WebAssembly.instantiateStreaming()

Baseline Widely available

This feature is well established and works across many devices and browser versions. It’s been available across browsers since October 2017.

The WebAssembly.instantiateStreaming() static method compiles and instantiates a WebAssembly module directly from a streamed underlying source. This is the most efficient, optimized way to load Wasm code.

Note: Webpages that have strict Content Security Policy (CSP) might block WebAssembly from compiling and executing modules. For more information on allowing WebAssembly compilation and execution, see the script-src CSP.

Syntax

js
WebAssembly.instantiateStreaming(source, importObject)

Parameters

source

A Response object or a promise that will fulfill with one, representing the underlying source of a Wasm module you want to stream, compile, and instantiate.

importObject Optional

An object containing the values to be imported into the newly-created Instance, such as functions or WebAssembly.Memory objects. There must be one matching property for each declared import of the compiled module or else a WebAssembly.LinkError is thrown.

Return value

A Promise that resolves to a ResultObject which contains two fields:

Exceptions

Examples

Instantiating streaming

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.

js
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 member is then accessed, and the contained exported function invoked.

Note: For this to work, .wasm files should be returned with an application/wasm MIME type by the server.

Specifications

Specification
WebAssembly Web API
# dom-webassembly-instantiatestreaming

Browser compatibility

BCD tables only load in the browser

See also