What is React and How Does it Work?

What is React?

React is an open-source JavaScript library built by Meta (Facebook) for building user interfaces out of reusable components. It powers Facebook, Instagram, Netflix, Airbnb, and millions of other apps.

The Core Idea: Components

function Welcome({ name }) {
  return <h1>Hello, {name}!</h1>;
}

function App() {
  return (
    <div>
      <Welcome name="Sara" />
      <Welcome name="Ali" />
    </div>
  );
}

How React Works Under the Hood

1. You describe UI as components that return JSX.
2. React builds a lightweight copy of the DOM called the Virtual DOM.
3. When state changes, React creates a new Virtual DOM and diffs it against the old one.
4. It computes the minimal set of real DOM changes and applies only those — this is what makes React fast.

State Makes It Interactive

import { useState } from "react";

function Counter() {
  const [count, setCount] = useState(0);
  return (
    <button onClick={() => setCount(count + 1)}>
      Clicked {count} times
    </button>
  );
}

Why Developers Love React

• Component reuse — build once, use everywhere.
• Huge ecosystem — Next.js, React Native, thousands of libraries.
• Declarative — describe what the UI looks like, not how to update it.
• Strong job market — one of the most in-demand frontend skills.

FAQs

Is React a framework or a library?

Technically a library (it only handles the view). Add routing and data-fetching libraries — or use a framework like Next.js that bundles them.

Do I need to know JavaScript first?

Yes — React is just JavaScript. Solid JS fundamentals make React much easier. See our React guides.