<pre class='metadata'>
Title: WebAssembly JavaScript Interface
Shortname: wasm-js-api
Group: wasm
Status: ED
Issue Tracking: GitHub https://github.com/WebAssembly/spec/issues
Level: 2
TR: https://www.w3.org/TR/wasm-js-api-2/
ED: https://webassembly.github.io/spec/js-api/
Implementation Report: https://webassembly.org/features/
Editor: Ms2ger, w3cid 46309, Igalia
Repository: WebAssembly/spec
Markup Shorthands: css no, markdown yes
Abstract: This document provides an explicit JavaScript API for interacting with WebAssembly.
Prepare For TR: true
Date: now
</pre>
<pre class='biblio'>
{
"WEBASSEMBLY": {
"href": "https://webassembly.github.io/spec/core/",
"title": "WebAssembly Core Specification",
"publisher": "W3C WebAssembly Community Group",
"status": "Draft"
},
"WASMWEB": {
"href": "https://webassembly.github.io/spec/js-api/",
"title": "WebAssembly Web API Specification",
"publisher": "W3C WebAssembly Community Group",
"status": "Draft"
}
}
</pre>
<pre class="anchors">
urlPrefix: https://tc39.github.io/ecma262/; spec: ECMASCRIPT
type: interface; for: ECMAScript
text: ArrayBuffer; url: sec-arraybuffer-objects
type: exception; for: ECMAScript
text: Error; url: sec-error-objects
text: NativeError; url: sec-nativeerror-constructors
text: TypeError; url: sec-native-error-types-used-in-this-standard-typeerror
text: RangeError; url: sec-native-error-types-used-in-this-standard-rangeerror
type: dfn
url: sec-returnifabrupt-shorthands
text: !
text: ?
url: sec-ecmascript-language-types-bigint-type
text: is a BigInt
text: is not a BigInt
url: sec-ecmascript-language-types-boolean-type
text: is a Boolean
text: is not a Boolean
url: sec-ecmascript-language-types-number-type
text: is a Number
text: is not a Number
url: sec-ecmascript-language-types-string-type
text: is a String
text: is not a String
url: sec-ecmascript-language-types-symbol-type
text: is a Symbol
text: is not a Symbol
url: sec-object-type
text: is an Object
text: is not an Object
text: current Realm; url: current-realm
text: Built-in Function Objects; url: sec-built-in-function-objects
text: NativeError Object Structure; url: sec-nativeerror-object-structure
text: 𝔽; url: #𝔽
text: ℤ; url: #ℤ
text: SameValue; url: sec-samevalue
text: Array; url: sec-array-exotic-objects
text: BigInt; url: sec-ecmascript-language-types-bigint-type
urlPrefix: https://webassembly.github.io/spec/core/; spec: WebAssembly; type: dfn
text: embedding interface; url: appending/embedding.html
text: scope; url: intro/introduction.html#scope
url: valid/modules.html#valid-module
text: valid
text: WebAssembly module validation
text: valid limits; url: valid/types.html#valid-limits
text: valid memtype; url: valid/types.html#valid-memtype
text: valid tabletype; url: valid/types.html#valid-tabletype
text: module grammar; url: binary/modules.html#binary-module
text: custom section; url: binary/modules.html#custom-section
text: customsec; url: binary/modules.html#binary-customsec
text: memory instance; url: exec/runtime.html#memory-instances
text: table instance; url: exec/runtime.html#table-instances
text: global instance; url: exec/runtime.html#global-instances
text: trap; url: exec/runtime.html#syntax-trap
url: exec/runtime.html#values
text: WebAssembly value
text: i64.const
text: i32.const
text: f32.const
text: f64.const
text: v128.const
text: ref.null
text: ref.func
text: ref.extern
text: function index; url: syntax/modules.html#syntax-funcidx
text: function instance; url: exec/runtime.html#function-instances
text: store_init; url: appendix/embedding.html#embed-store-init
text: module_decode; url: appendix/embedding.html#embed-module-decode
text: module_validate; url: appendix/embedding.html#embed-module-validate
text: module_instantiate; url: appendix/embedding.html#embed-module-instantiate
text: module_imports; url: appendix/embedding.html#embed-module-imports
text: module_exports; url: appendix/embedding.html#embed-module-exports
text: instance_export; url: appendix/embedding.html#embed-instance-export
text: func_alloc; url: appendix/embedding.html#embed-func-alloc
text: func_type; url: appendix/embedding.html#embed-func-type
text: func_invoke; url: appendix/embedding.html#embed-func-invoke
text: table_alloc; url: appendix/embedding.html#embed-table-alloc
text: table_type; url: appendix/embedding.html#embed-table-type
text: table_read; url: appendix/embedding.html#embed-table-read
text: table_write; url: appendix/embedding.html#embed-table-write
text: table_size; url: appendix/embedding.html#embed-table-size
text: table_grow; url: appendix/embedding.html#embed-table-grow
text: mem_alloc; url: appendix/embedding.html#embed-mem-alloc
text: mem_type; url: appendix/embedding.html#embed-mem-type
text: mem_read; url: appendix/embedding.html#embed-mem-read
text: mem_write; url: appendix/embedding.html#embed-mem-write
text: mem_size; url: appendix/embedding.html#embed-mem-size
text: mem_grow; url: appendix/embedding.html#embed-mem-grow
text: global_alloc; url: appendix/embedding.html#embed-global-alloc
text: global_type; url: appendix/embedding.html#embed-global-type
text: global_read; url: appendix/embedding.html#embed-global-read
text: global_write; url: appendix/embedding.html#embed-global-write
text: error; url: appendix/embedding.html#embed-error
text: store; url: exec/runtime.html#syntax-store
text: limits; url: syntax/types.html#syntax-limits
text: table type; url: syntax/types.html#syntax-tabletype
text: table address; url: exec/runtime.html#syntax-tableaddr
text: function address; url: exec/runtime.html#syntax-funcaddr
text: memory type; url: syntax/types.html#syntax-memtype
text: memory address; url: exec/runtime.html#syntax-memaddr
text: global address; url: exec/runtime.html#syntax-globaladdr
text: extern address; url: exec/runtime.html#syntax-externaddr
text: page size; url: exec/runtime.html#page-size
url: syntax/types.html#syntax-numtype
text: i32
text: i64
text: f32
text: f64
url: syntax/types.html#vector-types
text: v128
url: syntax/types.html#syntax-reftype
text: reftype
text: funcref
text: externref
url: syntax/values.html#syntax-float
text: +∞
text: −∞
text: nan
text: canon
text: signif
text: function element; url: exec/runtime.html#syntax-funcelem
text: import component; url: syntax/modules.html#imports
text: external value; url: exec/runtime.html#syntax-externval
text: host function; url: exec/runtime.html#syntax-hostfunc
text: the instantiation algorithm; url: exec/modules.html#instantiation
text: module; url: syntax/modules.html#syntax-module
text: imports; url: syntax/modules.html#syntax-module
text: import; url: syntax/modules.html#syntax-import
url: syntax/types.html#external-types
text: external type
text: func
text: table
text: mem
text: global
text: global type; url: syntax/types.html#syntax-globaltype
url: syntax/types.html#syntax-mut
text: var
text: const
text: address; url: exec/runtime.html#addresses
text: signed_32; url: exec/numerics.html#aux-signed
text: memory.grow; url: exec/instructions.html#exec-memory-grow
text: current frame; url: exec/conventions.html#exec-notation-textual
text: module; for: frame; url: exec/runtime.html#syntax-frame
text: memaddrs; for: moduleinst; url: exec/runtime.html#syntax-moduleinst
text: signed_64; url: exec/numerics.html#aux-signed
text: sequence; url: syntax/conventions.html#grammar-notation
urlPrefix: https://heycam.github.io/webidl/; spec: WebIDL
type: dfn
text: create a namespace object; url: create-a-namespace-object
urlPrefix: https://tc39.es/proposal-resizablearraybuffer/; spec: ResizableArrayBuffer proposal
type: dfn
text: handled; url: sec-hostresizearraybuffer
text: IsFixedLengthArrayBuffer; url: sec-isfixedarraybuffer
text: HostResizeArrayBuffer; url: sec-hostresizearraybuffer
</pre>
<pre class='link-defaults'>
spec:infra; type:dfn; text:list
spec:ecma-262; type:exception; for:ECMAScript; text:Error
spec:ecmascript; type:exception; for:ECMAScript; text:TypeError
spec:ecmascript; type:exception; for:ECMAScript; text:RangeError
spec:ecmascript; type:interface; for:ECMAScript; text:ArrayBuffer
spec:ecmascript; type:dfn; text:agent
spec:webidl; type:dfn; text:resolve
</pre>
<style>
emu-const {
font-family: serif;
}
</style>
<h2 id="intro">Introduction</h2>
By design, the [=scope=] of the WebAssembly core specification [[WEBASSEMBLY]] does not include a description of how WebAssembly programs interact with their surrounding execution environment.
