WebAssembly.Instance() constructor

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

Syntax

Important: 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.

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.

Examples

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 = {
  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();
})

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

Specifications

Browser compatibility

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See also