jscan-rs

High-performance zero-allocation JSON iterator and validator for Rust.

A faithful port of romshark/jscan/v2 (Go).

What it does

jscan traverses JSON without deserializing it. Instead of building a DOM or mapping to structs, it calls your closure for every value it encounters — giving you the type, key, array index, raw value slice, nesting depth, and JSON Pointer path. Validation happens as a side effect.

This is useful when you need to:

• Extract a few fields from large JSON without parsing the whole thing
• Validate JSON syntax at high speed
• Stream-process JSON values with minimal allocations
• Build custom deserializers

Usage

use jscan::{scan, ValueType};

let json = br#"{"name":"Alice","scores":[98,76,100]}"#;

scan(json, |iter| {
    match iter.value_type() {
        ValueType::Number => {
            println!("{} = {}", iter.pointer(), std::str::from_utf8(iter.value()).unwrap());
        }
        _ => {}
    }
    false // return true to stop early
});
// Output:
// /scores/0 = 98
// /scores/1 = 76
// /scores/2 = 100

Validation only

use jscan::valid;

assert!(valid(br#"{"key": [1, 2, 3]}"#));
assert!(!valid(br#"{"trailing": 1,}"#));

Scanning multiple concatenated values

use jscan::scan_one;

let input = br#"123"hello"null"#;
let (rest, err) = scan_one(input, |iter| { /* ... */ false });
// rest = b#""hello"null"#, err = None

Reusable parser (avoids repeated allocation)

use jscan::Parser;

let mut parser = Parser::new(64);
for json_doc in &[br#""a""#.as_slice(), br#"[1,2]"#.as_slice()] {
    let err = parser.scan(json_doc, |iter| {
        // ...
        false
    });
}

Iterator API

Inside the callback, the Iterator provides:

License

MIT — same as the original Go library.