What is React Fiber and why was it introduced?

React Fiber is the reimplementation of React’s reconciliation algorithm that enables incremental rendering, prioritization, and interruption of work.

Before Fiber, React used a synchronous stack-based reconciliation which could block the main thread. Fiber introduced a linked-list tree structure where work is split into units, allowing React to pause, resume, or abort rendering. This enables features like concurrent rendering, prioritization of updates (e.g., animations vs data loading), and smoother UI.