React JavaScript UI Library

React is a free and open-source JavaScript library for building user interfaces, maintained by Meta (formerly Facebook). Created by Jordan Walke at Facebook in 2011 (open-sourced at JSConf US in May 2013), React has become the most popular frontend library in the world, used by millions of developers and powering applications at Facebook, Instagram, WhatsApp, Netflix, Airbnb, and countless others. Key features: component-based: UIs are built from reusable, composable components. Each component manages its own state and props (inputs from parent), and can be composed hierarchically to build complex interfaces. JSX: a syntax extension that allows writing HTML-like markup directly in JavaScript, making component templates readable and maintainable. JSX is compiled to JavaScript by Babel or SWC. Virtual DOM: React maintains an in-memory representation of the DOM. When state changes, React computes the minimal set of DOM updates via a diffing algorithm (reconciliation) and applies them in a single batch, minimizing expensive browser reflows. Hooks: introduced in React 16.8 (2019), hooks (useState, useEffect, useContext, useReducer, useMemo, useCallback, useRef) allow using state and lifecycle features in functional components, replacing class components. State management: component-level state via useState/useReducer; application-level state via Context API, Redux, Zustand, or Jotai. Server Components: React Server Components (RSC, introduced in React 18) render on the server, sending only the result to the client, reducing bundle size. Concurrent features: React 18 introduced concurrent rendering with useTransition and useDeferredValue for non-blocking UI updates. Suspense for declarative loading states. React Native: build native iOS and Android apps using React. Next.js: the most popular React framework for production apps. MIT.