💚 What is Node.js?

📖 Definition

Node.js is a JavaScript runtime built on Chrome's V8 engine. It enables JavaScript execution outside browsers with asynchronous event-driven architecture for high performance and scalability. Provides vast library ecosystem through npm (Node Package Manager).

🎯 Simple Analogy

Restaurant Kitchen

Traditional Server (Synchronous)
└─ One chef serves customers one by one (slow)

Node.js (Asynchronous)
├─ One chef handles multiple orders simultaneously
├─ While steak cooks → prepare salad
└─ Fast and efficient!

⚙️ Event Loop

┌─────────────┐
│  Call Stack │  Executing code
└──────┬──────┘
       │
┌──────▼──────┐
│ Event Loop  │  Coordinator
└──────┬──────┘
       │
  ┌────┴─────┐
  │          │
Microtask   Task Queue
(Promise)   (setTimeout)

💡 Key Examples

Basic HTTP Server

const http = require('http');

const server = http.createServer((req, res) => {
  res.writeHead(200, { 'Content-Type': 'text/html' });
  res.end('<h1>Hello, Node.js!</h1>');
});

server.listen(3000, () => {
  console.log('Server running on http://localhost:3000');
});

Express API

const express = require('express');
const app = express();

app.use(express.json());

app.get('/api/users', (req, res) => {
  res.json([
    { id: 1, name: 'John' },
    { id: 2, name: 'Jane' }
  ]);
});

app.post('/api/users', (req, res) => {
  const newUser = req.body;
  res.status(201).json({ message: 'Created', user: newUser });
});

app.listen(3000);

Async/Await

const fs = require('fs').promises;

// ❌ Sequential (slow)
async function readSequential() {
  const file1 = await fs.readFile('file1.txt');
  const file2 = await fs.readFile('file2.txt');
  // Total: 200ms
}

// ✅ Parallel (fast)
async function readParallel() {
  const [file1, file2] = await Promise.all([
    fs.readFile('file1.txt'),
    fs.readFile('file2.txt')
  ]);
  // Total: 100ms (concurrent)
}

Database Integration

const express = require('express');
const mysql = require('mysql2/promise');

const pool = mysql.createPool({
  host: 'localhost',
  user: 'root',
  password: 'password',
  database: 'mydb'
});

app.get('/api/users', async (req, res) => {
  try {
    const [rows] = await pool.execute('SELECT * FROM users');
    res.json(rows);
  } catch (error) {
    res.status(500).json({ error: 'Database error' });
  }
});

🤔 FAQ

Q1. Why is Node.js fast?

// Non-blocking I/O
readFile('file.txt', (err, data) => {
  processData(data);
});
// Can handle other requests while reading!

// Event-driven
// Single thread handles thousands of connections
// No context switching overhead

Q2. What is npm?

# Package manager for Node.js
npm install express        # Install package
npm init -y                # Initialize project
npm list                   # List installed packages
npm audit                  # Security check

# package.json
{
  "dependencies": {
    "express": "^4.18.0"
  }
}

Q3. CommonJS vs ES Modules?

// CommonJS (traditional)
const { add } = require('./math');
module.exports = { add };

// ES Modules (modern)
import { add } from './math.js';
export function add(a, b) { return a + b; }

// Enable in package.json
{ "type": "module" }

🎬 Summary

• Asynchronous I/O: High performance and scalability
• JavaScript: Same language as frontend
• npm: Vast package ecosystem
• Event Loop: Efficient concurrent processing

Become a full-stack developer with Node.js! 💚