WebAssembly (Wasm)

WebAssembly (Wasm) is a low-level binary format that browsers execute in a sandboxed virtual machine alongside JavaScript. Code written in languages such as Rust, C, C++ or Go is compiled to a .wasm module, which the page loads and calls.

Why it matters

Wasm brings performance-sensitive work to the web: image and video processing, games, CAD tools, cryptography, and ports of desktop software. It is not a replacement for JavaScript. Wasm modules cannot touch the DOM directly; they exchange numbers and memory with JavaScript, which handles the user interface. Outside browsers it is also used on servers and edge platforms as a portable, sandboxed runtime.

Example

const { instance } = await WebAssembly.instantiateStreaming(
  fetch("/add.wasm")
);
console.log(instance.exports.add(2, 3)); // 5

instantiateStreaming compiles the module while it downloads. The server should send the file with the application/wasm MIME type, or streaming compilation fails.

Common mistakes

• Expecting Wasm to speed up ordinary page logic; crossing the JavaScript boundary has a cost.
• Shipping large modules without compression or caching.
• Serving the wrong MIME type.
• Assuming it is a new language rather than a compilation target.

Related

• JavaScript
• Compiler
• MIME type
• Web Workers

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