Instead it defines an abstract [=embedding interface=] between WebAssembly and its environment, (called the *embedder*).
It is only through this interface that an embedder interacts with the semantics of WebAssembly, and the embedder implements the connection between its host environment and the embedding API.
This document describes the embedding of WebAssembly into JavaScript [[ECMASCRIPT]] environments, including how WebAssembly modules can be constructed and instantiated, how imported and exported functions are called, how data is exchanged, and how errors are handled.
When the JavaScript environment is itself embedded in a Web browser, the Web API spec [[WASMWEB]] describes additional behavior relevant to the Web environment.
<h2 id="sample">Sample API Usage</h2>
<p><em>This section is non-normative.</em></p>
Given `demo.wat` (encoded to `demo.wasm`):
```lisp
(module
(import "js" "import1" (func $i1))
(import "js" "import2" (func $i2))
(func $main (call $i1))
(start $main)
(func (export "f") (call $i2))
)
```
and the following JavaScript, run in a browser:
```javascript
var importObj = {js: {
import1: () => console.log("hello,"),
import2: () => console.log("world!")
}};
fetch('demo.wasm').then(response =>
response.arrayBuffer()
).then(buffer =>
WebAssembly.instantiate(buffer, importObj)
).then(({module, instance}) =>
instance.exports.f()
);
```
<h2 id="notation">Notation</h2>
This specification depends on the Infra Standard. [[INFRA]]
The WebAssembly [=sequence=] type is equivalent to the [=list=] type defined there; values of one
are treated as values of the other transparently.
<h2 id="webassembly-storage">Internal storage</h2>
<h3 id="store">Interaction of the WebAssembly Store with JavaScript</h3>
Note: WebAssembly semantics are defined in terms of an abstract [=store=], representing the state of the WebAssembly abstract machine. WebAssembly operations take a store and return an updated store.
Each [=agent=] has an <dfn>associated store</dfn>. When a new agent is created, its associated store is set to the result of [=store_init=]().
Note: In this specification, no WebAssembly-related objects, memory or addresses can be shared among agents in an [=agent cluster=]. In a future version of WebAssembly, this may change.
Elements of the WebAssembly store may be <dfn>identified with</dfn> JavaScript values. In particular, each WebAssembly [=memory instance=] with a corresponding {{Memory}} object is identified with a JavaScript [=Data Block=]; modifications to this Data Block are identified to updating the agent's store to a store which reflects those changes, and vice versa.
<h3 id="object-caches">WebAssembly JS Object Caches</h3>
Note: There are several WebAssembly objects that may have a corresponding JavaScript object. The correspondence is stored in a per-agent mapping from WebAssembly [=address=]es to JavaScript objects.
This mapping is used to ensure that, for a given [=agent=], there exists at most one JavaScript object for a particular WebAssembly address. However, this property does not hold for shared objects.
Each [=agent=] is associated with the following [=ordered map=]s:
* The <dfn>Memory object cache</dfn>, mapping [=memory address=]es to {{Memory}} objects.
* The <dfn>Table object cache</dfn>, mapping [=table address=]es to {{Table}} objects.
* The <dfn>Exported Function cache</dfn>, mapping [=function address=]es to [=Exported Function=] objects.
* The <dfn>Global object cache</dfn>, mapping [=global address=]es to {{Global}} objects.
* The <dfn>Extern value cache</dfn>, mapping [=extern address=]es to values.
<h2 id="webassembly-namespace">The WebAssembly Namespace</h2>
<pre class="idl">
dictionary WebAssemblyInstantiatedSource {
required Module module;
required Instance instance;
};
[Exposed=*]
namespace WebAssembly {
boolean validate(BufferSource bytes);
Promise<Module> compile(BufferSource bytes);
Promise<WebAssemblyInstantiatedSource> instantiate(
BufferSource bytes, optional object importObject);
Promise<Instance> instantiate(
Module moduleObject, optional object importObject);
};
</pre>
<!--
Should we include notes describing what the functions do, as the HTML spec does? It could look like this:
Note:
WebAssembly.validate(|bytes|) synchronously validates bytes of WebAssembly, returning true if the validation was successful.
WebAssembly.compile(|bytes|) asynchronously validates and complies bytes of WebAssembly into a Module.
WebAssembly.instantiate(|bytes|, |importObject|) asynchronously compiles and instantiates a WebAssembly module from bytes of source.
The WebAssembly.instantiate(|moduleObject|, |importObject|) asynchronously instantiates a compiled module.
-->
<div algorithm>
To <dfn>compile a WebAssembly module</dfn> from source bytes |bytes|, perform the following steps:
1. Let |module| be [=module_decode=](|bytes|). If |module| is [=error=], return [=error=].
1. If [=module_validate=](|module|) is [=error=], return [=error=].
1. Return |module|.
</div>
<div algorithm>
The <dfn method for="WebAssembly">validate(|bytes|)</dfn> method, when invoked, performs the following steps:
1. Let |stableBytes| be a [=get a copy of the buffer source|copy of the bytes held by the buffer=] |bytes|.
1. [=Compile a WebAssembly module|Compile=] |stableBytes| as a WebAssembly module and store the results as |module|.
1. If |module| is [=error=], return false.
1. Return true.
</div>
A {{Module}} object represents a single WebAssembly module. Each {{Module}} object has the following internal slots:
* \[[Module]] : a WebAssembly [=/module=]
* \[[Bytes]] : the source bytes of \[[Module]].
<div algorithm>
To <dfn>construct a WebAssembly module object</dfn> from a module |module| and source bytes |bytes|, perform the following steps:
1. Let |moduleObject| be a new {{Module}} object.
1. Set |moduleObject|.\[[Module]] to |module|.
1. Set |moduleObject|.\[[Bytes]] to |bytes|.
1. Return |moduleObject|.
</div>
<div algorithm>
To <dfn>asynchronously compile a WebAssembly module</dfn> from source bytes |bytes|, using optional [=task source=] |taskSource|, perform the following steps:
1. Let |promise| be [=a new promise=].
1. Run the following steps [=in parallel=]:
1. [=compile a WebAssembly module|Compile the WebAssembly module=] |bytes| and store the result as |module|.
1. [=Queue a task=] to perform the following steps. If |taskSource| was provided, queue the task on that task source.
1. If |module| is [=error=], reject |promise| with a {{CompileError}} exception.
1. Otherwise,
1. [=Construct a WebAssembly module object=] from |module| and |bytes|, and let |moduleObject| be the result.
1. [=Resolve=] |promise| with |moduleObject|.
1. Return |promise|.
</div>
<div algorithm>
The <dfn method for="WebAssembly">compile(|bytes|)</dfn> method, when invoked, performs the following steps:
1. Let |stableBytes| be a [=get a copy of the buffer source|copy of the bytes held by the buffer=] |bytes|.
1. [=Asynchronously compile a WebAssembly module=] from |stableBytes| and return the result.
</div>
<div algorithm="read-the-imports">
To <dfn>read the imports</dfn> from a WebAssembly module |module| from imports object |importObject|, perform the following steps:
1. If |module|.[=imports=] [=list/is empty|is not empty=], and |importObject| is undefined, throw a {{TypeError}} exception.
1. Let |imports| be « ».
1. [=list/iterate|For each=] (|moduleName|, |componentName|, |externtype|) of [=module_imports=](|module|),
1. Let |o| be [=?=] [$Get$](|importObject|, |moduleName|).
1. If |o| [=is not an Object=], throw a {{TypeError}} exception.
1. Let |v| be [=?=] [$Get$](|o|, |componentName|).
1. If |externtype| is of the form [=func=] |functype|,
1. If [$IsCallable$](|v|) is false, throw a {{LinkError}} exception.
1. If |v| has a \[[FunctionAddress]] internal slot, and therefore is an [=Exported Function=],
1. Let |funcaddr| be the value of |v|'s \[[FunctionAddress]] internal slot.
1. Otherwise,
1. [=Create a host function=] from |v| and |functype|, and let |funcaddr| be the result.
1. Let |index| be the number of external functions in |imports|. This value |index| is known as the <dfn>index of the host function</dfn> |funcaddr|.
1. Let |externfunc| be the [=external value=] [=external value|func=] |funcaddr|.
1. [=list/Append=] |externfunc| to |imports|.
1. If |externtype| is of the form [=global=] <var ignore>mut</var> |valtype|,
1. If |v| [=is a Number=] or |v| [=is a BigInt=],
1. If |valtype| is [=i64=] and |v| [=is a Number=],
1. Throw a {{LinkError}} exception.
1. If |valtype| is not [=i64=] and |v| [=is a BigInt=],
1. Throw a {{LinkError}} exception.
1. If |valtype| is [=v128=],
1. Throw a {{LinkError}} exception.
1. Let |value| be [=ToWebAssemblyValue=](|v|, |valtype|).
1. Let |store| be the [=surrounding agent=]'s [=associated store=].
1. Let (|store|, |globaladdr|) be [=global_alloc=](|store|, [=const=] |valtype|, |value|).
1. Set the [=surrounding agent=]'s [=associated store=] to |store|.
1. Otherwise, if |v| [=implements=] {{Global}},
1. Let |globaladdr| be |v|.\[[Global]].
1. Otherwise,
1. Throw a {{LinkError}} exception.
1. Let |externglobal| be [=external value|global=] |globaladdr|.
1. [=list/Append=] |externglobal| to |imports|.
