React 16: A look inside an API-compatible rewrite of our frontend UI library

This post details the internal rewrite of React to React Fiber (later React 16), focusing on the engineering process. Key contributions include the use of feature flags for parallel development, the extensive use and evolution of the Jest test suite to achieve feature parity, the development of `isfiberreadyyet.com` for progress tracking, and a phased dogfooding and rollout strategy across Facebook's web and mobile applications. The rewrite enabled asynchronous rendering and introduced error boundaries.