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What is Node.js?

Node.js is an open-source, cross-platform JavaScript runtime that lets you run JavaScript outside the browser. Built on Chrome’s V8 engine, it brings JavaScript to the server, command line, and even IoT devices.

┌──────────────┐
│  JavaScript  │
│   (your      │
│    code)     │
├──────────────┤
│  Node.js     │
│  APIs        │
├──────────────┤
│  V8 Engine   │
├──────────────┤
│  OS Layer    │
└──────────────┘

Why Node.js?

Before Node.js (2009), JavaScript was trapped in the browser. Developers had to learn a different language (PHP, Python, Ruby, Java) to write backend code. Node.js changed that by:

• Unifying language — use JavaScript everywhere
• Non-blocking I/O — handle thousands of concurrent connections with a single thread
• NPM ecosystem — the largest package registry on earth

Who Uses Node.js?

Your First Node.js Script

Create a file hello.js:

// hello.js
console.log('Hello, Node.js!');
console.log('Running on', process.version);

Run it:

node hello.js
# Output:
# Hello, Node.js!
# Running on v22.x.x

Note: The exact version depends on what you have installed. This is fine — all code in this tutorial works across modern Node.js versions.

Node.js is Event-Driven

Node.js uses an event-driven, non-blocking I/O model. Instead of waiting for a file to read or a database to respond, it registers a callback and moves on:

// non-blocking.js
const fs = require('fs');

console.log('1. Start reading file...');

fs.readFile('hello.js', 'utf8', (err, data) => {
  if (err) throw err;
  console.log('3. File read complete');
});

console.log('2. Done! (but file read is still in progress)');

Output:

1. Start reading file...
2. Done! (but file is still in progress)
3. File read complete

Node.js vs Traditional Servers

Key Takeaways

• Node.js is a JavaScript runtime for the server, not a framework
• Built on the V8 JavaScript engine
• Uses non-blocking, event-driven I/O for high concurrency
• npm provides access to over 2 million packages
• Perfect for APIs, real-time apps, CLIs, and microservices