⚛️ What is React?

📖 Definition

React is a JavaScript library for building user interfaces developed by Facebook (Meta). It uses component-based architecture for reusable UI and Virtual DOM for efficient screen updates. Declarative programming simplifies complex UI management.

🎯 Simple Analogy

LEGO Blocks

React Components = LEGO Blocks
├─ Combine small blocks
├─ Reuse same blocks multiple times
├─ Build complex structures easily
└─ Replace individual blocks as needed

<House>
  <Roof />
  <Wall />
  <Door />
  <Window />
</House>

⚙️ Core Concepts

Component Structure

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

// Usage
<Welcome name="John" />

State Management

import { useState } from 'react';

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

  return (
    <div>
      <p>Count: {count}</p>
      <button onClick={() => setCount(count + 1)}>
        Increment
      </button>
    </div>
  );
}

useEffect Hook

import { useState, useEffect } from 'react';

function UserProfile({ userId }) {
  const [user, setUser] = useState(null);

  useEffect(() => {
    fetch(`/api/users/${userId}`)
      .then(res => res.json())
      .then(data => setUser(data));
  }, [userId]);  // Run when userId changes

  return <div>{user?.name}</div>;
}

💡 Key Examples

List Rendering

function TodoList() {
  const [todos, setTodos] = useState([
    { id: 1, text: 'Exercise', done: false },
    { id: 2, text: 'Study', done: true }
  ]);

  return (
    <ul>
      {todos.map(todo => (
        <li key={todo.id}>
          {todo.text}
        </li>
      ))}
    </ul>
  );
}

Form Handling

function LoginForm() {
  const [formData, setFormData] = useState({
    username: '',
    password: ''
  });

  const handleChange = (e) => {
    setFormData({
      ...formData,
      [e.target.name]: e.target.value
    });
  };

  const handleSubmit = (e) => {
    e.preventDefault();
    // API call...
  };

  return (
    <form onSubmit={handleSubmit}>
      <input name="username" onChange={handleChange} />
      <input name="password" type="password" onChange={handleChange} />
      <button type="submit">Login</button>
    </form>
  );
}

🤔 FAQ

Q1. Why use React?

// ❌ Vanilla JS (complex)
const button = document.createElement('button');
button.addEventListener('click', () => {
  // Manual DOM manipulation
});

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

Q2. What is JSX?

A: JavaScript + XML syntax for React elements:

// JSX
const element = <h1 className="title">Hello</h1>;

// Converted to
React.createElement('h1', { className: 'title' }, 'Hello');

Q3. What are Hooks?

A: Functions that let you use state and lifecycle in function components:

useState    // State management
useEffect   // Side effects (API calls)
useContext  // Context access
useRef      // DOM access
useMemo     // Memoization
useCallback // Function memoization

🎬 Summary

• Components: Reusable UI blocks
• Virtual DOM: Efficient updates
• JSX: Intuitive syntax
• Hooks: Powerful function components

Build interactive applications with React! ⚛️