WebAssembly

The WebAssembly JavaScript object acts as the namespace for all WebAssembly-related functionality.

Unlike most other global objects, WebAssembly is not a constructor (it is not a function object). You can compare it to Math, which is also a namespace object for mathematical constants and functions, or to Intl which is the namespace object for internationalization constructors and other language-sensitive functions.

Description

The primary uses for the WebAssembly object are:

Loading WebAssembly code, using the WebAssembly.instantiate() function.
Creating new memory and table instances via the WebAssembly.Memory()/WebAssembly.Table() constructors.
Providing facilities to handle errors that occur in WebAssembly via the WebAssembly.CompileError()/WebAssembly.LinkError()/WebAssembly.RuntimeError() constructors.

Interfaces

WebAssembly.CompileError: Indicates an error during WebAssembly decoding or validation.
WebAssembly.Global: Represents a global variable instance, accessible from both JavaScript and importable/exportable across one or more WebAssembly.Module instances. This allows dynamic linking of multiple modules.
WebAssembly.Instance: Is a stateful, executable instance of a WebAssembly.Module
WebAssembly.LinkError: Indicates an error during module instantiation (besides traps from the start function).
WebAssembly.Memory: An object whose buffer property is a resizable ArrayBuffer that holds the raw bytes of memory accessed by a WebAssembly Instance.
WebAssembly.Module: Contains stateless WebAssembly code that has already been compiled by the browser and can be efficiently shared with Workers, and instantiated multiple times.
WebAssembly.RuntimeError: Error type that is thrown whenever WebAssembly specifies a trap.
WebAssembly.Suspending: Represents a suspending function — an asynchronous (Promise-based) JavaScript function that, when imported into a Wasm module and called from inside, will result in execution being suspended until the promise resolves.
WebAssembly.Table: An array-like structure representing a WebAssembly Table, which stores references, such as function references.
WebAssembly.Tag: An object that represents a type of WebAssembly exception.
WebAssembly.Exception: A WebAssembly exception object that can be thrown, caught, and rethrown both within and across WebAssembly/JavaScript boundaries.

Static properties

WebAssembly.JSTag: A built-in WebAssembly.Tag representing exceptions thrown in the JavaScript host — it allows exceptions thrown in JavaScript to be handled from inside a Wasm module.

Static methods

WebAssembly.compile(): Compiles a WebAssembly.Module from WebAssembly binary code, leaving instantiation as a separate step.
WebAssembly.compileStreaming(): Compiles a WebAssembly.Module directly from a streamed underlying source, leaving instantiation as a separate step.
WebAssembly.instantiate(): The primary API for compiling and instantiating WebAssembly code, returning both a Module and its first Instance.
WebAssembly.instantiateStreaming(): Compiles and instantiates a WebAssembly module directly from a streamed underlying source, returning both a Module and its first Instance.
WebAssembly.promising(): Wraps an exported Wasm function that relies on an asynchronous operation (that is, an imported suspending function created via the WebAssembly.Suspending() constructor), turning it into a Promise.
WebAssembly.validate(): Validates a given typed array of WebAssembly binary code, returning whether the bytes are valid WebAssembly code (true) or not (false).

Examples

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Stream a Wasm module then compile and instantiate it

The following example (see our instantiate-streaming.html demo on GitHub, and view it live also) directly streams a Wasm module from an underlying source then compiles and instantiates it, the promise fulfilling with a ResultObject. Because the instantiateStreaming() function accepts a promise for a Response object, you can directly pass it a fetch() call, and it will pass the response into the function when it fulfills.

const importObject = {
  my_namespace: { imported_func: (arg) => console.log(arg) },
};

WebAssembly.instantiateStreaming(fetch("simple.wasm"), importObject).then(
  (obj) => obj.instance.exports.exported_func(),
);

The ResultObject's .instance property is then accessed, and the contained exported function invoked.

Specifications

WebAssembly JavaScript Interface