WebAssembly.Module() constructor

BaselineWidely available *

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

* Some parts of this feature may have varying levels of support.

AWebAssembly.Module() constructor creates a new Moduleobject containing stateless WebAssembly code that has already been compiled by thebrowser and can be efficientlyshared with Workers, and instantiated multiple times.

TheWebAssembly.Module() constructor function can be called tosynchronously compile given WebAssembly binary code. However, the primary way to get aModule is through an asynchronous compilation function likeWebAssembly.compile().

Warning:Since compilation for large modules can be expensive,developers should only use theModule() constructor when synchronouscompilation is absolutely required; the asynchronousWebAssembly.compileStreaming() method should be used at all other times.

Note:Webpages that have strictContent Security Policy (CSP) might block WebAssembly from compiling and executing modules.For more information on allowing WebAssembly compilation and execution, see thescript-src CSP.

Syntax

new WebAssembly.Module(bufferSource)new WebAssembly.Module(bufferSource, compileOptions)

Parameters

bufferSource

Atyped array orArrayBuffercontaining the binary code of the Wasm module you want to compile.

compileOptionsOptional

An object containing compilation options. Properties can include:

builtinsOptional: An array of strings that enables the usage ofJavaScript builtins in the compiled Wasm module. The strings define the builtins you want to enable. Currently the only available value is"js-string", which enables JavaScript string builtins.
importedStringConstantsOptional: A string specifying a namespace forimported global string constants. This property needs to be specified if you wish to use imported global string constants in the Wasm module.

Exceptions

If the parameter is not of the correct type or structure, aTypeError is thrown.
If compilation fails, the constructor rejects with aWebAssembly.CompileError.
Some browsers may throw aRangeError, as they prohibit compilation and instantiation of Wasm with large buffers on the UI thread.

Examples

Synchronously compiling a WebAssembly module

const importObject = {  my_namespace: {    imported_func(arg) {      console.log(arg);    },  },};function createWasmModule(bytes) {  return new WebAssembly.Module(bytes);}fetch("simple.wasm")  .then((response) => response.arrayBuffer())  .then((bytes) => {    const mod = createWasmModule(bytes);    WebAssembly.instantiate(mod, importObject).then((result) =>      result.exports.exported_func(),    );  });

Enabling JavaScript builtins and global string imports

This example enables JavaScript string builtins and imported global string constants when compiling a Wasm module via theModule() constructor, which is then instantiated withinstantiate(). It then calls the exportedmain() function, which logs"hello world!" to the console.See it running live.

const importObject = {  // Regular import  m: {    log: console.log,  },};const compileOptions = {  builtins: ["js-string"], // Enable JavaScript string builtins  importedStringConstants: "string_constants", // Enable imported global string constants};fetch("log-concat.wasm")  .then((response) => response.arrayBuffer())  .then((bytes) => {    const module = new WebAssembly.Module(bytes, compileOptions);    WebAssembly.instantiate(module, importObject).then((instance) =>      instance.exports.main(),    );  });