DEV Community

sudip khatiwada
sudip khatiwada

Posted on

# CommonJS vs. ES6 Modules: Understanding CJS and MJS in Node.js

#webdev #javascript #node #es6

Meta Description: Learn the key differences between CommonJS (CJS) and ES6 Modules (MJS) in Node.js. Discover when to use each module system with practical examples and setup guides.

Choosing the right module system in Node.js can make or break your development experience. Understanding the difference between CommonJS (CJS) and ES6 Modules (MJS) is crucial for modern JavaScript backend development.

What is CommonJS (CJS)?

CommonJS is Node.js's original module system, introduced when the platform launched in 2009. Think of CJS like checking out a book from a traditional library – you use require() to "borrow" functionality from other files, and module.exports to "lend out" your code to others.

CJS uses synchronous loading, meaning modules are loaded and executed immediately when required. This approach works perfectly for server-side applications where file access is fast and predictable.

Basic CJS Example:

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

// app.js
const { add } = require('./math');
console.log(add(2, 3)); // Output: 5
Enter fullscreen mode Exit fullscreen mode

What are ES6 Modules (MJS)?

ES6 Modules represent the modern JavaScript module standard, introduced in ES2015. If CJS is like a traditional library, then ES6 modules are like instant e-book downloads – they use import and export statements with a more elegant, standardized syntax.

MJS supports both synchronous and asynchronous loading, making it ideal for modern applications that need flexibility. The syntax is cleaner and more intuitive, especially for developers coming from frontend frameworks.

Basic MJS Example:

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

// app.mjs
import { add } from './math.mjs';
console.log(add(2, 3)); // Output: 5
Enter fullscreen mode Exit fullscreen mode

Key Differences Between CJS and MJS Module Systems

Understanding the practical differences helps you choose the right Node.js module system for your project:

Syntax and Loading:

  • CJS: Uses require() and module.exports with synchronous loading
  • MJS: Uses import/export with support for both sync and async loading

File Extensions:

  • CJS: Uses .js files by default
  • MJS: Requires .mjs extension or "type": "module" in package.json

Browser Compatibility:

  • CJS: Node.js only, requires bundlers for browser use
  • MJS: Native browser support and Node.js compatibility

Dynamic Imports:

  • CJS: Limited dynamic loading capabilities
  • MJS: Full support for dynamic imports with import() function

Setting Up CJS vs MJS in Node.js

Running CommonJS:

node app.js
Enter fullscreen mode Exit fullscreen mode

[Insert screenshot of terminal running CJS example]

Setting up ES6 Modules - Option 1 (File Extension):

node app.mjs
Enter fullscreen mode Exit fullscreen mode

Setting up ES6 Modules - Option 2 (Package.json):

{
  "type": "module",
  "scripts": {
    "start": "node app.js"
  }
}
Enter fullscreen mode Exit fullscreen mode 
npm start
Enter fullscreen mode Exit fullscreen mode

[Insert screenshot of terminal running MJS example]

When to Use Each Module System

Choose CommonJS when:

  • Working with legacy Node.js projects (pre-ES6)
  • Using older npm packages that don't support ES6 modules
  • Building simple server-side applications without modern tooling

Choose ES6 Modules when:

  • Starting new Node.js projects (recommended for Node.js 14+)
  • Building isomorphic applications (code that runs on both server and browser)
  • Working with modern JavaScript frameworks and tools
  • Needing dynamic import capabilities for code splitting

The JavaScript modules tutorial landscape is shifting toward ES6 modules as the standard, but CommonJS remains widely used in existing codebases.

Making the Right Choice for Your Project

Both CommonJS vs ES6 modules have their place in modern Node.js development. ES6 modules offer better syntax, browser compatibility, and future-proofing, making them ideal for new projects. However, CommonJS remains reliable for existing applications and provides excellent compatibility with the vast npm ecosystem.

Consider your project's requirements, team expertise, and long-term maintenance needs when choosing between CJS vs MJS. Many developers are gradually migrating from CommonJS to ES6 modules as the ecosystem matures.

Ready to level up your Node.js skills? Try converting a simple CJS project to MJS today and experience the cleaner syntax firsthand. Start with a small utility module and gradually adopt ES6 modules in your larger applications!

Top comments (0)