WebAssembly.Instance

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.

WebAssembly.Instance()

Creates a new Instance object.

Instance.prototype.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.

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

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

fetch('simple.wasm').then(response =>
  response.arrayBuffer()
).then(bytes => {
  let mod = new WebAssembly.Module(bytes);
  let 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:

const importObject = {
  imports: {
    imported_func: function(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.

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