1. If |externtype| is of the form [=mem=] <var ignore>memtype</var>,
1. If |v| does not [=implement=] {{Memory}}, throw a {{LinkError}} exception.
1. Let |externmem| be the [=external value=] [=external value|mem=] |v|.\[[Memory]].
1. [=list/Append=] |externmem| to |imports|.
1. If |externtype| is of the form [=table=] <var ignore>tabletype</var>,
1. If |v| does not [=implement=] {{Table}}, throw a {{LinkError}} exception.
1. Let |tableaddr| be |v|.\[[Table]].
1. Let |externtable| be the [=external value=] [=external value|table=] |tableaddr|.
1. [=list/Append=] |externtable| to |imports|.
1. Return |imports|.
Note: This algorithm only verifies the right kind of JavaScript values are passed.
The verification of WebAssembly type requirements is deferred to the
"[=instantiate the core of a WebAssembly module=]" algorithm.
</div>
<div algorithm>
To <dfn>create an exports object</dfn> from a WebAssembly module |module| and instance |instance|, perform the following steps:
1. Let |exportsObject| be [=!=] [$OrdinaryObjectCreate$](null).
1. [=list/iterate|For each=] (|name|, |externtype|) of [=module_exports=](|module|),
1. Let |externval| be [=instance_export=](|instance|, |name|).
1. Assert: |externval| is not [=error=].
1. If |externtype| is of the form [=func=] <var ignore>functype</var>,
1. Assert: |externval| is of the form [=external value|func=] |funcaddr|.
1. Let [=external value|func=] |funcaddr| be |externval|.
1. Let |func| be the result of creating [=a new Exported Function=] from |funcaddr|.
1. Let |value| be |func|.
1. If |externtype| is of the form [=global=] <var ignore>mut</var> <var ignore>globaltype</var>,
1. Assert: |externval| is of the form [=external value|global=] |globaladdr|.
1. Let [=external value|global=] |globaladdr| be |externval|.
1. Let |global| be [=create a global object|a new Global object=] created from |globaladdr|.
1. Let |value| be |global|.
1. If |externtype| is of the form [=mem=] <var ignore>memtype</var>,
1. Assert: |externval| is of the form [=external value|mem=] |memaddr|.
1. Let [=external value|mem=] |memaddr| be |externval|.
1. Let |memory| be [=create a memory object|a new Memory object=] created from |memaddr|.
1. Let |value| be |memory|.
1. If |externtype| is of the form [=table=] <var ignore>tabletype</var>,
1. Assert: |externval| is of the form [=external value|table=] |tableaddr|.
1. Let [=external value|table=] |tableaddr| be |externval|.
1. Let |table| be [=create a Table object|a new Table object=] created from |tableaddr|.
1. Let |value| be |table|.
1. Let |status| be [=!=] [$CreateDataProperty$](|exportsObject|, |name|, |value|).
1. Assert: |status| is true.
Note: the validity and uniqueness checks performed during [=WebAssembly module validation=] ensure that each property name is valid and no properties are defined twice.
1. Perform [=!=] [$SetIntegrityLevel$](|exportsObject|, `"frozen"`).
1. Return |exportsObject|.
</div>
<div algorithm>
To <dfn>initialize an instance object</dfn> |instanceObject| from a WebAssembly module |module| and instance |instance|, perform the following steps:
1. [=Create an exports object=] from |module| and |instance| and let |exportsObject| be the result.
1. Set |instanceObject|.\[[Instance]] to |instance|.
1. Set |instanceObject|.\[[Exports]] to |exportsObject|.
</div>
<div algorithm>
To <dfn>instantiate the core of a WebAssembly module</dfn> from a module |module| and imports |imports|, perform the following steps:
1. Let |store| be the [=surrounding agent=]'s [=associated store=].
1. Let |result| be [=module_instantiate=](|store|, |module|, |imports|).
1. If |result| is [=error=], throw an appropriate exception type:
* A {{LinkError}} exception for most cases which occur during linking.
* If the error came when running the start function, throw a {{RuntimeError}} for most errors which occur from WebAssembly, or the error object propagated from inner ECMAScript code.
* Another error type if appropriate, for example an out-of-memory exception, as documented in <a href="#errors">the WebAssembly error mapping</a>.
1. Let (|store|, |instance|) be |result|.
1. Set the [=surrounding agent=]'s [=associated store=] to |store|.
1. Return |instance|.
</div>
<div algorithm>
To <dfn>asynchronously instantiate a WebAssembly module</dfn> from a {{Module}} |moduleObject| and imports |importObject|, perform the following steps:
1. Let |promise| be [=a new promise=].
1. Let |module| be |moduleObject|.\[[Module]].
1. [=Read the imports=] of |module| with imports |importObject|, and let |imports| be the result.
If this operation throws an exception, catch it, [=reject=] |promise| with the exception, and return |promise|.
1. Run the following steps [=in parallel=]:
1. [=Queue a task=] to perform the following steps:
Note: Implementation-specific work may be performed here.
1. [=Instantiate the core of a WebAssembly module=] |module| with |imports|, and let |instance| be the result.
If this throws an exception, catch it, [=reject=] |promise| with the exception, and terminate these substeps.
1. Let |instanceObject| be a [=/new=] {{Instance}}.
1. [=initialize an instance object|Initialize=] |instanceObject| from |module| and |instance|.
If this throws an exception, catch it, [=reject=] |promise| with the exception, and terminate these substeps.
1. [=Resolve=] |promise| with |instanceObject|.
1. Return |promise|.
</div>
<div algorithm>
To <dfn>instantiate a promise of a module</dfn> |promiseOfModule| with imports |importObject|, perform the following steps:
1. Let |promise| be [=a new promise=].
1. [=Upon fulfillment=] of |promiseOfModule| with value |module|:
1. [=asynchronously instantiate a WebAssembly module|Instantiate the WebAssembly module=] |module| importing |importObject|, and let |innerPromise| be the result.
1. [=Upon fulfillment=] of |innerPromise| with value |instance|.
1. Let |result| be the {{WebAssemblyInstantiatedSource}} value «[ "{{WebAssemblyInstantiatedSource/module}}" → |module|, "{{WebAssemblyInstantiatedSource/instance}}" → |instance| ]».
1. [=Resolve=] |promise| with |result|.
1. [=Upon rejection=] of |innerPromise| with reason |reason|:
1. [=Reject=] |promise| with |reason|.
1. [=Upon rejection=] of |promiseOfModule| with reason |reason|:
1. [=Reject=] |promise| with |reason|.
1. Return |promise|.
</div>
<div algorithm>
The <dfn method for="WebAssembly">instantiate(|bytes|, |importObject|)</dfn> method, when invoked, performs the following steps:
1. Let |stableBytes| be a [=get a copy of the buffer source|copy of the bytes held by the buffer=] |bytes|.
1. [=Asynchronously compile a WebAssembly module=] from |stableBytes| and let |promiseOfModule| be the result.
1. [=Instantiate a promise of a module|Instantiate=] |promiseOfModule| with imports |importObject| and return the result.
</div>
<div algorithm>
The <dfn method for="WebAssembly">instantiate(|moduleObject|, |importObject|)</dfn> method, when invoked, performs the following steps:
1. [=asynchronously instantiate a WebAssembly module|Asynchronously instantiate the WebAssembly module=] |moduleObject| importing |importObject|, and return the result.
</div>
Note: A follow-on streaming API is documented in the <a href="https://webassembly.github.io/spec/web-api/index.html">WebAssembly Web API</a>.
<h3 id="modules">Modules</h3>
<pre class="idl">
enum ImportExportKind {
"function",
"table",
"memory",
"global"
};
dictionary ModuleExportDescriptor {
required USVString name;
required ImportExportKind kind;
// Note: Other fields such as signature may be added in the future.
