Facebook has completely rewritten React, its popular JavaScript library for building user interfaces. The company hasn’t previously talked much about React Fiber, as the project is called, but it has actually been working on it for a while. It’s now ready to talk about this project publicly in more detail.

Read more here: https://techcrunch.com/2017/04/18/facebook-announces-react-fiber-a-rewrite-of-its-react-framework/