Node.js lets you run JavaScript outside the browser — on servers, in build tools, in CLI scripts. It's built on Chrome's V8 engine and uses an event-driven, non-blocking I/O model that makes it fast for I/O-heavy workloads.

Your first server

import { createServer } from 'node:http';

const server = 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');
});

The npm ecosystem

Node comes with npm — the world's largest package registry. Installing a package:

npm install express

Building a REST API with Express

import express from 'express';

const app = express();
app.use(express.json());

const posts = [{ id: 1, title: 'Hello World' }];

app.get('/posts', (req, res) => res.json(posts));
app.get('/posts/:id', (req, res) => {
    const post = posts.find(p => p.id === Number(req.params.id));
    post ? res.json(post) : res.status(404).json({ error: 'Not found' });
});

app.listen(3000);

When to use Node.js

• REST APIs and GraphQL servers
• Real-time apps (chat, notifications) via WebSockets
• CLI tools and build scripts
• Serverless functions

Node is not ideal for CPU-intensive work (image processing, machine learning) — use Python or Go for those.