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39. Can you explain what React Fiber is?

React Fiber is a reimplementation of React's core algorithm for rendering, applied in React version 16 and above. It solves two major problems - improving the performance of updates on large component trees and enhancing support for other rendering targets like TV, VR and mobile devices.

The React Fiber works by breaking down the rendering process into smaller units of work called fibers, which can then be interrupted or resumed in-between rendering cycles. This allows React to prioritize rendering updates based on their importance or urgency and distribute work across multiple CPU cores, resulting in faster and smoother updates.

Fiber also introduces the concept of Time Slicing where it splits the work into smaller chunks to allow a more responsive UI. This means that higher priority tasks can be executed before lower priority tasks, making the application more interactive and performant.

In summary, React Fiber is a major update to React's rendering engine that makes it more adaptable, faster and flexible. It optimizes the rendering process, improves performance, enhances support for other targets, and allows developers to build more complex and interactive user interfaces.