WebAssembly.instantiate()

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:

• module: A WebAssembly.Module object representing the compiled WebAssembly module. This Module can be instantiated again, shared via postMessage() or cached in IndexedDB.
• 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.

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

• 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

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()
);

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

Browser compatibility

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

[2] Currently supported behind the “Experimental JavaScript Features” flag. See this blog post for more details.

See also

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