What is Node.js?

Introduction: The JavaScript Revolution

Node.js is a JavaScript runtime built on Chrome's V8 engine. It lets you run JavaScript on your computer/server, not just in browsers.

Created: 2009 by Ryan Dahl
Purpose: Run JavaScript everywhere
Impact: Revolutionized backend development

Why Node.js Matters

Before Node.js:
- Frontend: JavaScript
- Backend: PHP, Python, Java, Ruby
- Result: Learn 2+ languages

After Node.js:
- Frontend: JavaScript
- Backend: JavaScript (Node.js)
- Result: One language for everything!

Benefits:

• One Language - JavaScript for frontend and backend
• Fast - V8 engine compiles to machine code
• Non-Blocking - Handles thousands of requests
• Huge Ecosystem - npm has millions of packages
• Easy to Learn - If you know JavaScript, you're ready!

Complete Node.js Overview

                    ┌─────────────────────┐
                    │   JavaScript Code    │
                    │   (Your Application) │
                    └──────────┬───────────┘
                               │
                    ┌──────────▼───────────┐
                    │    Node.js Runtime   │
                    │  ┌────────────────┐ │
                    │  │  V8 Engine     │ │ ← Executes JavaScript
                    │  │  (Chrome's)    │ │
                    │  └────────────────┘ │
                    │  ┌────────────────┐ │
                    │  │  libuv         │ │ ← Handles async I/O
                    │  │  (Event Loop)  │ │
                    │  └────────────────┘ │
                    │  ┌────────────────┐ │
                    │  │  Node.js APIs  │ │ ← fs, http, crypto
                    │  └────────────────┘ │
                    └──────────┬───────────┘
                               │
                    ┌──────────▼───────────┐
                    │  Operating System   │
                    │  (File System,       │
                    │   Network, etc.)      │
                    └──────────────────────┘

What Node.js Does:
├── Runs JavaScript outside browser
├── Accesses file system
├── Creates web servers
├── Connects to databases
├── Handles network requests
└── Much more!

The Problem: JavaScript Was Limited

Before Node.js (2009), JavaScript could only run in browsers:

┌─────────────────────────────────┐
│         Browser                 │
│  ┌──────────────────────────┐  │
│  │   JavaScript Engine       │  │
│  │   (V8, SpiderMonkey)      │  │
│  └──────────────────────────┘  │
│                                 │
│  ✅ Can manipulate DOM          │
│  ✅ Can handle events           │
│  ❌ Can't access files          │
│  ❌ Can't create servers        │
│  ❌ Can't connect to databases  │
└─────────────────────────────────┘

Backend Development:
├── PHP
├── Python
├── Java
├── Ruby
└── C# / .NET

Result: Different languages for frontend and backend!

The Solution: Node.js

Node.js lets you run JavaScript outside the browser on your computer/server.

┌─────────────────────────────────┐
│         Node.js Runtime          │
│  ┌──────────────────────────┐   │
│  │   V8 JavaScript Engine    │   │
│  │   (Same as Chrome!)       │   │
│  └──────────────────────────┘   │
│  ┌──────────────────────────┐   │
│  │   System APIs             │   │
│  │   - File System (fs)      │   │
│  │   - Network (http)        │   │
│  │   - OS (os)               │   │
│  │   - Crypto                │   │
│  └──────────────────────────┘   │
│  ┌──────────────────────────┐   │
│  │   Event Loop              │   │
│  │   (Non-blocking I/O)      │   │
│  └──────────────────────────┘   │
└─────────────────────────────────┘

✅ Can access files
✅ Can create servers
✅ Can connect to databases
✅ Can do everything backend languages do!

How Node.js Works

Architecture Diagram

┌─────────────────────────────────────────┐
│         Your JavaScript Code             │
└─────────────────┬───────────────────────┘
                  │
┌─────────────────▼───────────────────────┐
│         Node.js Runtime                 │
│  ┌──────────────────────────────────┐  │
│  │   V8 Engine (JavaScript Parser)  │  │
│  │   - Compiles JS to machine code   │  │
│  │   - Executes code                 │  │
│  └──────────────────────────────────┘  │
│  ┌──────────────────────────────────┐  │
│  │   libuv (C++ Library)           │  │
│  │   - Handles async operations    │  │
│  │   - Thread pool for heavy tasks │  │
│  │   - Event loop                  │  │
│  └──────────────────────────────────┘  │
│  ┌──────────────────────────────────┐  │
│  │   Node.js APIs                   │  │
│  │   - fs, http, crypto, etc.       │  │
│  └──────────────────────────────────┘  │
└─────────────────────────────────────────┘
                  │
┌─────────────────▼───────────────────────┐
│         Operating System                │
│   (File System, Network, etc.)          │
└─────────────────────────────────────────┘

Key Components:

1. V8 Engine - Executes JavaScript (same engine Chrome uses)
2. libuv - Handles asynchronous operations
3. Node.js APIs - Provides system access (fs, http, etc.)

