WebAssembly.instance

The Instance() constructor of the WebAssembly global object instantiates a Module instance to create a new Instance object instance.

An Instance instance can be thought of as a JavaScript wrapper around a WebAssembly module instance. You can create one using the WebAssembly.Instance() constructor, or the WebAssembly.instantiate() method.

Syntax

var myInstance = new WebAssembly.Instance(module, importObject);

Parameters

module

A WebAssembly Module instance e.g. created using WebAssembly.compile() or the WebAssembly.Module() constructor.

importObject Optional

An object containing definitions of functions that you want to import into the WebAssembly module instance, if any.

Instance constructor methods

None.

Instance instances

All Instance instances inherit from the Instance() constructor's prototype object — this can be modified to affect all Module instances.

Instance properties

Instance.prototype.constructor

Returns the function that created this object's instance. By default this is the WebAssembly.Instance() constructor.

Instance.prototype.exports Read only

Returns an object containing as its members all the functions exported from the WebAssembly module instance, to allow them to be accessed and used by JavaScript.

Instance methods

None.

Examples

The following example compiles and instantiates the loaded simple.wasm byte code using the WebAssembly.instantiate() method, importing a JavaScript function into the WebAssembly Module in the process — this promise resolves to an object (obj) containing the compiled Module instance, and it's instantiated Instance instance. We then export a function from the module via Instance.exports.exported_func().

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

fetch('simple.wasm').then(function(response) {
  response.arrayBuffer().then(function(bytes) {
    WebAssembly.instantiate(bytes, importObject).then(function(obj) {
      obj.instance.exports.exported_func();
    })
  })
});

Specifications

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

Browser compatibility

[1] Experimental support can be enabled in Chrome 51+ and Opera 38+ by going to chrome://flags and enabling the Experimental WebAssembly flag.

[2] Experimental support can be enabled in Firefox 47+ by enabling the javascript.options.wasm flag in about:config.

See also

• WebAssembly landing page.
• WebAssembly concepts
• Using the WebAssembly JavaScript API