WebAssembly.Instance

Baseline Widely available

This feature is well established and works across many devices and browser versions. It’s been available across browsers since October 2017.

A WebAssembly.Instance object is a stateful, executable instance of a WebAssembly.Module. Instance objects contain all the Exported WebAssembly functions that allow calling into WebAssembly code from JavaScript.

Constructor

WebAssembly.Instance()

Creates a new Instance object.

Instance properties

exports

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. Read-only.

Examples

Synchronously instantiating a WebAssembly module

The WebAssembly.Instance() constructor function can be called to synchronously instantiate a given WebAssembly.Module object, for example:

js
const importObject = {
  my_namespace: {
    imported_func(arg) {
      console.log(arg);
    },
  },
};

fetch("simple.wasm")
  .then((response) => response.arrayBuffer())
  .then((bytes) => {
    const mod = new WebAssembly.Module(bytes);
    const instance = new WebAssembly.Instance(mod, importObject);
    instance.exports.exported_func();
  });

The preferred way to get an Instance is asynchronously, for example using the WebAssembly.instantiateStreaming() function like this:

js
const importObject = {
  my_namespace: {
    imported_func(arg) {
      console.log(arg);
    },
  },
};

WebAssembly.instantiateStreaming(fetch("simple.wasm"), importObject).then(
  (obj) => obj.instance.exports.exported_func(),
);

This also demonstrates how the exports property is used to access exported functions.