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React's process of comparing old and new UI descriptions to figure out the smallest set of changes to make on screen — like a spot-the-difference game.
The algorithm React uses to diff the previous and next virtual DOM trees. It compares element types, props, and keys to determine which DOM nodes to create, update, or remove, minimizing expensive DOM operations.
React's fiber reconciler splits reconciliation into interruptible units of work. The beginWork phase compares fibers by type and key, bailing out when props are referentially equal and no pending updates exist. The completeWork phase constructs or updates host instances. Concurrent mode enables time-slicing and priority-based scheduling via the lane model, allowing urgent updates (input) to preempt deferred ones (transitions).