CommonJS vs ESM: The Great JavaScript Module Battle!

JavaScript has had an interesting journey when it comes to module systems. CommonJS (CJS) has been the traditional standard for Node.js, but with the rise of ECMAScript Modules (ESM), the landscape has started shifting.

In this post, we’ll break down CommonJS vs ESM, their differences, problems with CommonJS, and when you should choose one over the other. Let’s dive in!

🔍 What Are JavaScript Modules?

Modules in JavaScript allow you to break code into reusable files, making it more maintainable and scalable. Before modules, developers had to rely on global variables or external libraries like RequireJS.

Today, we have two main module systems:

• CommonJS (CJS) – The default in Node.js before ES modules.
• ECMAScript Modules (ESM) – The modern standard, introduced in ES6.

---

📌 CommonJS (CJS)

CommonJS was designed for server-side JavaScript (Node.js). It uses require() to import modules and module.exports to export them.

Example:

math.js (exporting a function)

// CommonJS module
function add(a, b) {
    return a + b;
}

module.exports = add;

app.js (importing the module)

// Importing a CommonJS module
const add = require('./math');
console.log(add(2, 3)); // Output: 5

✅ Pros of CommonJS:

• Synchronous loading (good for server-side apps).
• Works well in older Node.js versions.
• Simple and widely adopted in the Node.js ecosystem.

❌ Problems with CommonJS:

• Synchronous Imports: Blocking nature makes it unsuitable for browsers.
• Bundling Issues: Not tree-shakable, leading to larger bundle sizes.
• Incompatibility with ES Modules: Interoperability with modern ESM can be tricky.

---

📌 ECMAScript Modules (ESM)

ESM is the official module system of JavaScript. It uses import and export keywords.

Example:

math.js (exporting functions)

// ESM module
export function add(a, b) {
    return a + b;
}

app.js (importing the module)

// Importing an ESM module
import { add } from './math.js';
console.log(add(2, 3)); // Output: 5

✅ Pros of ESM:

• Asynchronous Loading: Great for the browser and modern environments.
• Tree Shaking Support: Removes unused code, reducing bundle sizes.
• Better Compatibility with Modern JavaScript.

❌ Problems with ESM:

• Not Supported in Older Node.js Versions: Requires "type": "module" in package.json.
• Cannot Use require(): Legacy CommonJS modules require special handling.

---

⚖️ Key Differences Between CommonJS and ESM

Feature	CommonJS (CJS)	ECMAScript Modules (ESM)
Syntax	require() / module.exports	import / export
Execution	Synchronous	Asynchronous
Browser Support	❌ No (requires bundlers)	✅ Yes
Tree Shaking	❌ No	✅ Yes
Default in Node.js	✅ Yes (by default)	🚧 Requires "type": "module"

---

🛠️ Mixing CommonJS and ESM

Sometimes, you might need to use both CJS and ESM in a project. Here’s how:

📌 Importing CommonJS in ESM

Use import() for dynamic import:

const fs = await import('fs');

📌 Importing ESM in CommonJS

Use import() inside CommonJS:

async function loadModule() {
    const { add } = await import('./math.js');
    console.log(add(2, 3));
}
loadModule();

---

🔮 What Should You Use in 2025?

• Use ESM for new projects: It’s the standard and works in both Node.js and browsers.
• Stick with CommonJS for legacy projects: If you have existing codebases relying on CJS, no need to migrate immediately.
• Use a hybrid approach: If transitioning, use both and gradually move towards ESM.

---

🎯 Conclusion

CommonJS served JavaScript well for years, but ESM is the future. If you’re working on modern projects, adopting ECMAScript Modules is the way to go.

👉 Which module system do you prefer? Have you faced any challenges migrating to ESM? Drop your thoughts in the comments! 🚀