React Fiber is a complete rewrite of React’s core architecture, introduced starting with React 16. It was designed to address limitations in the original React stack and enable new powerful features.

Main Goals of React Fiber:

1. Incremental Rendering
   Fiber allows rendering work to be split into units and spread out over multiple frames. This means React can pause work, come back to it later, and prioritize important updates, leading to smoother apps.

2. Prioritization of Updates
   Not all updates are equally important. Fiber enables update prioritization, meaning critical updates (like animations or user input) are handled first, while less important tasks (like background data fetching) can be delayed.

3. Better Error Handling
   Fiber introduces error boundaries, allowing React components to gracefully handle rendering errors without crashing the entire app.

4. Concurrency and Scheduling
   Fiber prepares React for concurrent rendering, where multiple versions of the UI can exist simultaneously. This is the foundation for features like Suspense and Concurrent Mode.

5. Improved Support for Portals
   Fiber made it easier and more reliable to render children into a DOM node outside of their parent hierarchy using portals.

6. More Flexible and Fine-Grained Control
   The new architecture gives React developers deeper control over the rendering process, enabling features like timeouts, yields, and abortable updates.

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Key Benefits of React Fiber:

• Smoother user experience, even during heavy updates.
• Faster perceived performance thanks to task prioritization.
• Resilient applications with built-in error recovery.
• Readiness for future innovations like Suspense for Data Fetching and Concurrent Rendering.
• Better developer tools and debugging capabilities.

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In simple words:
React Fiber made React smarter, faster, and more resilient — preparing it for the complex needs of modern web applications.

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