Node.js basics

What Node.js adds

In the browser, JavaScript can change web pages but cannot read your files. Node.js removes the page and adds access to the computer: files, folders, network ports, environment variables and other programs. It is used for web servers and APIs, command-line tools, build tools (Vite, ESLint and TypeScript's compiler all run on Node) and scripts.

Built-in modules

import { readFile } from 'node:fs/promises';
import path from 'node:path';
import os from 'node:os';

console.log(path.join('data', 'orders', '2026.csv'));  // data/orders/2026.csv (\ on Windows)
console.log(path.extname('invoice.pdf'));               // '.pdf'
console.log(os.cpus().length, 'CPU cores');
console.log(process.platform, process.version);        // e.g. 'linux' 'v24.x.x'

process: arguments, environment and exit codes

// node greet.js Asha
const name = process.argv[2] ?? 'friend';   // argv[0] is node, argv[1] the script path
console.log(`Namaste, ${name}!`);
console.log(process.env.NODE_ENV);           // an environment variable (or undefined)
if (!name) process.exit(1);                   // a non-zero exit code means failure

npm and package.json

npm (Node Package Manager) comes with Node. It installs packages (other people's code) from the npm registry and records them in package.json, the file that describes your project. package-lock.json records the exact versions installed, so every developer and server gets the same ones; commit it to Git.

{
  "name": "shop-api",
  "version": "1.0.0",
  "type": "module",
  "scripts": {
    "start": "node src/server.js",
    "dev": "node --watch src/server.js",
    "test": "vitest run"
  },
  "dependencies": { "express": "^5.1.0" },
  "devDependencies": { "vitest": "^3.2.0" }
}

Semantic versioning

Versions look like MAJOR.MINOR.PATCH (5.1.0). PATCH fixes bugs, MINOR adds features without breaking anything, MAJOR may break things. ^5.1.0 allows any 5.x.x from 5.1.0 up (not 6.0.0); ~5.1.0 allows only 5.1.x patches.

LTS releases

Node.js releases a new major version twice a year. Even-numbered versions become LTS (Long-Term Support) and get fixes for about two and a half years in total; odd-numbered ones are short-lived. For anything real, install the version marked LTS on nodejs.org. node --watch app.js restarts your program automatically when files change (handy during development).