expo

Expo is a platform that lets you build mobile apps and web apps all from the same JavaScript code. Instead of learning separate languages and tools for iPhone, Android, and web, you write your app once using React and JavaScript, and Expo handles the complexity of making it work on all three platforms. It's like having a universal translator for app development.

The way it works is through a combination of a runtime environment and pre-built libraries. When you write code in Expo, you're using React—a popular JavaScript framework—along with a set of tools and libraries that Expo provides. These libraries give you access to phone features like the camera, location, and notifications without having to write platform-specific code. Expo includes a client app (available on both iPhone and Android) that you can use to preview your app while developing, plus the underlying engine that makes your code actually run on each device.

This repo is where the core Expo platform is developed. It contains the client apps themselves, all the libraries and modules that power the platform, documentation, and templates you can use to start new projects. If you're building an app with Expo, you're using code that comes directly from this repository.

Who uses Expo? Developers who want to ship an app to iOS, Android, and the web without tripling their workload. Startups moving quickly on a budget. Designers who want to prototype app ideas with code. Anyone who already knows JavaScript and React and wants to avoid learning native development. The platform is free to use and open source, so anyone can contribute improvements or see exactly how it works under the hood. If you want to dive deeper, there's also a separate Expo CLI repository that handles the development tools and command-line interface.