Introduction to ReactJS

🌟 What is ReactJS?

ReactJS is a JavaScript library (not a full-fledged framework) that helps developers build fast and dynamic user interfaces (UIs), especially for Single Page Applications (SPAs)—web apps that update content without reloading the entire page.

🔍 Real-world analogy: Think of React as LEGO blocks (components). You can create small pieces (like buttons, cards, menus), and then assemble them into a complete website or app UI.

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📈 Why Use ReactJS?

🚀 Benefits of ReactJS:

Feature	What It Means
🧱 Component-Based	Reuse small pieces of UI across the app
⚡ Fast Rendering	Uses Virtual DOM for performance
🔁 One-Way Data Flow	Makes code easier to debug
🔄 Real-time Updates	UI updates automatically with data changes
🛠️ Developer Tools	Excellent tooling and community support

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🔍 Key Concepts in ReactJS – In Depth

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1️⃣ Components – The Building Blocks

A component is a self-contained piece of the UI. It can be as small as a button or as large as an entire webpage section.

🧪 Example:

function Header() {
  return <h1>Welcome to React</h1>;
}

You can reuse this Header anywhere in your app!

🔄 Two types of components:

• Functional Components (modern and simple)
• Class Components (older style, but still used)

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2️⃣ JSX – JavaScript + XML

JSX allows writing HTML-like syntax in JavaScript.

📄 Example:

const title = <h2>Hello MCA Students</h2>;

✅ JSX gets compiled to React.createElement(...) calls behind the scenes.

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3️⃣ Props – Component Communication

Props (short for properties) are used to pass data from one component to another.

📌 Example:

function Greeting(props) {
  return <h2>Hello, {props.name}</h2>;
}

// Usage:
<Greeting name="Anjali" />

✅ Props are read-only. They help make components dynamic.

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4️⃣ State – Managing Data in Components

State is used to store values that can change over time, like user input or counter values.

✅ Example using useState Hook:

import { useState } from 'react';

function Counter() {
  const [count, setCount] = useState(0); // Initialize state

  return (
    <div>
      <p>You clicked {count} times</p>
      <button onClick={() => setCount(count + 1)}>Click Me</button>
    </div>
  );
}

✅ useState() is a Hook—a special function to use state in functional components.

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5️⃣ Virtual DOM – Speed Booster

Instead of changing the real DOM directly (which is slow), React uses a Virtual DOM—a lightweight copy of the real DOM.

⚙️ How it works:

• React creates a virtual copy of the UI.
• When data changes, React compares the old and new virtual DOM using Diffing Algorithm.
• It updates only the changed parts in the actual DOM.

✅ Result: High performance and fast user experience.

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6️⃣ React Lifecycle (for Class Components)

Lifecycle methods allow you to run code at specific points in a component’s life.

⏱ Common methods:

• componentDidMount() – After component is added to DOM
• componentDidUpdate() – After data (props or state) changes
• componentWillUnmount() – Before component is removed

In modern React, you achieve similar results using useEffect() in functional components.

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🧠 React Folder Structure – Organized Codebase

my-app/
├── public/           # index.html (root of your app)
├── src/
│   ├── App.js        # Main component
│   ├── index.js      # Entry point for rendering React
│   └── components/   # All reusable components
├── package.json      # Project metadata and dependencies

✅ npx create-react-app my-app sets up everything automatically.

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🌐 Real World Apps Built with React

App/Website	React Usage
Facebook	Feeds, notifications, dynamic chat
Netflix	Fast rendering UI for TV, movies
Instagram	Realtime image feeds, stories
WhatsApp Web	Messaging interface
Zomato, Swiggy	Dynamic filtering, menus, cart updates

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🧪 React Mini Projects for Practice

1. 📋 To-Do List App – Add, delete, and mark tasks as complete.
2. 🏫 Student Portal – Show subjects, attendance, and marks using components.
3. 📊 Dashboard – Cards, graphs, tables using React and Bootstrap.
4. 📅 Calendar/Event App – Show events with dynamic date handling.
5. 🛒 Shopping Cart – Use props/state to add/remove products.

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🔍 ReactJS vs AngularJS vs VueJS

Feature	ReactJS	AngularJS	VueJS
Type	Library	Framework	Framework
Learning Curve	Easy	Steep	Moderate
DOM	Virtual DOM	Real DOM	Virtual DOM
Flexibility	High	Moderate	Moderate
Data Flow	One-way	Two-way binding	Two-way

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📚 Summary

Concept	Description
Component	Reusable block of UI
JSX	HTML + JS in one syntax
Props	Pass data from parent to child
State	Internal, dynamic data
Hooks	Modern way to use features in functional components
Virtual DOM	React’s fast UI rendering system