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.


WebAssembly.instantiateStreaming(source, importObject)



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:



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.

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.


WebAssembly Web API
# dom-webassembly-instantiatestreaming

Browser compatibility

BCD tables only load in the browser

See also