React Fiber is the complete rewrite of the React core algorithm, introduced with React 16. Its main purpose was to address the limitations of the old React reconciliation algorithm and to enable powerful new features.

Why Was React Fiber Introduced?

1. Better Handling of Asynchronous Rendering
   The old React rendering was synchronous — once started, it couldn't be paused. For complex UIs, this could cause slow and janky user experiences.
   Fiber introduced the ability to pause, interrupt, and prioritize work, leading to smoother and more responsive applications.

2. Improved Performance and User Experience
   Fiber allowed React to break rendering work into small units called "fibers" and schedule them based on priority (high-priority updates like animations could happen first).

3. Support for New Features
   Fiber was necessary to enable advanced features like:

   • Concurrent Mode (doing work in parallel without blocking the main thread)
   • Suspense (waiting for some code/data to load before rendering)
   • Time slicing (breaking work into chunks to avoid freezing the UI)

4. More Fine-Grained Control
   Fiber provided React with a more sophisticated update system: it could start rendering, pause in the middle, and come back later — just like a video game loop.

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In Short:

React Fiber makes React faster, smarter, and more flexible by giving it the ability to split, prioritize, and manage rendering tasks efficiently. Without Fiber, features like Concurrent Mode and Suspense would not have been possible.

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