How to create a CJS module

How to create a CJS module

Use modules to write reusable code in Node.js

ByMario Kandut

honey pot logo

Europe’s developer-focused job platform

Let companies apply to you

Developer-focused, salary and tech stack upfront.

Just one profile, no job applications!

This article is based on Node v16.14.0.

Modules are building blocks of code structures and allow Node.js developers to build better structure, reuse, and distribute code. Since Node v14, there are two kinds of modules, CommonJS Modules (CJS) and EcmaScript Modules (ESM) . This article is about CommonJS modules (CJS).

💰 The Pragmatic Programmer: journey to mastery. 💰 One of the best books in software development, sold over 200,000 times.

When importing a CJS module, you have to require it. The result of require is in some cases a function, which creates a new instance, but this is not guaranteed. The require function will always return what is exported from the module.

Let's create a local module, which will convert any input string to uppercase and use it in five steps.

  1. Create a file for the module.
touch format.js
  1. Create a simple function which converts any input string to uppercase.
'use strict'
const toUpper = str => {
  if (typeof str === 'symbol') str = str.toString(); // convert to string if symbol is used as input
  str += '';
  return str.toUpperCase();

module.exports = { toUpper };
  1. Create an index.js to import the CSJ module.
touch index.js
  1. Import module with destructuring.
const { toUpper } = require('./format');
console.log(toUpper('this will be uppercase'));
  1. Execute it.
node index.js

The output to the terminal should be THIS WILL BE UPPERCASE.


  • The require function will only return what is exported from the module.
  • CommonJS modules are imported with the require function.
  • Modularization helps to abstract complexity.

Thanks for reading and if you have any questions, use the comment function or send me a message @mariokandut.

If you want to know more about Node, have a look at these Node Tutorials.

References (and Big thanks):


More node articles:

Getting started with Webpack

How to list/debug npm packages?

How to specify a Node.js version

How to create a web server in Node.js

How to dynamically load ESM in CJS

How to convert a CJS module to an ESM

How to create a CJS module

How to stream to an HTTP response

How to handle binary data in Node.js?

How to use streams to ETL data?

How to connect streams with pipeline?

How to handle stream errors?

How to connect streams with pipe?

What Is a Node.js Stream?

Handling Errors in Node (asynchronous)

Handling Errors in Node.js (synchronous)

Introduction to errors in Node.js

Callback to promise-based functions

ETL: Load Data to Destination with Node.js

ETL: Transform Data with Node.js

ETL: Extract Data with Node.js

Event Emitters in Node.js

How to set up SSL locally with Node.js?

How to use async/await in Node.js

What is an API proxy?

How to make an API request in Node.js?

How does the Event Loop work in Node.js

How to wait for multiple Promises?

How to organize Node.js code

Understanding Promises in Node.js

How does the Node.js module system work?

Set up and test a .env file in Node

How to Use Environment Variables in Node

How to clean up node modules?

Restart a Node.js app automatically

How to update a Node dependency - NPM?

What are NPM scripts?

How to uninstall npm packages?

How to install npm packages?

How to create a package.json file?

What Is the Node.js ETL Pipeline?

What is data brokering in Node.js?

How to read and write JSON Files with Node.js?

What is package-lock.json?

How to install Node.js locally with nvm?

How to update Node.js?

How to check unused npm packages?

What is the Node.js fs module?

What is Semantic versioning?

The Basics of Package.json explained

How to patch an NPM dependency

What is NPM audit?

Beginner`s guide to NPM

Getting started with Node.js

Scroll to top ↑