};
dictionary ModuleImportDescriptor {
required USVString module;
required USVString name;
required ImportExportKind kind;
};
[LegacyNamespace=WebAssembly, Exposed=*]
interface Module {
constructor(BufferSource bytes);
static sequence<ModuleExportDescriptor> exports(Module moduleObject);
static sequence<ModuleImportDescriptor> imports(Module moduleObject);
static sequence<ArrayBuffer> customSections(Module moduleObject, DOMString sectionName);
};
</pre>
<div algorithm>
The <dfn>string value of the extern type</dfn> |type| is
* "function" if |type| is of the form [=func=] <var ignore>functype</var>
* "table" if |type| is of the form [=table=] <var ignore>tabletype</var>
* "memory" if |type| is of the form [=mem=] <var ignore>memtype</var>
* "global" if |type| is of the form [=global=] <var ignore>globaltype</var>
</div>
<div algorithm>
The <dfn method for="Module">exports(|moduleObject|)</dfn> method, when invoked, performs the following steps:
1. Let |module| be |moduleObject|.\[[Module]].
1. Let |exports| be « ».
1. [=list/iterate|For each=] (|name|, |type|) of [=module_exports=](|module|),
1. Let |kind| be the [=string value of the extern type=] |type|.
1. Let |obj| be «[ "{{ModuleExportDescriptor/name}}" → |name|, "{{ModuleExportDescriptor/kind}}" → |kind| ]».
1. [=list/Append=] |obj| to |exports|.
1. Return |exports|.
</div>
<div algorithm>
The <dfn method for="Module">imports(|moduleObject|)</dfn> method, when invoked, performs the following steps:
1. Let |module| be |moduleObject|.\[[Module]].
1. Let |imports| be « ».
1. [=list/iterate|For each=] (|moduleName|, |name|, |type|) of [=module_imports=](|module|),
1. Let |kind| be the [=string value of the extern type=] |type|.
1. Let |obj| be «[ "{{ModuleImportDescriptor/module}}" → |moduleName|, "{{ModuleImportDescriptor/name}}" → |name|, "{{ModuleImportDescriptor/kind}}" → |kind| ]».
1. [=list/Append=] |obj| to |imports|.
1. Return |imports|.
</div>
<div algorithm>
The <dfn method for="Module">customSections(|moduleObject|, |sectionName|)</dfn> method, when invoked, performs the following steps:
1. Let |bytes| be |moduleObject|.\[[Bytes]].
1. Let |customSections| be « ».
1. [=list/iterate|For each=] [=custom section=] |customSection| of |bytes|, interpreted according to the [=module grammar=],
1. Let |name| be the <code>name</code> of |customSection|, [=UTF-8 decode without BOM or fail|decoded as UTF-8=].
1. Assert: |name| is not failure (|moduleObject|.\[[Module]] is [=valid=]).
1. If |name| equals |sectionName| as string values,
1. [=list/Append=] a new {{ArrayBuffer}} containing a copy of the bytes in |bytes| for the range matched by this [=customsec=] production to |customSections|.
1. Return |customSections|.
</div>
<div algorithm>
The <dfn constructor for="Module">Module(|bytes|)</dfn> constructor, when invoked, performs the following steps:
1. Let |stableBytes| be a [=get a copy of the buffer source|copy of the bytes held by the buffer=] |bytes|.
1. [=Compile a WebAssembly module|Compile the WebAssembly module=] |stableBytes| and store the result as |module|.
1. If |module| is [=error=], throw a {{CompileError}} exception.
1. Set **this**.\[[Module]] to |module|.
1. Set **this**.\[[Bytes]] to |stableBytes|.
Note: Some implementations enforce a size limitation on |bytes|. Use of this API is discouraged, in favor of asynchronous APIs.
</div>
<h3 id="instances">Instances</h3>
<pre class="idl">
[LegacyNamespace=WebAssembly, Exposed=*]
interface Instance {
constructor(Module module, optional object importObject);
readonly attribute object exports;
};
</pre>
<div algorithm>
The <dfn constructor for="Instance">Instance(|module|, |importObject|)</dfn> constructor, when invoked, runs the following steps:
1. Let |module| be |module|.\[[Module]].
1. [=Read the imports=] of |module| with imports |importObject|, and let |imports| be the result.
1. [=Instantiate the core of a WebAssembly module=] |module| with |imports|, and let |instance| be the result.
1. [=initialize an instance object|Initialize=] **this** from |module| and |instance|.
Note: The use of this synchronous API is discouraged, as some implementations sometimes do long-running compilation work when instantiating.
</div>
<div algorithm>
The getter of the <dfn attribute for="Instance">exports</dfn> attribute of {{Instance}} returns **this**.\[[Exports]].
</div>
<h3 id="memories">Memories</h3>
<pre class="idl">
dictionary MemoryDescriptor {
required [EnforceRange] unsigned long initial;
[EnforceRange] unsigned long maximum;
};
[LegacyNamespace=WebAssembly, Exposed=*]
interface Memory {
constructor(MemoryDescriptor descriptor);
unsigned long grow([EnforceRange] unsigned long delta);
ArrayBuffer toFixedLengthBuffer();
ArrayBuffer toResizableBuffer();
readonly attribute ArrayBuffer buffer;
};
</pre>
A {{Memory}} object represents a single [=memory instance=]
which can be simultaneously referenced by multiple {{Instance}} objects. Each
{{Memory}} object has the following internal slots:
* \[[Memory]] : a [=memory address=]
* \[[BufferObject]] : an {{ArrayBuffer}} whose [=Data Block=] is [=identified with=] the above memory address
<div algorithm>
To <dfn>create a fixed length memory buffer</dfn> from a [=memory address=] |memaddr|, perform the following steps:
1. Let |block| be a [=Data Block=] which is [=identified with=] the underlying memory of |memaddr|.
1. Let |buffer| be a new {{ArrayBuffer}} with the internal slots \[[ArrayBufferData]], \[[ArrayBufferByteLength]], and \[[ArrayBufferDetachKey]].
1. Set |buffer|.\[[ArrayBufferData]] to |block|.
1. Set |buffer|.\[[ArrayBufferByteLength]] to the length of |block|.
1. Set |buffer|.\[[ArrayBufferDetachKey]] to "WebAssembly.Memory".
1. Return |buffer|.
</div>
<div algorithm>
To <dfn>create a resizable memory buffer</dfn> from a [=memory address=] |memaddr| and a |maxsize|, perform the following steps:
1. Let |block| be a [=Data Block=] which is [=identified with=] the underlying memory of |memaddr|.
1. Let |length| be the length of |block|.
1. If |maxsize| > (65536 × 65536),
1. Throw a {{RangeError}} exception.
1. Let |buffer| be a new {{ArrayBuffer}} with the internal slots \[[ArrayBufferData]], \[[ArrayBufferByteLength]], \[[ArrayBufferMaxByteLength]], and \[[ArrayBufferDetachKey]].
1. Set |buffer|.\[[ArrayBufferData]] to |block|.
1. Set |buffer|.\[[ArrayBufferByteLength]] to |length|.
1. Set |buffer|.\[[ArrayBufferMaxByteLength]] is |maxsize|.
1. Set |buffer|.\[[ArrayBufferDetachKey]] to "WebAssembly.Memory".
1. Return |buffer|.
</div>
<div algorithm>
To <dfn>initialize a memory object</dfn> |memory| from a [=memory address=] |memaddr|, perform the following steps:
1. Let |map| be the [=surrounding agent=]'s associated [=Memory object cache=].
1. Assert: |map|[|memaddr|] doesn't [=map/exist=].
1. Let |buffer| be the result of [=create a fixed length memory buffer|creating a fixed length memory buffer=] from |memaddr|.
1. Set |memory|.\[[Memory]] to |memaddr|.
1. Set |memory|.\[[BufferObject]] to |buffer|.
1. [=map/Set=] |map|[|memaddr|] to |memory|.
</div>
<div algorithm>
To <dfn>create a memory object</dfn> from a [=memory address=] |memaddr|, perform the following steps:
1. Let |map| be the [=surrounding agent=]'s associated [=Memory object cache=].
1. If |map|[|memaddr|] [=map/exists=],
1. Return |map|[|memaddr|].
1. Let |memory| be a [=/new=] {{Memory}}.
1. [=initialize a memory object|Initialize=] |memory| from |memaddr|.
1. Return |memory|.
</div>
<div algorithm>
The <dfn constructor for="Memory">Memory(|descriptor|)</dfn> constructor, when invoked, performs the following steps:
1. Let |initial| be |descriptor|["initial"].
1. If |descriptor|["maximum"] [=map/exists=], let |maximum| be |descriptor|["maximum"]; otherwise, let |maximum| be empty.
1. Let |memtype| be the [=memory type=] { <b>[=limits|min=]</b> |initial|, <b>[=limits|max=]</b> |maximum| }.
1. If |memtype| is not [=valid memtype|valid=], throw a {{RangeError}} exception.
1. Let |store| be the [=surrounding agent=]'s [=associated store=].
1. Let (|store|, |memaddr|) be [=mem_alloc=](|store|, |memtype|). If allocation fails, throw a {{RangeError}} exception.
1. Set the [=surrounding agent=]'s [=associated store=] to |store|.
1. [=initialize a memory object|Initialize=] **this** from |memaddr|.
