Node.js – Running JavaScript on the Server

// Download from nodejs.org or use a version manager:
// nvm install 20   (Node Version Manager — recommended)
// nvm use 20

// Check versions
// $ node --version   → v20.x.x
// $ npm --version    → 10.x.x

// REPL (Read-Eval-Print Loop) — interactive shell
// $ node
// > 2 + 2
// 4
// > 'hello'.toUpperCase()
// 'HELLO'
// > .exit

// Run a file
// $ node app.js

// Basic Node.js script — hello.js
console.log('Hello from Node.js!');
console.log('Node version:', process.version);
console.log('Platform:', process.platform);   // 'linux', 'darwin', 'win32'
console.log('Working dir:', process.cwd());

// process.argv — command-line arguments
// $ node greet.js Alice
const name = process.argv[2] || 'World';
console.log(`Hello, ${name}!`);

// __dirname — absolute path of current file's directory
console.log(__dirname);  // /home/user/project/src

// __filename — absolute path of current file
console.log(__filename); // /home/user/project/src/app.js

// process — information about the running Node process
process.env.NODE_ENV = 'development';     // set env variable
console.log(process.env.PORT || 3000);    // read env variable
process.exit(0);                          // exit with code 0 (success)

// Buffer — handle binary data
const buf = Buffer.from('Hello');
console.log(buf);           // <Buffer 48 65 6c 6c 6f>
console.log(buf.toString()); // "Hello"

// global — equivalent of window in browsers (rarely used directly)
global.myConfig = { debug: true };

// setTimeout, setInterval, clearTimeout, clearInterval — same as browser
const timer = setTimeout(() => console.log('fired!'), 1000);
clearTimeout(timer);

// No npm install needed — built into Node.js

// fs — File System
const fs = require('fs');
// Or with ES modules: import fs from 'fs';

// Read file (async with callback)
fs.readFile('data.txt', 'utf8', (err, content) => {
  if (err) throw err;
  console.log(content);
});

// Read file (Promise-based — preferred)
const fsPromises = require('fs/promises');
const content = await fsPromises.readFile('data.txt', 'utf8');
console.log(content);

// Write file
await fsPromises.writeFile('output.txt', 'Hello Node!');

// path — cross-platform file path utilities
const path = require('path');
const filePath = path.join(__dirname, 'data', 'users.json');
console.log(path.basename(filePath)); // "users.json"
console.log(path.extname(filePath));  // ".json"
console.log(path.dirname(filePath));  // "/home/.../data"

// os — operating system info
const os = require('os');
console.log(os.platform());  // "linux"
console.log(os.cpus().length); // number of CPU cores
console.log(os.freemem());   // free memory in bytes
console.log(os.homedir());   // "/home/user"

// Initialize a project — creates package.json
// $ npm init -y

// Install a package (saved to dependencies)
// $ npm install express

// Install a dev dependency (only for development/build)
// $ npm install --save-dev nodemon

// Install globally (CLI tools)
// $ npm install -g typescript

// Remove a package
// $ npm uninstall lodash

// package.json structure
// {
//   "name": "my-app",
//   "version": "1.0.0",
//   "main": "index.js",
//   "scripts": {
//     "start": "node index.js",
//     "dev": "nodemon index.js",
//     "test": "jest"
//   },
//   "dependencies": { "express": "^4.18.0" },
//   "devDependencies": { "nodemon": "^3.0.0" }
// }

// Run scripts
// $ npm run dev
// $ npm start      (shortcut — no "run" needed)
// $ npm test       (shortcut — no "run" needed)

// npx — run a package without installing it globally
// $ npx create-react-app my-app
// $ npx nodemon index.js

const http = require('http');
const fs   = require('fs/promises');
const path = require('path');

const server = http.createServer(async (req, res) => {
  const url = req.url;
  const method = req.method;

  console.log(`${method} ${url}`);

  if (url === '/' && method === 'GET') {
    res.writeHead(200, { 'Content-Type': 'text/html' });
    res.end('<h1>Hello from Node.js!</h1>');

  } else if (url === '/api/users' && method === 'GET') {
    const users = [{ id: 1, name: 'Alice' }, { id: 2, name: 'Bob' }];
    res.writeHead(200, { 'Content-Type': 'application/json' });
    res.end(JSON.stringify(users));

  } else {
    res.writeHead(404, { 'Content-Type': 'application/json' });
    res.end(JSON.stringify({ error: 'Not Found' }));
  }
});

const PORT = process.env.PORT || 3000;
server.listen(PORT, () => {
  console.log(`Server running at http://localhost:${PORT}`);
});

// npm install express
const express = require('express');
const app = express();

// Built-in middleware — parse JSON request bodies
app.use(express.json());
// Serve static files from /public folder
app.use(express.static('public'));

// Routes
app.get('/', (req, res) => {
  res.send('<h1>Welcome to Express!</h1>');
});

app.get('/api/users', async (req, res) => {
  try {
    const users = await getUsersFromDB();
    res.json(users);
  } catch (err) {
    res.status(500).json({ error: err.message });
  }
});

// Route parameters
app.get('/api/users/:id', (req, res) => {
  const { id } = req.params;
  res.json({ id, name: 'Alice' });
});

// POST with body
app.post('/api/users', (req, res) => {
  const { name, email } = req.body;
  if (!name || !email) {
    return res.status(400).json({ error: 'name and email required' });
  }
  const newUser = { id: Date.now(), name, email };
  res.status(201).json(newUser);
});

// Custom middleware — runs before route handlers
app.use((req, res, next) => {
  console.log(`[${new Date().toISOString()}] ${req.method} ${req.url}`);
  next(); // pass to next middleware or route
});

app.listen(3000, () => console.log('Express server on port 3000'));