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WebAssembly.instantiate()

This is an experimental technology
Because this technology's specification has not stabilized, check the compatibility table for usage in various browsers. Also note that the syntax and behavior of an experimental technology is subject to change in future versions of browsers as the specification changes.

The WebAssembly.instantiate() function is the primary API for compiling and instantiating WebAssembly code.  This function has two overloads:

  • The primary overload takes the WebAssembly binary code, in the form of a typed array or ArrayBuffer, and performs both compilation and instantiation in one step.  The returned Promise resolves to both a compiled WebAssembly.Module and its first WebAssembly.Instance.
  • The secondary overload takes an already-compiled WebAssembly.Module and returns a Promise that resolves to an Instance of that Module. This overload is useful if the Module has already been compiled or retrieved from cache.

Syntax

Primary overload — taking wasm binary code

Promise<ResultObject> WebAssembly.instantiate(bufferSource, importObject);

Parameters

bufferSource
A typed array or ArrayBuffer containing the binary code of the .wasm module you want to compile.
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

Secondary overload — taking a module object instance

Promise<WebAssembly.Instance> WebAssembly.instantiate(module, importObject);

Parameters

module
The WebAssembly.Module object to be instantiated.
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 module or else a WebAssembly.LinkError is thrown.

Return value

A Promise that resolves to an WebAssembly.Instance object.

Exceptions

Examples

First overload example

After fetching some WebAssembly bytecode using fetch, we compile and instantiate the module using the WebAssembly.instantiate() function, importing a JavaScript function into the WebAssembly Module in the process. We then call an Exported WebAssembly function that is exported by the Instance.

var importObject = {
  imports: {
    imported_func: function(arg) {
      console.log(arg);
    }
  }
};

fetch('simple.wasm').then(response =>
  response.arrayBuffer()
).then(bytes =>
  WebAssembly.instantiate(bytes, importObject)
).then(result =>
  result.instance.exports.exported_func()
);

Note: See index.html on GitHub (view it live also) for a similar example that makes use of our fetchAndInstantiate() library function.

Second overload example

The following example (see our index-compile.html demo on GitHub, and view it live also) compiles the loaded simple.wasm byte code using the compile() function and then sends it to a worker using postMessage().

var worker = new Worker("wasm_worker.js");

fetch('simple.wasm').then(response =>
  response.arrayBuffer()
).then(bytes =>
  WebAssembly.compile(bytes)
).then(mod =>
  worker.postMessage(mod)
);

In the worker (see wasm_worker.js) we define an import object for the module to use, then set up an event handler to receive the module from the main thread. When the module is received, we create an instance from it using the WebAssembly.instantiate() method and invoke an exported function from inside it.

var importObject = {
  imports: {
    imported_func: function(arg) {
      console.log(arg);
    }
  }
};

onmessage = function(e) {
  console.log('module received from main thread');
  var mod = e.data;

  WebAssembly.instantiate(mod, importObject).then(function(instance) {
    instance.exports.exported_func();
  });
};

Specifications

Specification Status Comment
Web Assembly JavaScript API
The definition of 'instantiate()' in that specification.
Draft Initial draft definition.

Browser compatibility

Feature Chrome Edge Firefox (Gecko) Internet Explorer Opera Safari (WebKit)
Basic support 57 No support 52 (52)[1] No support (Yes) No support
Feature Chrome for Android Android Webview Edge Mobile Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile
Basic support 57 57 No support 52.0 (52)[1] No support No support No support

[1] WebAssembly is enabled in Firefox 52+, although disabled in the Firefox 52 Extended Support Release (ESR.)

See also

Document Tags and Contributors

 Contributors to this page: chrisdavidmills, lukewagner, fscholz
 Last updated by: chrisdavidmills,