</div>
<div algorithm>
To <dfn>refresh the Memory buffer</dfn> of |memaddr|, perform the following steps:
1. Let |map| be the [=surrounding agent=]'s associated [=Memory object cache=].
1. Assert: |map|[|memaddr|] [=map/exists=].
1. Let |memory| be |map|[|memaddr|].
1. Let |buffer| be |memory|.\[[BufferObject]].
1. If [=IsFixedLengthArrayBuffer=](|buffer|) is true,
1. Perform [=!=] [$DetachArrayBuffer$](|buffer|, "WebAssembly.Memory").
1. Let |buffer| be the result of [=create a fixed length memory buffer|creating a fixed length memory buffer=] from |memaddr|.
1. Set |memory|.\[[BufferObject]] to |buffer|.
1. Otherwise,
1. Let |block| be a [=Data Block=] which is [=identified with=] the underlying memory of |memaddr|.
1. Set |buffer|.\[[ArrayBufferData]] to |block|.
1. Set |buffer|.\[[ArrayBufferByteLength]] to the length of |block|.
</div>
<div algorithm>
To <dfn>grow the memory buffer</dfn> associated with a [=memory address=] |memaddr| by |delta|, perform the following steps:
1. Let |store| be the [=surrounding agent=]'s [=associated store=].
1. Let |ret| be the [=mem_size=](|store|, |memaddr|).
1. Let |store| be [=mem_grow=](|store|, |memaddr|, |delta|).
1. If |store| is [=error=], throw a {{RangeError}} exception.
1. Set the [=surrounding agent=]'s [=associated store=] to |store|.
1. [=Refresh the memory buffer=] of |memaddr|.
1. Return |ret|.
</div>
<div algorithm=dom-Memory-grow>
The <dfn method for="Memory">grow(|delta|)</dfn> method, when invoked, performs the following steps:
1. Let |memaddr| be **this**.\[[Memory]].
1. Return the result of [=grow the memory buffer|growing the memory buffer=] associated with |memaddr| by |delta|.
</div>
Immediately after a WebAssembly [=memory.grow=] instruction executes, perform the following steps:
<div algorithm="memory.grow">
1. If the top of the stack is not [=i32.const=] (−1),
1. Let |frame| be the [=current frame=].
1. Assert: due to validation, |frame|.[=frame/module=].[=moduleinst/memaddrs=][0] exists.
1. Let |memaddr| be the memory address |frame|.[=frame/module=].[=moduleinst/memaddrs=][0].
1. [=Refresh the memory buffer=] of |memaddr|.
</div>
<div algorithm=dom-Memory-toFixedLengthBuffer>
The <dfn method for="Memory">toFixedLengthBuffer()</dfn> method, when invoked, performs the following steps:
1. Let |buffer| be **this**.\[[BufferObject]].
1. If [=IsFixedLengthArrayBuffer=](|buffer|) is true, return |buffer|.
1. Let |memaddr| be **this**.\[[Memory]].
1. Let |fixedBuffer| be the result of [=create a fixed length memory buffer|creating a fixed length memory buffer=] from |memaddr|.
1. Perform [=!=] [$DetachArrayBuffer$](|buffer|, "WebAssembly.Memory").
1. Set **this**.\[[BufferObject]] to |fixedBuffer|.
1. Return |fixedBuffer|.
</div>
<div algorithm=dom-Memory-toResizableBuffer>
The <dfn method for="Memory">toResizableBuffer()</dfn> method, when invoked, performs the following steps:
1. Let |buffer| be **this**.\[[BufferObject]].
1. If [=IsFixedLengthArrayBuffer=](|buffer|) is false, return |buffer|.
1. Let |memaddr| be **this**.\[[Memory]].
1. Let |store| be the [=surrounding agent=]'s [=associated store=].
1. Let |memtype| be [=mem_type=](|store|, |memaddr|).
1. If |memtype| has a max,
1. Let |maxsize| be the max value in |memtype|.
1. Otherwise,
1. Let |maxsize| be 65536 × 65536.
1. Let |resizableBuffer| be the result of [=create a resizable memory buffer|creating a resizable memory buffer=] from |memaddr| and |maxsize|.
1. Perform [=!=] [$DetachArrayBuffer$](|buffer|, "WebAssembly.Memory").
1. Set **this**.\[[BufferObject]] to |resizableBuffer|.
1. Return |resizableBuffer|.
</div>
{{ArrayBuffer}} objects returned by a {{Memory}} object must have a size that is a multiple of a WebAssembly [=page size=] (the constant 65536). For this reason [=HostResizeArrayBuffer=] is redefined as follows.
<div algorithm>
The <dfn id=HostResizeArrayBuffer export>abstract operation [=HostResizeArrayBuffer=]</dfn> takes arguments |buffer| (an {{ArrayBuffer}}) and |newLength|. It performs the following steps when called.
1. If |buffer|.\[[ArrayBufferDetachKey]] is "WebAssembly.Memory",
1. Let |map| be the [=surrounding agent=]'s associated [=Memory object cache=].
1. Assert: |buffer| is the \[[BufferObject]] of exactly one value in |map|.
1. [=map/iterate|For each=] |memaddr| → |mem| in |map|,
1. If [=SameValue=](|mem|.\[[BufferObject]], |buffer|) is true,
1. Assert: |buffer|.\[[ArrayBufferByteLength]] modulo 65536 is 0.
1. Let |lengthDelta| be |newLength| - |buffer|.\[[ArrayBufferByteLength]].
1. If |lengthDelta| < 0 or |lengthDelta| modulo 65536 is not 0,
1. Throw a {{RangeError}} exception.
1. Let |delta| be |lengthDelta| ÷ 65536.
1. [=Grow the memory buffer=] associated with |memaddr| by |delta|.
1. Return <emu-const>handled</emu-const>.
1. Otherwise, return <emu-const>unhandled</emu-const>.
</div>
<div algorithm>
The getter of the <dfn attribute for="Memory">buffer</dfn> attribute of {{Memory}} returns **this**.\[[BufferObject]].
</div>
<h3 id="tables">Tables</h3>
<pre class="idl">
enum TableKind {
"externref",
"anyfunc",
// Note: More values may be added in future iterations,
// e.g., typed function references, typed GC references
};
dictionary TableDescriptor {
required TableKind element;
required [EnforceRange] unsigned long initial;
[EnforceRange] unsigned long maximum;
};
[LegacyNamespace=WebAssembly, Exposed=*]
interface Table {
constructor(TableDescriptor descriptor, optional any value);
unsigned long grow([EnforceRange] unsigned long delta, optional any value);
any get([EnforceRange] unsigned long index);
undefined set([EnforceRange] unsigned long index, optional any value);
readonly attribute unsigned long length;
};
</pre>
A {{Table}} object represents a single [=table instance=] which can be simultaneously referenced by
multiple {{Instance}} objects.
Each {{Table}} object has a \[[Table]] internal slot, which is a [=table address=].
<div algorithm>
To <dfn>initialize a table object</dfn> |table| from a [=table address=] |tableaddr|, perform the following steps:
1. Let |map| be the [=surrounding agent=]'s associated [=Table object cache=].
1. Assert: |map|[|tableaddr|] doesn't [=map/exist=].
1. Set |table|.\[[Table]] to |tableaddr|.
1. [=map/Set=] |map|[|tableaddr|] to |table|.
</div>
<div algorithm>
To <dfn>create a table object</dfn> from a [=table address=] |tableaddr|, perform the following steps:
1. Let |map| be the [=surrounding agent=]'s associated [=Table object cache=].
1. If |map|[|tableaddr|] [=map/exists=],
1. Return |map|[|tableaddr|].
1. Let |table| be a [=/new=] {{Table}}.
1. [=initialize a table object|Initialize=] |table| from |tableaddr|.
1. Return |table|.
</div>
<div algorithm>
The <dfn constructor for="Table">Table(|descriptor|, |value|)</dfn> constructor, when invoked, performs the following steps:
1. Let |elementType| be [=ToValueType=](|descriptor|["element"]).
1. If |elementType| is not a [=reftype=],
1. [=Throw=] a {{TypeError}} exception.
1. Let |initial| be |descriptor|["initial"].
1. If |descriptor|["maximum"] [=map/exists=], let |maximum| be |descriptor|["maximum"]; otherwise, let |maximum| be empty.
1. Let |type| be the [=table type=] { <b>[=limits|min=]</b> |initial|, <b>[=limits|max=]</b> |maximum| } |elementType|.
1. If |type| is not [=valid tabletype|valid=], throw a {{RangeError}} exception.
Note: Because tables may have up to 2<sup>32</sup> - 1 elements in validation, the checks for [=limits|min=] ≤ 2<sup>32</sup> - 1 and [=limits|max=] ≤ 2<sup>32</sup> - 1 in [=valid limits|limits validation=] cannot fail.
1. If |value| is missing,
1. Let |ref| be [=DefaultValue=](|elementType|).
1. Otherwise,
1. Let |ref| be [=?=] [=ToWebAssemblyValue=](|value|, |elementType|).
1. Let |store| be the [=surrounding agent=]'s [=associated store=].
1. Let (|store|, |tableaddr|) be [=table_alloc=](|store|, |type|, |ref|). If allocation fails, throw a {{RangeError}} exception.
