What is the Virtual DOM and how does React use it?

The Virtual DOM is an in-memory representation of the real DOM — a tree of plain JavaScript objects describing what the UI should look like. React uses it to make UI updates fast: instead of touching the slow real DOM on every change, React builds a new virtual tree, diffs it against the previous one, and applies the smallest set of real DOM changes.

• Reconciliation — the process of comparing two virtual trees and computing the minimum diff.

• Diffing algorithm — O(n) heuristic that compares same-type elements and uses keys to match list items efficiently.

• Batched updates — multiple state changes within an event are coalesced into a single render and DOM commit.

• Why it's fast — in-memory object compares are cheap; the actual DOM mutations are kept small.

• Fiber architecture (16+) — lets React pause, abort, and prioritize rendering work for smoother UIs.

• Not magic — you can still force unnecessary renders; React.memo, useMemo, and stable keys help when needed.