What is expo?

Expo is a platform that lets you write a single React-based app that works on Android phones, iPhones, and the web. Instead of building three separate apps with three different languages and toolsets, you write your code once in JavaScript and React, and it runs natively on all three platforms. That's the core promise, less duplication, faster development. Under the hood, Expo provides a runtime (a piece of software that executes your code on each device) and a library of pre-built tools so you don't have to build everything from scratch. The repository itself is the backbone: it contains all the core libraries and modules, a command-line tool (CLI) for managing your project, built-in routing for navigation, documentation, and the Expo Go app, a mobile app you can use to instantly preview and test your work on a real phone without going through the full build-and-deploy cycle. The people who'd use this are mobile app developers who want to save time, startup founders building their first app, or teams that want to ship to multiple platforms without hiring separate iOS and Android specialists. For example, a founder could prototype a social app and get it running on both Android and iOS in weeks instead of months. The project also includes Expo Application Services (EAS), a paid hosting platform that handles building, signing, and deploying your finished app, though the core open-source project is free. The README shows this is a mature, well-maintained project with clear documentation, an active community on Discord, and a structured codebase. Developers can contribute by modifying libraries in the packages folder, test changes in the apps folder, and reference guides for advanced work. The project is licensed under MIT, meaning you can use and modify it freely.