Node.js Modules

// CommonJS (default in Node.js)
// math.js
const PI = 3.14159;
function add(a, b) { return a + b; }
function multiply(a, b) { return a * b; }
module.exports = { add, multiply, PI };

// main.js - import with require
const { add, multiply, PI } = require('./math');
console.log(add(2, 3));       // 5
console.log(multiply(4, PI)); // 12.566

// ES Modules (use "type": "module" in package.json)
// utils.mjs
export const VERSION = '1.0.0';
export function formatDate(date) {
    return date.toISOString().split('T')[0];
}
export default function greet(name) {
    return `Hello, ${name}!`;
}

// app.mjs
import greet, { VERSION, formatDate } from './utils.mjs';

// Built-in modules (no install needed)
const path = require('path');
const fs   = require('fs');
const os   = require('os');

console.log(path.join(__dirname, 'data', 'file.json'));
console.log(os.platform()); // 'win32' or 'linux'