1. Set the [=surrounding agent=]'s [=associated store=] to |store|.
1. [=initialize a table object|Initialize=] **this** from |tableaddr|.
</div>
<div algorithm=dom-Table-grow>
The <dfn method for="Table">grow(|delta|, |value|)</dfn> method, when invoked, performs the following steps:
1. Let |tableaddr| be **this**.\[[Table]].
1. Let |store| be the [=surrounding agent=]'s [=associated store=].
1. Let |initialSize| be [=table_size=](|store|, |tableaddr|).
1. Let (<var ignore>limits</var>, |elementType|) be [=table_type=](|tableaddr|).
1. If |value| is missing,
1. Let |ref| be [=DefaultValue=](|elementType|).
1. Otherwise,
1. Let |ref| be [=?=] [=ToWebAssemblyValue=](|value|, |elementType|).
1. Let |result| be [=table_grow=](|store|, |tableaddr|, |delta|, |ref|).
1. If |result| is [=error=], throw a {{RangeError}} exception.
Note: The above exception can happen due to either insufficient memory or an invalid size parameter.
1. Set the [=surrounding agent=]'s [=associated store=] to |result|.
1. Return |initialSize|.
</div>
<div algorithm>
The getter of the <dfn attribute for="Table">length</dfn> attribute of {{Table}}, when invoked, performs the following steps:
1. Let |tableaddr| be **this**.\[[Table]].
1. Let |store| be the [=surrounding agent=]'s [=associated store=].
1. Return [=table_size=](|store|, |tableaddr|).
</div>
<div algorithm>
The <dfn method for="Table">get(|index|)</dfn> method, when invoked, performs the following steps:
1. Let |tableaddr| be **this**.\[[Table]].
1. Let |store| be the [=surrounding agent=]'s [=associated store=].
1. Let |result| be [=table_read=](|store|, |tableaddr|, |index|).
1. If |result| is [=error=], throw a {{RangeError}} exception.
1. Return [=ToJSValue=](|result|).
</div>
<div algorithm>
The <dfn method for="Table">set(|index|, |value|)</dfn> method, when invoked, performs the following steps:
1. Let |tableaddr| be **this**.\[[Table]].
1. Let (<var ignore>limits</var>, |elementType|) be [=table_type=](|tableaddr|).
1. If |value| is missing,
1. Let |ref| be [=DefaultValue=](|elementType|).
1. Otherwise,
1. Let |ref| be [=?=] [=ToWebAssemblyValue=](|value|, |elementType|).
1. Let |store| be the [=surrounding agent=]'s [=associated store=].
1. Let |store| be [=table_write=](|store|, |tableaddr|, |index|, |ref|).
1. If |store| is [=error=], throw a {{RangeError}} exception.
1. Set the [=surrounding agent=]'s [=associated store=] to |store|.
</div>
<h3 id="globals">Globals</h3>
<pre class="idl">
enum ValueType {
"i32",
"i64",
"f32",
"f64",
"v128",
"externref",
"anyfunc",
};
</pre>
Note: this type may be extended with additional cases in future versions of WebAssembly.
<pre class="idl">
dictionary GlobalDescriptor {
required ValueType value;
boolean mutable = false;
};
[LegacyNamespace=WebAssembly, Exposed=*]
interface Global {
constructor(GlobalDescriptor descriptor, optional any v);
any valueOf();
attribute any value;
};
</pre>
A {{Global}} object represents a single [=global instance=]
which can be simultaneously referenced by multiple {{Instance}} objects. Each
{{Global}} object has one internal slot:
* \[[Global]] : a [=global address=]
<div algorithm>
To <dfn>initialize a global object</dfn> |global| from a [=global address=] |globaladdr|, perform the following steps:
1. Let |map| be the [=surrounding agent=]'s associated [=Global object cache=].
1. Assert: |map|[|globaladdr|] doesn't [=map/exist=].
1. Set |global|.\[[Global]] to |globaladdr|.
1. [=map/Set=] |map|[|globaladdr|] to |global|.
</div>
<div algorithm>
To <dfn>create a global object</dfn> from a [=global address=] |globaladdr|, perform the following steps:
1. Let |map| be the [=surrounding agent=]'s associated [=Global object cache=].
1. If |map|[|globaladdr|] [=map/exists=],
1. Return |map|[|globaladdr|].
1. Let |global| be a [=/new=] {{Global}}.
1. [=initialize a global object|Initialize=] |global| from |globaladdr|.
1. Return |global|.
</div>
<div algorithm>
The algorithm <dfn>ToValueType</dfn>(|s|) performs the following steps:
1. If |s| equals "i32", return [=i32=].
1. If |s| equals "i64", return [=i64=].
1. If |s| equals "f32", return [=f32=].
1. If |s| equals "f64", return [=f64=].
1. If |s| equals "v128", return [=v128=].
1. If |s| equals "anyfunc", return [=funcref=].
1. If |s| equals "externref", return [=externref=].
1. Assert: This step is not reached.
</div>
<div algorithm>
The algorithm <dfn>DefaultValue</dfn>(|valuetype|) performs the following steps:
1. If |valuetype| equals [=i32=], return [=i32.const=] 0.
1. If |valuetype| equals [=i64=], return [=i64.const=] 0.
1. If |valuetype| equals [=f32=], return [=f32.const=] 0.
1. If |valuetype| equals [=f64=], return [=f64.const=] 0.
1. If |valuetype| equals [=funcref=], return [=ref.null=] [=funcref=].
1. If |valuetype| equals [=externref=], return [=ToWebAssemblyValue=](undefined, |valuetype|).
1. Assert: This step is not reached.
</div>
<div algorithm>
The <dfn constructor for="Global">Global(|descriptor|, |v|)</dfn> constructor, when invoked, performs the following steps:
1. Let |mutable| be |descriptor|["mutable"].
1. Let |valuetype| be [=ToValueType=](|descriptor|["value"]).
1. If |valuetype| is [=v128=],
1. Throw a {{TypeError}} exception.
1. If |v| is missing,
1. Let |value| be [=DefaultValue=](|valuetype|).
1. Otherwise,
1. Let |value| be [=ToWebAssemblyValue=](|v|, |valuetype|).
1. If |mutable| is true, let |globaltype| be [=var=] |valuetype|; otherwise, let |globaltype| be [=const=] |valuetype|.
1. Let |store| be the current agent's [=associated store=].
1. Let (|store|, |globaladdr|) be [=global_alloc=](|store|, |globaltype|, |value|). <!-- TODO(littledan): Report allocation failure https://github.com/WebAssembly/spec/issues/584 -->
1. Set the current agent's [=associated store=] to |store|.
1. [=initialize a global object|Initialize=] **this** from |globaladdr|.
</div>
<div algorithm>
The algorithm <dfn>GetGlobalValue</dfn>({{Global}} |global|) performs the following steps:
1. Let |store| be the current agent's [=associated store=].
1. Let |globaladdr| be |global|.\[[Global]].
1. Let |globaltype| be [=global_type=](|store|, |globaladdr|).
1. If |globaltype| is of the form <var ignore>mut</var> [=v128=], throw a {{TypeError}}.
1. Let |value| be [=global_read=](|store|, |globaladdr|).
1. Return [=ToJSValue=](|value|).
</div>
<div algorithm>
The getter of the <dfn attribute for="Global">value</dfn> attribute of {{Global}}, when invoked, performs the following steps:
1. Return [=GetGlobalValue=](**this**).
The setter of the value attribute of {{Global}}, when invoked, performs the following steps:
1. Let |store| be the current agent's [=associated store=].
1. Let |globaladdr| be **this**.\[[Global]].
1. Let |mut| |valuetype| be [=global_type=](|store|, |globaladdr|).
1. If |valuetype| is [=v128=], throw a {{TypeError}}.
1. If |mut| is [=const=], throw a {{TypeError}}.
1. Let |value| be [=ToWebAssemblyValue=](**the given value**, |valuetype|).
1. Let |store| be [=global_write=](|store|, |globaladdr|, |value|).
1. If |store| is [=error=], throw a {{RangeError}} exception.
1. Set the current agent's [=associated store=] to |store|.
</div>
<div algorithm>
The <dfn method for="Global">valueOf()</dfn> method, when invoked, performs the following steps:
1. Return [=GetGlobalValue=](**this**).
</div>
<h3 id="exported-function-exotic-objects">Exported Functions</h3>
A WebAssembly function is made available in JavaScript as an <dfn>Exported Function</dfn>.
Exported Functions are [=Built-in Function Objects=] which are not constructors, and which have a \[[FunctionAddress]] internal slot.
This slot holds a [=function address=] relative to the [=surrounding agent=]'s [=associated store=].
