WebAssembly.Instance() constructor

The WebAssembly.Instance() constructor creates a new Instance object which is a stateful, executable instance of a WebAssembly.Module.

Warning: Since instantiation for large modules can be expensive, developers should only use the Instance() constructor when synchronous instantiation is absolutely required; the asynchronous WebAssembly.instantiateStreaming() method should be used at all other times.

Syntax

new WebAssembly.Instance(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.

Exceptions

If either of the parameters are not of the correct type or structure, a TypeError is thrown.
If the operation fails, one of WebAssembly.CompileError, WebAssembly.LinkError, or WebAssembly.RuntimeError are thrown, depending on the cause of the failure.
Some browsers may throw a RangeError, as they prohibit compilation and instantiation of Wasm with large buffers on the UI thread.

Examples

undefined

Synchronously instantiating a WebAssembly module

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

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();
  });

However, the preferred way to get an Instance is through the asynchronous WebAssembly.instantiateStreaming() function, for example like this:

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

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

Specifications

WebAssembly JavaScript Interface