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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
  • A corresponding text representation called WebAssembly Text format with a .wat extension

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Key concepts

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]
}
*/

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Generating WebAssembly modules

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.

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How to use it

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
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

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Interacting with the OS

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

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