What is React?

React is a JavaScript library for building user interfaces. It was created by engineers at Facebook and is now maintained by Meta and a large open-source community. You use React to describe what your UI should look like — React figures out how to make it happen.

The core idea in React is the component. A component is a small, self-contained piece of your interface — a button, a header, a form. You build complex UIs by combining simple components the same way you build a sentence from words.

React uses something called a virtual DOM — a lightweight copy of the real DOM kept in memory. When your data changes, React compares the virtual DOM with the real one and updates only what actually changed. This makes React fast, even in large apps.

You write React components as JavaScript functions. Each function returns a description of what should appear on screen. That description is written in JSX — a syntax that looks like HTML but lives inside JavaScript.