Node.js is an open-source, cross-platform, JavaScript runtime environment that allows you to run JavaScript outside the browser.

✅ Traditionally, JavaScript was used only in browsers (for front-end).
✅ With Node.js, you can use JavaScript on the server-side to build scalable and fast network applications.

🧩 Why Use Node.js?

Here’s why Node.js became so popular:

1. Uses JavaScript Everywhere – One language for both front-end and back-end.
2. Fast Execution – Built on Google’s V8 JavaScript engine (used in Chrome).
3. Asynchronous & Non-blocking I/O – Handles multiple requests efficiently.
4. Event-driven architecture – Great for real-time applications (like chat apps).
5. Huge npm Ecosystem – Access to thousands of open-source packages.

⚙️ How Node.js Works

Node.js runs on the V8 engine, which converts JavaScript into machine code.
It uses an event loop to handle many connections simultaneously without creating multiple threads.

🔁 Event Loop Concept:

• Node.js is single-threaded.
• When a task (like reading a file or database query) takes time, Node.js delegates it and continues executing other code.
• Once the task finishes, a callback is triggered.

💻 Installing Node.js

1. Download from: https://nodejs.org
2. Verify installation: node -v npm -v
3. Create a new file: touch app.js
4. Run your file: node app.js

🧠 Basic Example

// app.js
console.log("Hello from Node.js!");

Run:

node app.js

Output:

Hello from Node.js!

📦 npm – Node Package Manager

npm is the world’s largest package library.
It comes bundled with Node.js and lets you install packages easily.

Example:

npm install express

• Installs the Express.js web framework.
• Packages are stored in the node_modules folder.

To initialize a Node project:

npm init -y

This creates a package.json file to track dependencies and metadata.

🧱 Core Modules in Node.js

Node.js provides built-in modules — no need to install them.

Module	Description
fs	File System operations
http	Create web servers
path	Handle file paths
os	Information about operating system
url	URL utilities
events	Work with event-driven programming

Example – Using fs module:

const fs = require('fs');

// Write to a file
fs.writeFileSync('hello.txt', 'Welcome to Node.js!');

// Read the file
const data = fs.readFileSync('hello.txt', 'utf8');
console.log(data);

🌐 Building a Simple Server

Let’s create a basic HTTP server using Node’s built-in http module:

const http = require('http');

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

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

Run:

node app.js

Then open your browser at http://localhost:3000.

⚡ Asynchronous Programming

Node.js excels at non-blocking, asynchronous code.

Example:

const fs = require('fs');

console.log("Start reading...");

fs.readFile('hello.txt', 'utf8', (err, data) => {
  if (err) throw err;
  console.log(data);
});

console.log("End of program");

👉 Output order:

Start reading...
End of program
Welcome to Node.js!

Because file reading happens asynchronously!

🔥 Frameworks Built on Node.js

Framework	Purpose
Express.js	Simplifies server creation and routing
NestJS	Enterprise-level framework using TypeScript
Next.js	Full-stack React framework (runs on Node)
Socket.io	Real-time communication (e.g., chat apps)
Electron	Build desktop apps using JS, HTML, and CSS

🚀 Summary

Concept	Description
Type	JavaScript runtime environment
Created By	Ryan Dahl (2009)
Built On	Google Chrome V8 engine
Architecture	Event-driven, non-blocking I/O
Language Used	JavaScript
Ideal For	APIs, web servers, real-time apps

✅ Example Use Cases of Node.js

• Building REST APIs
• Real-time chat applications
• Streaming services (like Netflix)
• Command-line tools
• Backend for single-page apps