What is React.js?

React.js is a popular open-source JavaScript library used for building fast and interactive user interfaces (UI), especially for single-page applications (SPA). It was created by Jordan Walke at Facebook and officially released in 2013.

React allows developers to build applications using reusable components. Instead of updating the entire webpage, React updates only the parts that change using its powerful Virtual DOM system.

Today, React powers thousands of modern applications including Facebook, Instagram, Netflix, Airbnb, and WhatsApp Web. It is considered the #1 frontend JavaScript library in the world.

History of React.js

▶
2011 — React was first used internally at Facebook.
▶
2013 — React.js officially released as open-source.
▶
2015 — React Native launched for mobile app development.
▶
2018 — React Hooks introduced in React 16.8.
▶
2022 — React 18 released with concurrent rendering.
▶
2024-2026 — React Server Components and AI-powered frontend tooling became mainstream.

Key Features of React.js

⚛️

Component-Based

React applications are built using reusable components.

⚡

Virtual DOM

React updates only changed elements, improving performance.

🧩

Reusable UI

Components can be reused across the application.

🔄

One-Way Data Flow

Data flows from parent to child components.

🪝

Hooks Support

Hooks like useState and useEffect simplify development.

🌐

SEO Friendly

Works perfectly with frameworks like Next.js for server-side rendering.

How React.js Works

👨‍💻 Write Components

⚛️ JSX Compilation

📦 Virtual DOM

🔍 Compare Changes

🖥️ Update Real DOM

✅ Fast UI Rendering

Code Execution Flow — from source to output

What is JSX?

JSX (JavaScript XML) is a syntax extension for JavaScript used in React. JSX allows developers to write HTML-like code inside JavaScript.

⚛️ React JSXApp.jsx

function App() {
  return <h1>Hello React!</h1>;
}

export default App;

React Components

Components are the building blocks of React applications. Every UI part can be created as a separate reusable component.

Type	Description
Functional Component	Modern React component using functions
Class Component	Older React component using ES6 classes
Reusable Component	Used multiple times in applications
UI Component	Represents UI elements like Navbar or Card

React Hooks

Hooks are special functions introduced in React 16.8 that allow functional components to use state and lifecycle features.

▶
useState() — Manages component state.
▶
useEffect() — Handles side effects like API calls.
▶
useContext() — Shares data globally.
▶
useRef() — Accesses DOM elements directly.
▶
useReducer() — Manages complex state logic.

Your First React Program

⚛️ ReactApp.jsx

function App() {
  return (
    <h1>Hello, React.js!</h1>
  );
}

export default App;

Output

Hello, React.js!

React vs Angular vs Vue

Feature	React	Angular	Vue
Type	Library	Framework	Framework
Language	JavaScript	TypeScript	JavaScript
Learning Curve	Easy	Hard	Easy
Performance	Very Fast	Fast	Fast
DOM	Virtual DOM	Real DOM	Virtual DOM
Popularity	Highest	High	High

Advantages and Disadvantages of React.js

✅ Advantages

Fast RenderingVirtual DOM improves UI performance.

Reusable ComponentsWrite once and reuse everywhere.

Huge CommunityMillions of developers use React worldwide.

SEO SupportNext.js enables server-side rendering.

❌ Disadvantages

Fast Changing EcosystemReact ecosystem evolves very quickly.

Only UI LibraryNeeds extra libraries for routing and state management.

JSX ComplexityBeginners may initially find JSX confusing.

Where is React.js Used?

▶
🌐 Web Applications — React is used to build modern frontend applications.
▶
📱 Mobile Apps — React Native is used for Android and iOS apps.
▶
🛒 E-Commerce — Used in online shopping platforms.
▶
📊 Dashboards — Perfect for admin panels and analytics dashboards.
▶
🎮 Interactive UI — Used for dynamic interfaces and gaming UIs.

Why Learn React.js in 2026?

▶
💼 High Job Demand — React developers are highly demanded worldwide.
▶
⚛️ Most Popular Frontend Library — Used by startups and big tech companies.
▶
🚀 Great Career Growth — Essential skill for frontend engineers.
▶
🌐 Works with Next.js — Enables full-stack React applications.
▶
📚 Huge Ecosystem — Thousands of packages and tools available.

React.js Interview Questions

Conclusion

React.js is one of the most powerful and widely-used frontend technologies in the world. Its component-based architecture, Virtual DOM, hooks system, and massive ecosystem make it the best choice for modern web development.

If you want to become a frontend developer, full-stack developer, or build modern web applications in 2026, learning React.js is one of the smartest decisions you can make.