Seems really similar to the React Native tutorials where you can already type in a text box and see changes in a phone preview immediately: https://facebook.github.io/react-native/docs/tutorial.html
Those examples are using this library: https://github.com/necolas/react-native-web. It's really fast but the downside is that you can't use any native APIs, so things like gyroscope and push notifications won't work. Since we have an actual app we're able to support all those APIs.