The Module System

// math.js — export functions
function add(a, b) { return a + b; }
function multiply(a, b) { return a * b; }

module.exports = { add, multiply };  // export as an object
// or: module.exports.add = add;
// or: exports.add = add;  (shorthand)

// app.js — import them
const { add, multiply } = require("./math");
console.log(add(2, 3));       // 5
console.log(multiply(4, 5));  // 20

// Import a whole module (like built-in 'path')
const path = require("path");
const fs   = require("fs");

// math.js
export function add(a, b) { return a + b; }
export const PI = 3.14159;
export default function main() { ... }

// app.js
import { add, PI }   from "./math.js";
import main          from "./math.js";    // default import
import * as mathLib  from "./math.js";    // import everything

// To use ES modules in Node.js:
// 1. Add "type": "module" to package.json
// 2. OR rename files to .mjs

const fs    = require("fs");     // file system
const path  = require("path");   // file paths
const os    = require("os");     // operating system info
const http  = require("http");   // HTTP server
const https = require("https");  // HTTPS
const url   = require("url");    // URL parsing
const crypto = require("crypto"); // cryptography
const events = require("events"); // EventEmitter
const stream = require("stream"); // streams

// These are built in — no npm install needed