Expo is an open-source platform for building universal native apps: one React project in JavaScript or TypeScript that runs natively on Android, iOS and the web. It is made by 650 Industries, Inc., and the React Native project recommends it as the framework for new apps.
Expo builds on React Native and adds a universal runtime and libraries. The Expo SDK supplies over 100 modules for device features such as the camera, images and notifications, and developers can create their own modules. Expo Router gives file-based routing, in which the files in a folder define the app's screens. Expo Go, a phone app, runs a project on a real device during development. Expo Application Services (EAS), the hosted part, builds the app in the cloud, submits it to the app stores, sends over-the-air updates (new JavaScript code delivered to installed apps without a store release), runs CI/CD workflows and reports crashes and performance.
The Expo SDK, CLI and Router are open source under the MIT license. EAS has a free plan with a limited number of builds, and paid monthly plans plus usage costs.