What is React?

React is a JavaScript library for building user interfaces from small, reusable pieces called components. Each component is a function that returns what its piece of the page should look like, written in JSX, a syntax that lets you put HTML-like tags inside JavaScript. When your data changes, React works out what's different and updates the page for you.

Jordan Walke created React at Facebook, and it was open-sourced in May 2013. React 19 is the current major version. Here's a component that remembers how many times it's been clicked:

import { useState } from 'react';

export default function LikeButton() {
  const [likes, setLikes] = useState(0);
  return (
    <button onClick={() => setLikes(likes + 1)}>
      {likes} likes
    </button>
  );
}

useState gives the component a piece of memory, and calling setLikes tells React to render it again with the new value. React covers the UI layer only, so routing and data loading come from other tools. The React docs recommend starting new apps with a framework such as Next.js or React Router.