<div algorithm>
The <dfn>name of the WebAssembly function</dfn> |funcaddr| is found by performing the following steps:
1. Let |store| be the [=surrounding agent=]'s [=associated store=].
1. Let |funcinst| be |store|.funcs[|funcaddr|].
1. If |funcinst| is of the form {type <var ignore>functype</var>, hostcode |hostfunc|},
1. Assert: |hostfunc| is a JavaScript object and [$IsCallable$](|hostfunc|) is true.
1. Let |index| be the [=index of the host function=] |funcaddr|.
1. Otherwise,
1. Let |moduleinst| be |funcinst|.module.
1. Assert: |funcaddr| is contained in |moduleinst|.funcaddrs.
1. Let |index| be the index of |moduleinst|.funcaddrs where |funcaddr| is found.
1. Return [=!=] [$ToString$](|index|).
</div>
<div algorithm>
To create <dfn>a new Exported Function</dfn> from a WebAssembly [=function address=] |funcaddr|, perform the following steps:
1. Let |map| be the [=surrounding agent=]'s associated [=Exported Function cache=].
1. If |map|[|funcaddr|] [=map/exists=],
1. Return |map|[|funcaddr|].
1. Let |steps| be "[=call an Exported Function|call the Exported Function=] |funcaddr| with arguments."
1. Let |realm| be the [=current Realm=].
1. Let |store| be the [=surrounding agent=]'s [=associated store=].
1. Let |functype| be [=func_type=](|store|, |funcaddr|).
1. Let [|paramTypes|] → [<var ignore>resultTypes</var>] be |functype|.
1. Let |arity| be |paramTypes|'s [=list/size=].
1. Let |name| be the [=name of the WebAssembly function=] |funcaddr|.
1. Let |function| be [=!=] [$CreateBuiltinFunction$](|steps|, |arity|, |name|, « \[[FunctionAddress]] », |realm|).
1. Set |function|.\[[FunctionAddress]] to |funcaddr|.
1. [=map/Set=] |map|[|funcaddr|] to |function|.
1. Return |function|.
</div>
<div algorithm>
To <dfn>call an Exported Function</dfn> with [=function address=] |funcaddr| and a [=list=] of JavaScript arguments |argValues|, perform the following steps:
1. Let |store| be the [=surrounding agent=]'s [=associated store=].
1. Let |functype| be [=func_type=](|store|, |funcaddr|).
1. Let [|parameters|] → [|results|] be |functype|.
1. If |parameters| or |results| contain [=v128=], throw a {{TypeError}}.
Note: the above error is thrown each time the \[[Call]] method is invoked.
1. Let |args| be « ».
1. Let |i| be 0.
1. [=list/iterate|For each=] |t| of |parameters|,
1. If |argValues|'s [=list/size=] > |i|, let |arg| be |argValues|[|i|].
1. Otherwise, let |arg| be undefined.
1. [=list/Append=] [=ToWebAssemblyValue=](|arg|, |t|) to |args|.
1. Set |i| to |i| + 1.
1. Let (|store|, |ret|) be the result of [=func_invoke=](|store|, |funcaddr|, |args|).
1. Set the [=surrounding agent=]'s [=associated store=] to |store|.
1. If |ret| is [=error=], throw an exception. This exception should be a WebAssembly {{RuntimeError}} exception, unless otherwise indicated by <a href="#errors">the WebAssembly error mapping</a>.
1. Let |outArity| be the [=list/size=] of |ret|.
1. If |outArity| is 0, return undefined.
1. Otherwise, if |outArity| is 1, return [=ToJSValue=](|ret|[0]).
1. Otherwise,
1. Let |values| be « ».
1. [=list/iterate|For each=] |r| of |ret|,
1. [=list/Append=] [=ToJSValue=](|r|) to |values|.
1. Return [$CreateArrayFromList$](|values|).
</div>
Note: [=call an Exported Function|Calling an Exported Function=] executes in the \[[Realm]] of the callee Exported Function, as per the definition of [=built-in function objects=].
Note: Exported Functions do not have a \[[Construct]] method and thus it is not possible to call one with the `new` operator.
<div algorithm>
To <dfn>run a host function</dfn> from the JavaScript object |func|, type |functype|, and [=list=] of [=WebAssembly values=] |arguments|, perform the following steps:
1. Let [|parameters|] → [|results|] be |functype|.
1. If |parameters| or |results| contain [=v128=], throw a {{TypeError}}.
1. Let |jsArguments| be « ».
1. [=list/iterate|For each=] |arg| of |arguments|,
1. [=list/Append=] [=!=] [=ToJSValue=](|arg|) to |jsArguments|.
1. Let |ret| be [=?=] [$Call$](|func|, undefined, |jsArguments|).
1. Let |resultsSize| be |results|'s [=list/size=].
1. If |resultsSize| is 0, return « ».
1. Otherwise, if |resultsSize| is 1, return « [=?=] [=ToWebAssemblyValue=](|ret|, |results|[0]) ».
1. Otherwise,
1. Let |method| be [=?=] [$GetMethod$](|ret|, {{%Symbol.iterator%}}).
1. If |method| is undefined, [=throw=] a {{TypeError}}.
1. Let |values| be [=?=] [$IteratorToList$]([=?=] [$GetIteratorFromMethod$](|ret|, |method|)).
1. Let |wasmValues| be a new, empty [=list=].
1. If |values|'s [=list/size=] is not |resultsSize|, throw a {{TypeError}} exception.
1. For each |value| and |resultType| in |values| and |results|, paired linearly,
1. [=list/Append=] [=ToWebAssemblyValue=](|value|, |resultType|) to |wasmValues|.
1. Return |wasmValues|.
</div>
<div algorithm>
To <dfn>create a host function</dfn> from the JavaScript object |func| and type |functype|, perform the following steps:
1. Assert: [$IsCallable$](|func|).
1. Let |stored settings| be the <a spec=HTML>incumbent settings object</a>.
1. Let |hostfunc| be a [=host function=] which performs the following steps when called with arguments |arguments|:
1. Let |realm| be |func|'s [=associated Realm=].
1. Let |relevant settings| be |realm|'s [=realm/settings object=].
1. [=Prepare to run script=] with |relevant settings|.
1. [=Prepare to run a callback=] with |stored settings|.
1. Let |result| be the result of [=run a host function|running a host function=] from |func|, |functype|, and |arguments|.
1. [=Clean up after running a callback=] with |stored settings|.
1. [=Clean up after running script=] with |relevant settings|.
1. Assert: |result|.\[[Type]] is <emu-const>throw</emu-const> or <emu-const>normal</emu-const>.
1. If |result|.\[[Type]] is <emu-const>throw</emu-const>, then trigger a WebAssembly trap, and propagate |result|.\[[Value]] to the enclosing JavaScript.
1. Otherwise, return |result|.\[[Value]].
1. Let |store| be the [=surrounding agent=]'s [=associated store=].
1. Let (|store|, |funcaddr|) be [=func_alloc=](|store|, |functype|, |hostfunc|).
1. Set the [=surrounding agent=]'s [=associated store=] to |store|.
1. Return |funcaddr|.
</div>
<div algorithm>
The algorithm <dfn>ToJSValue</dfn>(|w|) coerces a [=WebAssembly value=] to a JavaScript value by performing the following steps:
1. Assert: |w| is not of the form [=v128.const=] <var ignore>v128</var>.
1. If |w| is of the form [=i64.const=] |i64|,
1. Let |v| be [=signed_64=](|i64|).
1. Return [=ℤ=](|v| interpreted as a mathematical value).
1. If |w| is of the form [=i32.const=] |i32|, return [=𝔽=]([=signed_32=](|i32| interpreted as a mathematical value)).
1. If |w| is of the form [=f32.const=] |f32|,
1. If |f32| is [=+∞=] or [=−∞=], return **+∞**<sub>𝔽</sub> or **-∞**<sub>𝔽</sub>, respectively.
1. If |f32| is [=nan=], return **NaN**.
1. Return [=𝔽=](|f32| interpreted as a mathematical value).
1. If |w| is of the form [=f64.const=] |f64|,
1. If |f64| is [=+∞=] or [=−∞=], return **+∞**<sub>𝔽</sub> or **-∞**<sub>𝔽</sub>, respectively.
1. If |f64| is [=nan=], return **NaN**.
1. Return [=𝔽=](|f64| interpreted as a mathematical value).
1. If |w| is of the form [=ref.null=] <var ignore>t</var>, return null.
1. If |w| is of the form [=ref.func=] |funcaddr|, return the result of creating [=a new Exported Function=] from |funcaddr|.
1. If |w| is of the form [=ref.extern=] |externaddr|, return the result of [=retrieving an extern value=] from |externaddr|.
Note: Number values which are equal to NaN may have various observable NaN payloads; see [$NumericToRawBytes$] for details.
