What is Node.js? How does it differ from JavaScript running in a browser?

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Accepted Answer

## What is Node.js?

Node.js is an **open-source, cross-platform JavaScript runtime environment** that allows you to run JavaScript code outside of a web browser. It's built on Chrome's V8 JavaScript engine, written in C++.

```javascript
// You can run this in Node.js (not in browser)
const fs = require('fs');
const data = fs.readFileSync('file.txt', 'utf8');
console.log(data);
```

## Key Characteristics

**1. Server-Side JavaScript**
- Run JavaScript on the server
- Build APIs, web servers, CLI tools
- Access file system, network, databases

**2. Non-Blocking I/O**
- Handles many connections simultaneously
- Doesn't wait for operations to complete
- Great for I/O-intensive applications

**3. Single-Threaded Event Loop**
- Uses one thread for JavaScript execution
- Delegates I/O to the operating system
- Efficient for concurrent connections

## Browser vs Node.js

| Feature | Browser | Node.js |
|---------|---------|---------|
| DOM Access | Yes (`document`, `window`) | No |
| File System | No | Yes (`fs` module) |
| Network (low-level) | Limited | Full (`net`, `http`) |
| Global Object | `window` | `global` |
| Module System | ES Modules | CommonJS + ES Modules |
| Purpose | User Interface | Server/Backend |

## Example: Simple HTTP Server

```javascript
const http = require('http');

const server = http.createServer((req, res) => {
  res.writeHead(200, { 'Content-Type': 'text/plain' });
  res.end('Hello from Node.js!');
});

server.listen(3000, () => {
  console.log('Server running on port 3000');
});
```

## Common Use Cases

- REST APIs and GraphQL servers
- Real-time applications (chat, gaming)
- Command-line tools
- Microservices
- Server-side rendering