Before vs After Node.js

Before Node.js (2009)

Frontend Development:
┌─────────────┐
│ JavaScript  │ → Browser only
└─────────────┘

Backend Development:
┌──────┐  ┌────────┐  ┌──────┐  ┌──────┐
│ PHP  │  │ Python │  │ Java │  │ Ruby │
└──────┘  └────────┘  └──────┘  └──────┘

Problem: Learn 2+ languages!

After Node.js

Full-Stack Development:
┌─────────────────────────────────┐
│         JavaScript               │
│  ┌─────────────┐  ┌──────────┐ │
│  │  Frontend   │  │ Backend   │ │
│  │  (Browser)  │  │ (Node.js) │ │
│  └─────────────┘  └──────────┘ │
└─────────────────────────────────┘

Solution: One language for everything!

The Event Loop: How Node.js Handles Multiple Requests

Traditional Server (Blocking)

Request 1 → Process → Wait for DB → Response (2 seconds)
Request 2 → Wait... → Wait... → Process → Response (4 seconds)
Request 3 → Wait... → Wait... → Wait... → Response (6 seconds)

Problem: Each request blocks others!

Node.js Server (Non-Blocking)

Request 1 → Start DB query → Don't wait → Continue
Request 2 → Start DB query → Don't wait → Continue
Request 3 → Start DB query → Don't wait → Continue

DB responds → Handle Request 1 → Send Response
DB responds → Handle Request 2 → Send Response
DB responds → Handle Request 3 → Send Response

All requests handled in ~2 seconds!

Event Loop Flow

┌─────────────────────────────────────┐
│         Event Loop                  │
│                                     │
│  1. Execute JavaScript code         │
│  2. If async operation (DB, file): │
│     → Send to thread pool          │
│     → Continue to next line        │
│  3. When async completes:          │
│     → Callback added to queue      │
│  4. Process callbacks              │
│  5. Repeat                          │
└─────────────────────────────────────┘

Result: Single thread handles thousands of requests!

What Node.js is Good For

✅ Great For:

1. APIs and Web Servers

┌──────────┐      ┌──────────┐
│  Client  │─────▶│ Node.js  │
│          │◀────│  Server  │
└──────────┘      └──────────┘
                  Fast response!

2. Real-Time Applications

• Chat apps (WhatsApp, Slack)
• Gaming servers
• Live notifications
• Collaborative editing

3. Microservices

• Small, independent services
• Easy to scale
• Fast deployment

4. CLI Tools

• npm, yarn
• Build tools
• Automation scripts

5. Streaming Applications

• Video processing
• File uploads
• Data pipelines

❌ Not Ideal For:

1. CPU-Intensive Tasks

❌ Video encoding
❌ Image processing
❌ Machine learning
❌ Heavy calculations

Why? Node.js is single-threaded.
Use: Worker Threads or other languages

2. Heavy Computation

• Mathematical calculations
• Data analysis
• Scientific computing

Your First Node.js Code

// hello.js
console.log('Hello from Node.js!');

// Run it
// $ node hello.js
// Output: Hello from Node.js!

Simple HTTP Server

const http = require('http');

const server = http.createServer((req, res) => {
  res.writeHead(200, { 'Content-Type': 'text/plain' });
  res.end('Hello World!');
});

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

What happens:

1. Import http module
2. Create server
3. Handle requests
4. Listen on port 3000

Node.js vs Other Technologies

Feature	Node.js	PHP	Python	Java
Language	JavaScript	PHP	Python	Java
Speed	Fast (V8)	Medium	Medium	Fast
Async	Native	Limited	Limited	Limited
Learning	Easy (if you know JS)	Easy	Easy	Hard
Ecosystem	Huge (npm)	Large	Large	Large
Use Case	APIs, Real-time	Web pages	Data science	Enterprise

Key Takeaway

Node.js = JavaScript + V8 Engine + System APIs + Event Loop. It enables full-stack JavaScript development with a non-blocking architecture perfect for I/O operations. Single-threaded but handles thousands of concurrent connections through event loop. Start with Node.js if you know JavaScript and want to build backends.