A mobile app development tool built on React Native that enables you to build and deploy native iOS and Android applications using JavaScript. According to the creators, Expo is to React Native as Rails is to Ruby. Writing an Expo app is like writing a React Native app, except you never need to open Xcode or Android Studio, or write any native code. The Expo SDK provides access to system functionality such as contacts, camera and social login. It also provides UI components to handle a variety of use-cases that are not part React Native core including icons, an image lightbox, a loading overlay and more.