What is React and what are its main features?

React is an open-source JavaScript library for building user interfaces, created by Facebook (Meta) in 2013. It is component-based, declarative, and unidirectional — you describe what the UI should look like for a given state, and React figures out how to update the DOM efficiently.

• Component-based — UI is built from small, reusable components that manage their own state and compose into larger ones.

• Declarative — you describe the desired UI; React diffs the previous and next descriptions and applies minimal DOM changes.

• Virtual DOM — an in-memory tree of UI elements; React reconciles changes against the real DOM in batches for performance.

• JSX — HTML-like syntax that compiles to JavaScript function calls, making component markup readable.

• Unidirectional data flow — data flows from parent to child via props; events bubble back up via callbacks. Predictable to reason about.

• Hooks — modern functional components manage state, side effects, and context with useState, useEffect, and friends.

• Ecosystem — React Router, Redux/Zustand, React Query, Next.js, React Native — the largest UI library ecosystem on the web.