metro

Metro is the tool that packages up JavaScript code for React Native apps. Think of it like a compiler: when you write your app's code across multiple files, Metro takes all those pieces and bundles them together into a format that can run on iOS and Android phones.

The key advantage of Metro is speed. It's designed to reload your code almost instantly when you make changes during development, which means you can see what you're building without waiting around. It also handles large projects well—apps with thousands of individual code files—without getting bogged down. Because it's built specifically for React Native (not adapted from a general-purpose tool), it works with every React Native project out of the box, with no extra configuration needed.

If you're a React Native developer, you're probably already using Metro without thinking about it. When you run your development server or build an app for release, Metro is the thing doing the heavy lifting behind the scenes. The tool was originally part of React Native itself, but the team split it out into its own project to make it easier to improve and fix bugs faster.

The README itself points to the Metro website for the actual documentation and setup instructions, so this repository is the source code rather than a beginner-friendly tutorial.