Modules and require

As applications grow, keeping all code in a single file becomes unmanageable. Node.js solves this with a module system that lets you split code into separate files and share functionality between them.

What is a Module?

A module is simply a file. Every file in a Node.js application is treated as its own module with its own scope. Variables and functions defined in one file are not visible to other files unless explicitly exported.

CommonJS: require and module.exports

The traditional Node.js module format is CommonJS (CJS). You export values using module.exports and import them using require().

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

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

module.exports = { add, subtract };

// app.js — using the module
const math = require("./math");

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

The path ./math is relative to the current file. Node.js automatically appends .js if the extension is omitted.

The exports Shorthand

You can also add properties directly to the exports object:

// greet.js
exports.hello = function (name) {
  return "Hello, " + name + "!";
};

exports.goodbye = function (name) {
  return "Goodbye, " + name + "!";
};

Be careful: reassigning exports directly (e.g. exports = function() {}) breaks the reference. Use module.exports for that case.

Core Modules

Node.js ships with built-in core modules that you can require by name (no path needed):

const path = require("path");
const os   = require("os");
const fs   = require("fs");

console.log(path.join("/users", "john", "file.txt")); // /users/john/file.txt
console.log(os.platform());                           // 'darwin', 'linux', etc.

Third-Party Modules

Packages installed via npm are required by their package name:

const express = require("express");
const lodash  = require("lodash");

Node.js looks for these in the node_modules folder in your project.

ES Modules (ESM)

Modern JavaScript supports ES Modules using import and export syntax. Node.js supports ESM in files with the .mjs extension or when "type": "module" is set in package.json:

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

// app.mjs
import { add } from "./math.mjs";
console.log(add(2, 3)); // 5

Most existing Node.js codebases still use CommonJS. Both systems are important to understand as you work with different projects and packages.