React makes it simple to build functional, component-based user interfaces on web and mobile; at Facebook, we have more than 30,000 React components in our main web repo alone. React's simplicity and functionality has led to its adoption by hundreds of thousands of developers outside Facebook.
With today's release of React 16, we've completely rewritten the internals of React while keeping the public API essentially unchanged. From an engineering standpoint, it's a bit like swapping out the engine of a running car: since hundreds of other companies (including Facebook) use React in production every day, we wanted to do the swap without forcing people to rewrite their components built in React.
... read the whole story at code.facebook.com.