WebAssembly.instantiateStreaming()

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

Syntax

Promise<ResultObject> 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:

• module: A WebAssembly.Module object representing the compiled WebAssembly module. This Module can be instantiated again or shared via postMessage().
• instance: A WebAssembly.Instance object that contains all the Exported WebAssembly functions.

Exceptions

• If either of the parameters are not of the correct type or structure, a TypeError is thrown.
• If the operation fails, the promise rejects with a WebAssembly.CompileError, WebAssembly.LinkError, or WebAssembly.RuntimeError, depending on the cause of the failure.

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 WindowOrWorkerGlobalScope.fetch() call, and it will pass the response into the function when it fulfills.

var importObject = { imports: { 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.

Specifications

Browser compatibility

See also

• WebAssembly overview page
• WebAssembly concepts
• Using the WebAssembly JavaScript API