https://github.com/necolas/react-native-web
If you're doing cross-platform prototyping, it can actually be easier than writing the app in React (or vanilla JS) and then porting to React Native, because the React Native elements are at a slightly higher semantic level of abstraction than stock DOM elements. The markup it generates is pretty atrocious, but if you just want to get something up on the screen, you can work out your design compromises first and then optimize it.
