What Prepack Does

Prepack is a tool that makes JavaScript code run faster by doing some of the work ahead of time. Instead of waiting for your code to execute in the browser or server, Prepack analyzes your JavaScript file before it runs and pre-computes parts of it—similar to how a baker might prep ingredients before the restaurant opens. The result is a new, optimized version of your code that has less work to do when it actually runs.

How It Works

The tool reads your JavaScript code and looks for calculations or operations that can be figured out before runtime. It then rewrites your code with those results already baked in. For example, if your code calls a function multiple times during startup to initialize variables, Prepack can run that initialization once during the preparation phase and just store the final result. This is most effective for code that does a lot of setup work—think loading data structures, computing values, or building objects that don't change based on user input.

Who Would Use It

Web developers and app teams use Prepack to make their applications faster, especially for initialization-heavy code. If you have a JavaScript bundle that takes a long time to start up because it's running lots of setup logic, Prepack can reduce that startup time. It integrates with popular bundling tools like Webpack and Rollup, so you can drop it into your build pipeline alongside your existing tooling. The project also includes an interactive playground on its website where you can experiment with simple examples to see how Prepack transforms code.

Development Status

Prepack is a Facebook project that's actively tested against JavaScript standards. The README emphasizes that conformance testing is ongoing work in progress, meaning it handles most common JavaScript correctly but may not yet support every edge case or newer language feature. The project is open for contributions and provides development setup instructions for anyone wanting to extend or improve it.