What is SWC?

SWC is a compiler for JavaScript and TypeScript, written in Rust. It’s a tool I see mentioned more and more.

It takes any JavaScript or TypeScript code and outputs JavaScript that works also on older browsers, as well as (of course) modern browsers.

In short it’s the new Babel.

But faster.

They say 20x+ faster.

Why is it so much faster?

Babel is written in JavaScript. It runs on a single thread, and it parses, transforms and prints your code using JavaScript itself.

SWC is written in Rust, a language compiled to native code, and it can work on multiple files in parallel. That’s where the speed comes from.

We’re seeing an incremental focus on speed everywhere these days. Mostly because tools are rewritten in more optimized system languages like Rust instead of less optimized languages like JavaScript, which is more optimal for other use cases.

On a small project you barely notice. On a large codebase, where every save triggers a rebuild, the difference between Babel and SWC is the difference between waiting and not waiting.

Who uses it?

As a “normal dev”, you’re not going to use it yourself.

But it’s used by other tools.

Next.js adopted SWC in version 12 to replace Babel for compiling and Terser for minifying.

Vite 4, released in December 2022, now has support for SWC instead of Babel. Which in turn makes it faster.

It’s used by Turbopack, which is the Vercel-made JavaScript/TypeScript bundler/build system, alternative to Vite and Webpack.

You can also use it to speed up tests, through the @swc/jest package, which compiles your test files much faster than the Babel-based default.

Trying it directly

If you’re curious, you can run it on its own:

npm install -D @swc/cli @swc/core
npx swc src -d dist

This compiles everything in src and writes the output to dist. Configuration lives in a .swcrc file, where you set the syntax (TypeScript or JavaScript) and the target environment.

One thing to know about TypeScript

Here’s the pitfall people hit when a tool switches them to SWC: it compiles TypeScript, but it does not type-check it.

SWC strips the types and outputs JavaScript, without verifying them. Your build succeeds even if the types are wrong.

The fix is to keep running the TypeScript compiler for checking only:

tsc --noEmit

You get SWC’s speed for the build, and tsc catches the type errors, in your editor or in CI.