On this page

WebAssembly is a high-performance assembly-like language that can be compiled from various languages, including C/C++, Rust, and AssemblyScript. Currently, it is supported by Chrome, Firefox, Safari, Edge, and Node.js!

The WebAssembly specification details two file formats, a binary format called a WebAssembly Module with a .wasm extension and corresponding text representation called WebAssembly Text format with a .wat extension.

  • Module - A compiled WebAssembly binary, i.e. a .wasm file.
  • Memory - A resizable ArrayBuffer.
  • Table - A resizable typed array of references not stored in Memory.
  • Instance - An instantiation of a Module with its Memory, Table, and variables.

In order to use WebAssembly, you need a .wasm binary file and a set of APIs to communicate with WebAssembly. Node.js provides the necessary APIs via the global WebAssembly object.

console.log(WebAssembly);
/*
Object [WebAssembly] {
  compile: [Function: compile],
  validate: [Function: validate],
  instantiate: [Function: instantiate]
}
*/
JavaScript

There are multiple methods available to generate WebAssembly binary files including:

  • Writing WebAssembly (.wat) by hand and converting to binary format using tools such as wabt
  • Using emscripten with a C/C++ application
  • Using wasm-pack with a Rust application
  • Using AssemblyScript if you prefer a TypeScript-like experience

Some of these tools generate not only the binary file, but the JavaScript "glue" code and corresponding HTML files to run in the browser.

Once you have a WebAssembly module, you can use the Node.js WebAssembly object to instantiate it.

// Assume add.wasm file exists that contains a single function adding 2 provided arguments
const fs = require('node:fs');

// Use the readFileSync function to read the contents of the "add.wasm" file
const wasmBuffer = fs.readFileSync('/path/to/add.wasm');

// Use the WebAssembly.instantiate method to instantiate the WebAssembly module
WebAssembly.instantiate(wasmBuffer).then(wasmModule => {
  // Exported function lives under instance.exports object
  const { add } = wasmModule.instance.exports;
  const sum = add(5, 6);
  console.log(sum); // Outputs: 11
});
JavaScript
// Assume add.wasm file exists that contains a single function adding 2 provided arguments
import fs from 'node:fs/promises';

// Use readFile to read contents of the "add.wasm" file
const wasmBuffer = await fs.readFile('/path/to/add.wasm');

// Use the WebAssembly.instantiate method to instantiate the WebAssembly module
const wasmModule = await WebAssembly.instantiate(wasmBuffer);

// Exported function lives under instance.exports object
const { add } = wasmModule.instance.exports;

const sum = add(5, 6);

console.log(sum); // Outputs: 11
JavaScript

WebAssembly modules cannot directly access OS functionality on its own. A third-party tool Wasmtime can be used to access this functionality. Wasmtime utilizes the WASI API to access the OS functionality.