CommonJS vs es modules in Node.js

Module Systems in Node.js

Node.js supports two module systems: the original CommonJS (CJS) and the modern ES Modules (ESM). Understanding the differences is essential for writing compatible Node.js code.

CommonJS (CJS)

The default module system in Node.js since its inception.

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

// main.js — importing
const { add } = require('./math');
console.log(add(2, 3)); // 5

Key characteristics:

Uses require() / module.exports
Synchronous loading (blocks until module is loaded)
Loaded at runtime
Default in .js files unless "type": "module" in package.json

ES Modules (ESM)

The official ECMAScript standard, now fully supported in Node.js 12+.

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

// main.mjs — importing
import { add } from './math.mjs';
console.log(add(2, 3)); // 5

Key characteristics:

Uses import / export syntax
Asynchronous loading (better for tree-shaking)
Statically analyzed at parse time
Use .mjs extension OR set "type": "module" in package.json

Comparison Table

Feature	CommonJS	ES Modules
Syntax	`require()` / `module.exports`	`import` / `export`
Loading	Synchronous	Asynchronous
Analysis	Runtime	Static (parse time)
Tree-shaking	❌	✅
Top-level `await`	❌	✅
`__dirname` / `__filename`	✅	❌ (use `import.meta.url`)
Default in Node.js	✅	With `"type":"module"`

Interoperability

// ESM can import CJS:
import { add } from './math.cjs'; // ✅

// CJS cannot require() ESM:
const { add } = require('./math.mjs'); // ❌ Error!

// CJS can use dynamic import() for ESM:
const { add } = await import('./math.mjs'); // ✅

Enabling ESM

Option 1: Use .mjs extension Option 2: Add to package.json:

json

{
  "type": "module"
}

Getting `__dirname` in ESM

import { fileURLToPath } from 'url';
import { dirname } from 'path';

const __filename = fileURLToPath(import.meta.url);
const __dirname = dirname(__filename);

Summary

Use ESM for new projects (it's the standard, supports tree-shaking and top-level await). Use CommonJS when working with legacy code or packages that don't support ESM yet.

Short Answer

Interview ready

Premium

A concise answer to help you respond confidently on this topic during an interview.

Finished reading?

Practice Problems