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WebAssembly basics

In short

Some jobs, like video editing or image compression, push JavaScript to its limit. WebAssembly lets code written in Rust, C++ or Go run in the browser close to native speed.

A binary instruction format that lets languages like C++, Rust and Go run in the browser at near-native speed — used for performance-critical tasks JS isn't well suited for.

The jaadu stunt double leaps a motorbike over a ramp while a camera records.

Think of it like this

Hiring a stunt double

The hero (JavaScript) does the acting, dialogue and talking to the audience (the DOM). For the dangerous bike jump, a trained stunt double (WebAssembly) steps in, does that one heavy scene, and hands back to the hero.

Watch it happen

From Rust to the browser

Rust code

Compile

.wasm file

Load

JS calls

1/5You write a fast add or image-blur function in Rust.

The big ideas

What you need to know

A compiled binary format

You write Rust, C or C++, compile it to a .wasm file, and the browser runs it safely in a sandbox.

Works with JavaScript

JS loads the module and calls its exported functions. Wasm can't touch the DOM directly, it goes through JS.

For heavy computing

It shines at image and video processing, games, physics and running existing C++ libraries on the web.

Not a React replacement

For normal UI work, JavaScript is simpler and fast enough. Wasm adds build complexity.

See it in code

Walk through the code

1// Load a compiled module and call its exported function2const { instance } = await WebAssembly.instantiateStreaming(3  fetch("/math.wasm")4);5 6console.log(instance.exports.add(2, 3)); // 5

1/2Fetch the .wasm file and compile it as it streams in.

instantiateStreaming needs the server to send .wasm files with the application/wasm content type. Tools like wasm-pack (Rust) and Emscripten (C/C++) create these files.

Save yourself the pain

Common mistakes

Oops

Rewriting a whole form-heavy React app in Rust and Wasm for speed.

Do this instead

Profile first. Move only the heavy, number-crunching parts to Wasm and keep the UI in JavaScript.

Oops

Calling into Wasm thousands of times per frame for tiny tasks.

Do this instead

Crossing between JS and Wasm has a cost. Send larger chunks of work in fewer calls.

Your mission

Run your first .wasm file

~30 min

Quick check

Which task is the best fit for WebAssembly in a web app?

Go deeper

1Run a simple pre-built Wasm demo in the browser and inspect the network payload.
2Learn typical use cases: video/image processing, games, cryptography, CAD tools.
3Understand Wasm complements JS for hot paths — it doesn't replace it for typical UI work.