(|| with_host(|h| h.to_number(v)) as u64);
    with_host(|h| h.cancel_timer(id));
}