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React App: Build interactive UIs with components

React is the world's most popular UI library. It lets you build web apps as a collection of reusable components — each managing its own look and behavior.

Overview

React was created by Facebook in 2013 and is now used by millions of developers worldwide. Instead of building one big HTML page, you build small, self-contained components — like LEGO bricks — and combine them into complex UIs. React automatically updates only the parts of the page that change, making apps feel fast and responsive.

Core concepts

Components: Reusable building blocks for your UI. Think of each component as a custom HTML element you define yourself. State (useState): The data that can change in your component. When state updates, React re-renders only the affected parts. Props: Data passed into a component from its parent — like arguments to a function. JSX: A syntax extension that looks like HTML inside your JavaScript. React converts it to real DOM elements.

When to use React App

Building interactive single-page apps Apps that need frequent UI updates without a page reload When you want access to the huge React ecosystem of libraries

Quick-start tip

Look for the useState hook — it's how React remembers values that change over time. Try adding a button that increments a counter to see the magic.