CommonJS vs ESM differences in Node.js

Introduction

CommonJS and ESM are two ways to organize and share code in Node.js. They help your code talk to other files or libraries.

When you want to include code from another file in your project.

When you use third-party libraries that use either CommonJS or ESM format.

When you want to write modern JavaScript that works with import/export.

When you need compatibility with older Node.js versions or tools.

When you want to optimize loading and performance with static imports.

Syntax

Node.js

CommonJS:
const module = require('module')
module.exports = value

ESM:
import module from 'module'
export default value

CommonJS uses require() and module.exports.

ESM uses import and export keywords.

Examples

CommonJS example: load file system module and export data.

Node.js

const fs = require('fs')
const data = fs.readFileSync('file.txt', 'utf8')
module.exports = data

ESM example: import file system module and export data as default.

Node.js

import fs from 'fs'
const data = fs.readFileSync('file.txt', 'utf8')
export default data

CommonJS exports an object with a function.

Node.js

const add = (a, b) => a + b
module.exports = { add }

ESM exports a named function directly.

Node.js

export function add(a, b) {
  return a + b
}

Sample Program

This shows a CommonJS module exporting a function, and an ESM module importing and using it.

Note: Node.js requires special setup to mix CommonJS and ESM files.

Node.js

// CommonJS module: math.js
const multiply = (x, y) => x * y
module.exports = { multiply }

// ESM module: main.mjs
import { multiply } from './math.js'
console.log(multiply(3, 4))

Important Notes

CommonJS loads modules synchronously, ESM loads them asynchronously.

ESM supports static analysis and tree shaking for smaller bundles.

Node.js treats files with .mjs as ESM and .cjs as CommonJS by default.

Summary

CommonJS uses require and module.exports, ESM uses import and export.

ESM is the modern standard and supports better optimization.

Choose based on your project needs and Node.js version compatibility.

Practice

1. Which syntax is used to import modules in CommonJS in Node.js?

easy

A. const module = require('module')

B. import module from 'module'

C. module.import('module')

D. load('module')

Solution

1. Step 1: Understand CommonJS import syntax

   CommonJS uses the require function to import modules.

2. Step 2: Compare with other options

   Options A, B, and C are not valid CommonJS import syntax; B is ESM syntax.

3. Final Answer:

   const module = require('module') -> Option A

4. Quick Check:

   CommonJS imports use require() [OK]

Hint: CommonJS uses require(), ESM uses import [OK]

Common Mistakes:

• Confusing ESM import syntax with CommonJS
• Using import in CommonJS files
• Trying to use module.import() which doesn't exist

2. Which of the following is the correct way to export a function in ESM syntax?

easy

A. export = function() {}

B. module.exports = function() {}

C. exports.function = function() {}

D. export default function() {}

Solution

1. Step 1: Identify ESM export syntax

   ESM uses export default to export a default function or value.

2. Step 2: Eliminate CommonJS and invalid options

   module.exports = function() {} and C are CommonJS exports; D is invalid syntax.

3. Final Answer:

   export default function() {} -> Option D

4. Quick Check:

   ESM default export uses export default [OK]

Hint: ESM exports use export default or named export [OK]

Common Mistakes:

• Using module.exports in ESM files
• Trying to use export = which is invalid in ESM
• Confusing exports.function with ESM named exports

3. Given this CommonJS code, what will be the output?

const message = require('./message');
console.log(message);

And the file message.js contains:

module.exports = 'Hello from CommonJS';

medium

A. 'Hello from CommonJS'

B. undefined

C. SyntaxError

D. ReferenceError

Solution

1. Step 1: Understand module.exports in CommonJS

   The file exports a string 'Hello from CommonJS' using module.exports.

2. Step 2: Import and log the exported value

   The require call imports the string, so console.log prints it.

3. Final Answer:

   'Hello from CommonJS' -> Option A

4. Quick Check:

   CommonJS require returns module.exports value [OK]

Hint: require() returns module.exports value [OK]

Common Mistakes:

• Expecting require to return an object by default
• Confusing with ESM import behavior
• Thinking require throws error without .js extension

4. What is wrong with this ESM import statement in a Node.js file?

const fs = require('fs');

medium

A. Missing semicolon at the end

B. require is not defined in ESM modules

C. fs module cannot be imported in Node.js

D. Should use import fs from 'fs' instead

Solution

1. Step 1: Identify module system used

   The code uses require which is CommonJS syntax, but in ESM modules, require is not available.

2. Step 2: Understand ESM import rules

   ESM modules must use import statements; require is undefined.

3. Final Answer:

   require is not defined in ESM modules -> Option B

4. Quick Check:

   require() undefined in ESM [OK]

Hint: require() is undefined in ESM modules [OK]

Common Mistakes:

• Thinking require works in ESM without extra setup
• Confusing missing semicolon as error
• Believing fs cannot be imported in Node.js

5. You want to write a Node.js module that can be imported both by CommonJS and ESM users without errors. Which approach is best?

hard

A. Use module.exports and add a separate export default for ESM

B. Write only CommonJS syntax and use dynamic import() in ESM

C. Use a dual package approach with conditional exports in package.json

D. Write only ESM syntax and rename file to .mjs

Solution

1. Step 1: Understand compatibility challenges

   CommonJS and ESM have different import/export systems, so a single file often can't serve both seamlessly.

2. Step 2: Use Node.js dual package support

   Node.js supports conditional exports in package.json to provide separate entry points for CommonJS and ESM consumers.

3. Step 3: Evaluate other options

   Use module.exports and add a separate export default for ESM mixes syntaxes which doesn't work reliably, C requires dynamic import in consumers, D limits to ESM-only.

4. Final Answer:

   Use a dual package approach with conditional exports in package.json -> Option C

5. Quick Check:

   Dual package conditional exports solve CommonJS/ESM compatibility [OK]

Hint: Use package.json conditional exports for dual support [OK]

Common Mistakes:

• Trying to mix module.exports and export default in one file
• Using only .mjs files limits CommonJS users
• Relying on dynamic import() without package config