WebAssembly Explained: What It Is, Why It Matters, and When to Use It

WebAssembly (Wasm) is a binary instruction format that runs in the browser at near-native speed. It's not a replacement for JavaScript — it's a compilation target. You write code in Rust, C++, or Go, compile it to Wasm, and run it alongside JavaScript in the browser.

What problem does it solve?

JavaScript is fast for most web tasks. But there are categories of computation where it's genuinely slow: video encoding/decoding, image processing, cryptography, physics simulations, CAD calculations. These are tasks that require sustained high-throughput arithmetic. Wasm can run them 5–20x faster than equivalent JavaScript.

Real applications running on Wasm today

• Figma — their rendering engine is compiled to Wasm for performance
• Google Earth — runs in the browser via Wasm
• AutoCAD Web — runs its legacy C++ codebase via Wasm compilation
• Squoosh — Google's image compression tool uses Wasm for encoding/decoding
• ffmpeg.wasm — full video processing library compiled to run in-browser

How Wasm and JavaScript work together

Wasm modules don't have direct DOM access — they can't manipulate HTML elements. JavaScript handles the UI. Wasm handles the computation. The two communicate through a well-defined interface: JavaScript calls Wasm functions, passes data (numbers, arrays, strings via shared memory), gets results back. The heavy lifting goes to Wasm, the event handling stays in JS.

When to actually use it

Don't reach for Wasm to solve JavaScript slowness in general. Reach for it when you have a specific computation bottleneck — something you can profile and verify is the actual bottleneck — and you need more than what JavaScript can deliver. Or when you have an existing C/C++ library (like a codec or compression algorithm) that you want to run in the browser without a full rewrite.

Getting started

The easiest entry point is Rust + wasm-pack. Write a function in Rust, compile with wasm-pack build, import the resulting module in JavaScript. The toolchain is mature, the documentation is good, and there are working examples for most common use cases.