It’s a major reimplementation of the core algorithm. Fiber is just a name for the project, but practically it is just version number change that brings major performance updates.
Check this post out about the changes:
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# React Fiber Architecture
React Fiber is an ongoing reimplementation of React's core algorithm. It is the culmination of over two years of research by the React team.
The goal of React Fiber is to increase its suitability for areas like animation, layout, and gestures. Its headline feature is **incremental rendering**: the ability to split rendering work into chunks and spread it out over multiple frames.
Other key features include the ability to pause, abort, or reuse work as new updates come in; the ability to assign priority to different types of updates; and new concurrency primitives.
### About this document
Fiber introduces several novel concepts that are difficult to grok solely by looking at code. This document began as a collection of notes I took as I followed along with Fiber's implementation in the React project. As it grew, I realized it may be a helpful resource for others, too.
I'll attempt to use the plainest language possible, and to avoid jargon by explicitly defining key terms. I'll also link heavily to external resources when possible.
Please note that I am not on the React team, and do not speak from any authority. **This is not an official document**. I have asked members of the React team to review it for accuracy.
This is also a work in progress. **Fiber is an ongoing project that will likely undergo significant refactors before it's completed.** Also ongoing are my attempts at documenting its design here. Improvements and suggestions are highly welcome.