Node Modules

Last Updated : Oct 2026

In Node.js, you don't write all your code in one file. You split it into multiple files called modules. Modules are three types:

User-Defined Modules: Created by you.
Built-in Modules: Provided by the language itself.
Third-Party Modules: External libraries you install using npm or yarn.

User - Defined Modules

You can create your own modules by writing JavaScript code in separate .js files.

Node has two ways to import and export code between files:

CommonJS (require / module.exports) — The traditional, default way Node worked for years.
ES Modules (import / export) — The modern JavaScript standard (same syntax used in React/Vue/frontend).

1. CommonJS (The Node.js Classic)

By default, Node treats .js files as CommonJS modules.

You export values by attaching them to module.exports or exports. before the value.
You import values using the require() function.
require() loads files synchronously (at runtime).

Example

Create a helper file named mathCJS.js:

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

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

exports.subtract = (a, b) => a - b;

// Exporting an object containing our functions
module.exports = {
  add,
  multiply,
};

Now, create indexCJS.js in the same directory:

// indexCJS.js
// Import using relative path './'
const math = require("./mathCJS");

// Destructuring :
// const { add, multiply } = require("./mathCJS");

console.log("Add:", math.add(5, 3));
console.log("Multiply:", math.multiply(4, 2));

// if you  use Destructuring :
// console.log("Add:", add(5, 3));
// console.log("Multiply:", multiply(4, 2));

Run it in your terminal:

node indexCJS.js

2. ES Modules (ESM - The Modern Standard)

ES Modules use import and export statements. They are processed statically (before the code runs), making them cleaner and enabling optimizations like tree-shaking (removing unused code).

To use ESM in Node.js, you have two choices:

Option A: Name your files with .mjs extension (e.g., m.mjs).
Option B (Recommended): Add "type": "module" inside your package.json file. This tells Node to treat all .js files in the folder as ES Modules.

Example

Option A

Create a helper file named m.mjs:

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

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

Now, create index.mjs in the same directory:

// index.mjs
import { add, multiply } from "./math.mjs";
// Note: In ESM Node.js, you MUST include the file extension!

console.log("Add:", add(5, 3));
console.log("Multiply:", multiply(4, 2));

Run it in your terminal:

node index.mjs

Option B

Step 1: Initialize package.json

Run this in your terminal:

npm init -y

Open package.json and add "type": "module" at the top level:

{
  "name": "your-project-name/directory-name",
  "version": "1.0.0",
  "type": "module",
  "main": "index.js"
}

Step 2: Write ESM Code

Create mathESM.js:

// mathESM.js

// Named Exports
export function add(a, b) {
  return a + b;
}

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

// Default Export (optional main export)
export default function subtract(a, b) {
  return a - b;
}

Create indexESM.js:

// indexESM.js
import subtract, { add, multiply } from "./mathESM.js";

console.log("Add:", add(10, 5)); // 15
console.log("Multiply:", multiply(3, 3)); // 9
console.log("Subtract:", subtract(10, 4)); // 6

Run it in your terminal:

node indexESM.js

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