WebAssembly (wasm) is a virtual assembly language. It's a binary instruction format that can be executed at near native speeds by JavaScript engines like V8, which means it can run in a browser or Node.js app. Not intended as a JavaScript replacement, it instead works with it for performance critical pieces of code. You can make calls from JavaScript to WebAssembly and vice versa. It can be useful for games, VR, and AR, for example.
Rust is a fast and memory-efficient programming language with interesting type and thread-safety characteristics. I've never used it, but have wanted to check it out for a while. Along with Go and C, it can easily be compiled into WebAssembly. This post just touches the surface of Rust.
First, to learn how to use Rust in a JavaScript app, I followed Compiling from Rust to WebAssembly. They guide you through compiling Rust code, generating a JavaScript wrapper package, then using npm and webpack to run it. The end user only needs a modern browser and they can execute the code originally written in Rust none the wiser.
Building on that, as a simple experiment to see how much faster Rust code compiled to WebAssembly can be versus vanilla JavaScript, I compared calculating the nth Fibonacci number in each. I used the inefficient recursive algorithm intentionally, wanting it to be slow.
The function in Rust looks like this:
And the JavaScript code that calls it, times it, and compares it:
Surprisingly, although maybe it shouldn't be, the WebAssembly version of this simple function is orders of magnitude faster when executed in the browser. At n=35, where they really start to diverge, I'm seeing about .06 seconds versus 3.5 seconds.
I don't often have to write CPU-intensive code like this in JavaScript, but with such a stark performance difference possible it's a good tool to have in the toolbox.