What is React JS?

React is a declarative, component‑based JavaScript library for building user interfaces, primarily for single‑page applications. It was created by Facebook and is maintained as an open‑source project. The core idea is to break the UI into reusable components that manage their own state and render based on that state. React uses a virtual DOM to efficiently update the real DOM: when a component’s state changes, React computes a diff between the new virtual DOM and the previous one, then applies only the minimal set of changes to the actual DOM, which improves performance.

Components can be defined as functions or classes. Functional components are now preferred, especially with the introduction of Hooks (useState, useEffect, useContext, etc.) that allow stateful logic without classes. JSX is a syntax extension that lets developers write HTML‑like markup directly in JavaScript, which is then transpiled to React.createElement calls.

Typical use cases include building dynamic UIs, progressive web apps, and mobile apps via React Native. React’s ecosystem includes routing (React Router), state management (Redux, MobX, Context API), and tooling (Create React App, Next.js). Overall, React simplifies UI development by promoting modularity, reusability, and efficient rendering.