</div>
<div algorithm>
For <dfn>retrieving an extern value</dfn> from an [=extern address=] |externaddr|, perform the following steps:
1. Let |map| be the [=surrounding agent=]'s associated [=extern value cache=].
1. Assert: |map|[|externaddr|] [=map/exists=].
1. Return |map|[|externaddr|].
</div>
<div algorithm>
The algorithm <dfn>ToWebAssemblyValue</dfn>(|v|, |type|) coerces a JavaScript value to a [=WebAssembly value=] by performing the following steps:
1. Assert: |type| is not [=v128=].
1. If |type| is [=i64=],
1. Let |i64| be [=?=] [$ToBigInt64$](|v|).
1. Return [=i64.const=] |i64|.
1. If |type| is [=i32=],
1. Let |i32| be [=?=] [$ToInt32$](|v|).
1. Return [=i32.const=] |i32|.
1. If |type| is [=f32=],
1. Let |number| be [=?=] [$ToNumber$](|v|).
1. If |number| is **NaN**,
1. Let |n| be an implementation-defined integer such that [=canon=]<sub>32</sub> ≤ |n| < 2<sup>[=signif=](32)</sup>.
1. Let |f32| be [=nan=](n).
1. Otherwise,
1. Let |f32| be |number| rounded to the nearest representable value using IEEE 754-2019 round to nearest, ties to even mode. [[IEEE-754]]
1. Return [=f32.const=] |f32|.
1. If |type| is [=f64=],
1. Let |number| be [=?=] [$ToNumber$](|v|).
1. If |number| is **NaN**,
1. Let |n| be an implementation-defined integer such that [=canon=]<sub>64</sub> ≤ |n| < 2<sup>[=signif=](64)</sup>.
1. Let |f64| be [=nan=](n).
1. Otherwise,
1. Let |f64| be |number|.
1. Return [=f64.const=] |f64|.
1. If |type| is [=funcref=],
1. If |v| is null,
1. Return [=ref.null=] [=funcref=].
1. If |v| is an [=Exported Function=],
1. Let |funcaddr| be the value of |v|'s \[[FunctionAddress]] internal slot.
1. Return [=ref.func=] |funcaddr|.
1. Throw a {{TypeError}}.
1. If |type| is [=externref=],
1. If |v| is null,
1. Return [=ref.null=] [=externref=].
1. Let |map| be the [=surrounding agent=]'s associated [=extern value cache=].
1. If a [=extern address=] |externaddr| exists such that |map|[|externaddr|] is the same as |v|,
1. Return [=ref.extern=] |externaddr|.
1. Let [=extern address=] |externaddr| be the smallest address such that |map|[|externaddr|] [=map/exists=] is false.
1. [=map/Set=] |map|[|externaddr|] to |v|.
1. Return [=ref.extern=] |externaddr|.
1. Assert: This step is not reached.
</div>
<h3 id="error-objects">Error Objects</h3>
WebAssembly defines the following Error classes: <dfn exception>CompileError</dfn>, <dfn exception>LinkError</dfn>, and <dfn exception>RuntimeError</dfn>.
<div algorithm="create the WebAssembly namespace object">
When the [=namespace object=] for the {{WebAssembly}} namespace is [=create a namespace object|created=], the following steps must be run:
1. Let |namespaceObject| be the [=namespace object=].
1. [=list/iterate|For each=] |error| of « "CompileError", "LinkError", "RuntimeError" »,
1. Let |constructor| be a new object, implementing the [=NativeError Object Structure=], with <var ignore>NativeError</var> set to |error|.
1. [=!=] [$DefineMethodProperty$](|namespaceObject|, |error|, |constructor|, false).
</div>
Note: This defines {{CompileError}}, {{LinkError}}, and {{RuntimeError}} classes on the {{WebAssembly}} namespace, which are produced by the APIs defined in this specification.
They expose the same interface as native JavaScript errors like {{TypeError}} and {{RangeError}}.
Note: It is not currently possible to define this behavior using Web IDL.
<h2 id="errors">Error Condition Mappings to JavaScript</h2>
Running WebAssembly programs encounter certain events which halt execution of the WebAssembly code.
WebAssembly code (currently)
has no way to catch these conditions and thus an exception will necessarily
propagate to the enclosing non-WebAssembly caller (whether it is a browser,
JavaScript or another runtime system) where it is handled like a normal JavaScript exception.
If WebAssembly calls JavaScript via import and the JavaScript throws an
exception, the exception is propagated through the WebAssembly activation to the
enclosing caller.
Because JavaScript exceptions can be handled, and JavaScript can continue to
call WebAssembly exports after a trap has been handled, traps do not, in
general, prevent future execution.
<h3 id="stack-overflow">Stack Overflow</h3>
Whenever a stack overflow occurs in
WebAssembly code, the same class of exception is thrown as for a stack overflow in
JavaScript. The particular exception here is implementation-defined in both cases.
Note: ECMAScript doesn't specify any sort of behavior on stack overflow; implementations have been observed to throw {{RangeError}}, InternalError or Error. Any is valid here.
<h3 id="out-of-memory">Out of Memory</h3>
Whenever validation, compilation or instantiation run out of memory, the
same class of exception is thrown as for out of memory conditions in JavaScript.
The particular exception here is implementation-defined in both cases.
Note: ECMAScript doesn't specify any sort of behavior on out-of-memory conditions; implementations have been observed to throw OOMError and to crash. Either is valid here.
<div class="issue">
A failed allocation of a large table or memory may either result in
- a {{RangeError}}, as specified in the {{Memory}} {{Memory/grow()}} and {{Table}} {{Table/grow()}} operations
- returning -1 as the [=memory.grow=] instruction
- UA-specific OOM behavior as described in this section.
In a future revision, we may reconsider more reliable and recoverable errors for allocations of large amounts of memory.
See [Issue 879](https://github.com/WebAssembly/spec/issues/879) for further discussion.
</div>
<h2 id="limits">Implementation-defined Limits</h2>
The WebAssembly core specification allows an implementation to define limits on the syntactic structure of the module.
While each embedding of WebAssembly may choose to define its own limits, for predictability the standard WebAssembly JavaScript Interface described in this document defines the following exact limits.
An implementation must reject a module that exceeds one of the following limits with a {{CompileError}}:
In practice, an implementation may run out of resources for valid modules below these limits.
<ul>
<li>The maximum size of a module is 1,073,741,824 bytes (1 GiB).</li>
<li>The maximum number of types defined in the types section is 1,000,000.</li>
<li>The maximum number of functions defined in a module is 1,000,000.</li>
<li>The maximum number of imports declared in a module is 100,000.</li>
<li>The maximum number of exports declared in a module is 100,000.</li>
<li>The maximum number of globals defined in a module is 1,000,000.</li>
<li>The maximum number of data segments defined in a module is 100,000.</li>
<li>The maximum number of tables, including declared or imported tables, is 100,000.</li>
<li>The maximum size of a table is 10,000,000.</li>
<li>The maximum number of table entries in any table initialization is 10,000,000.</li>
<li>The maximum number of memories, including declared or imported memories, is 1.</li>
<li>The maximum number of parameters to any function or block is 1,000.</li>
<li>The maximum number of return values for any function or block is 1,000.</li>
<li>The maximum size of a function body, including locals declarations, is 7,654,321 bytes.</li>
<li>The maximum number of locals declared in a function, including implicitly declared as parameters, is 50,000.</li>
</ul>
An implementation must throw a {{RuntimeError}} if one of the following limits is exceeded during runtime:
In practice, an implementation may run out of resources for valid modules below these limits.
<ul>
<li>The maximum size of a table is 10,000,000.</li>
<li>The maximum number of pages of a memory is 65,536.</li>
</ul>
<h2 id="security-considerations">Security and Privacy Considerations</h2>
<p><em>This section is non-normative.</em></p>
This document defines a host environment for WebAssembly. It enables a WebAssembly instance to [=import=] JavaScript objects and functions from an [=read the imports|import object=], but otherwise provides no access to the embedding environment. Thus a WebAssembly instance is bound to the same constraints as JavaScript.
<h2 id="change-history">Change History</h2>
<p><em>This section is non-normative.</em></p>
<p>Since the original release 1.0 of the WebAssembly specification, a number of proposals for extensions have been integrated.
The following sections provide an overview of what has changed.</p>
<h3 id="release-20">Release 2.0</h3>
<h4 id="changes-multivalue" class="no-toc heading settled">Multiple Values</h4>
Multiple values can be returned from WebAssembly functions into JavaScript as an [=Array=] object.
<h4 id="changes-bigint" class="no-toc heading settled">BigInt Integration</h4>
WebAssembly [=i64=] values can be passed to and from JavaScript (via imported or exported globals, table get or set operations, function return values or arguments) as [=BigInt=] objects.
<h4 id="changes-reftypes" class="no-toc heading settled">Reference types</h4>
JavaScript values can be passed to and from WebAssembly (via imported or exported globals, table set or get operations, and function arguments or return values) as [=externref=] values.
Multiple tables can be exported and imported to and from JavaScript.