Modules & NPM

Understanding Modules

In Node.js, each file is treated as a separate module. Modules help you organize your code into reusable pieces. Node.js supports two module systems: CommonJS (the traditional way) and ES Modules (the modern standard).

CommonJS Modules

The original module system in Node.js. Uses require() and module.exports.

// math.js
function add(a, b) {
  return a + b;
}

function subtract(a, b) {
  return a - b;
}

function multiply(a, b) {
  return a * b;
}

// Export multiple functions
module.exports = {
  add,
  subtract,
  multiply
};

// Or export individually
// exports.add = add;
// exports.subtract = subtract;

// app.js
const math = require('./math');

console.log(math.add(5, 3));       // 8
console.log(math.subtract(10, 4)); // 6
console.log(math.multiply(3, 7));  // 21

// Destructuring import
const { add, multiply } = require('./math');
console.log(add(2, 2));            // 4

ES Modules (ESM)

The modern JavaScript module system. To use ES modules in Node.js, either use .mjs extension or set "type": "module" in package.json.

// utils.mjs
export function greet(name) {
  return `Hello, ${name}!`;
}

export const PI = 3.14159;

// Default export
export default class Calculator {
  add(a, b) {
    return a + b;
  }
}

// app.mjs
import Calculator, { greet, PI } from './utils.mjs';

console.log(greet('World'));  // Hello, World!
console.log(PI);              // 3.14159

const calc = new Calculator();
console.log(calc.add(5, 3));  // 8

// Dynamic import
const module = await import('./utils.mjs');

Built-in Modules

Node.js comes with many built-in modules that don't need installation:

// File System
const fs = require('fs');

// Path utilities
const path = require('path');

// HTTP server
const http = require('http');

// Operating system info
const os = require('os');

// Event handling
const EventEmitter = require('events');

// URL parsing
const url = require('url');

// Cryptography
const crypto = require('crypto');

// Example: Using the os module
console.log('Platform:', os.platform());
console.log('CPU cores:', os.cpus().length);
console.log('Free memory:', os.freemem() / 1024 / 1024, 'MB');
console.log('Home directory:', os.homedir());

Initializing a Project

# Create a new project with package.json
npm init

# Quick initialization with defaults
npm init -y

# This creates package.json with project metadata

{
  "name": "my-nodejs-app",
  "version": "1.0.0",
  "description": "My first Node.js application",
  "main": "index.js",
  "scripts": {
    "start": "node index.js",
    "dev": "node --watch index.js",
    "test": "jest"
  },
  "keywords": ["nodejs", "learning"],
  "author": "Your Name",
  "license": "MIT",
  "dependencies": {
    "express": "^4.18.2"
  },
  "devDependencies": {
    "jest": "^29.7.0"
  }
}

Installing Packages

# Install a package (adds to dependencies)
npm install express
npm i express  # shorthand

# Install as dev dependency
npm install --save-dev jest
npm i -D jest  # shorthand

# Install globally (CLI tools)
npm install -g nodemon

# Install specific version
npm install express@4.18.2

# Install all dependencies from package.json
npm install

# Uninstall a package
npm uninstall express

Using Installed Packages

// After: npm install lodash
const _ = require('lodash');

const numbers = [1, 2, 3, 4, 5];
console.log(_.sum(numbers));      // 15
console.log(_.shuffle(numbers));  // [3, 1, 5, 2, 4]

// Option 1: Native fetch (Node.js 18+)
async function fetchData() {
  const response = await fetch('https://api.example.com/data');
  const data = await response.json();
  console.log(data);
}

// Option 2: Using axios (npm install axios)
const axios = require('axios');

async function fetchDataAxios() {
  const response = await axios.get('https://api.example.com/data');
  console.log(response.data);
}

// After: npm install dayjs
const dayjs = require('dayjs');
console.log(dayjs().format('YYYY-MM-DD'));  // 2026-01-09

NPM Scripts

Define custom commands in package.json scripts section:

{
  "scripts": {
    "start": "node server.js",
    "dev": "nodemon server.js",
    "build": "tsc",
    "test": "jest",
    "lint": "eslint .",
    "format": "prettier --write ."
  }
}

# Run npm scripts
npm run start    # or just: npm start
npm run dev
npm test         # shorthand for npm run test
npm run build