What is React?

React (also called React.js) is a JavaScript library for building user interfaces (UIs), especially for single-page applications (SPAs) where the page updates dynamically without reloading.

It was created by Meta (formerly Facebook) and is now one of the most popular frontend technologies in modern web development.

Simple Definition

React is a JavaScript library used to build fast, interactive, and reusable user interface components.

Why React Was Created

Before React:

• Developers manually updated the DOM (Document Object Model).
• Code became messy as applications grew.
• UI updates were slow and hard to manage.

React Solves This By:

• Breaking UI into reusable components
• Using a Virtual DOM
• Efficiently updating only what changes

Core Concepts of React

1️⃣ Component-Based Architecture

React apps are built using components. A component is a reusable piece of UI, like a JavaScript function that returns HTML.

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

Think of components like LEGO blocks 🧱 — small pieces combined to build large applications.

2️⃣ JSX (JavaScript XML)

React uses JSX, which looks like HTML but is written inside JavaScript. It makes UI code easier to read and more expressive.

const element = <h1>Hello World</h1>;
	

3️⃣ Virtual DOM

React does not directly update the real DOM. Instead:

• Creates a Virtual DOM (a lightweight copy of the real DOM)
• Compares changes
• Updates only what is necessary

This makes React faster and more efficient.

4️⃣ State and Props

Props

• Data passed from parent to child components
• Read-only

State

• Data managed inside a component
• Can change over time
• Causes re-render when updated

Why React Is Popular

• ✅ Fast performance
• ✅ Reusable components
• ✅ Strong community
• ✅ Huge job market demand
• ✅ Used in real-world applications

Companies Using React

• Meta
• Netflix
• Instagram
• Airbnb