CommonJS vs ESM differences in Node.js - Performance Comparison

Performance: CommonJS vs ESM differences

MEDIUM IMPACT

This affects module loading speed and runtime execution efficiency in Node.js applications.

Loading modules efficiently in a Node.js application

Node.js

import fs from 'fs';
import path from 'path';
// ESM loads modules asynchronously and supports static analysis

ESM loads modules asynchronously, allowing parallel loading and better startup performance.

📈 Performance Gainnon-blocking module loading reduces startup delay

Loading modules efficiently in a Node.js application

Node.js

const fs = require('fs');
const path = require('path');
// multiple require calls synchronously load modules

CommonJS loads modules synchronously, blocking the event loop during startup and increasing initial load time.

📉 Performance Costblocks event loop during module loading, increasing startup latency

Performance Comparison

Pattern	Module Loading	Blocking	Bundle Size	Verdict
CommonJS require()	Synchronous	Blocks event loop	Includes all code	[X] Bad
ESM import	Asynchronous	Non-blocking	Supports tree shaking	[OK] Good

Rendering Pipeline

Module loading affects the Node.js runtime initialization phase before executing application code.

→Module Resolution

→Module Loading

→Execution

⚠️ BottleneckSynchronous blocking during CommonJS module loading delays execution start.

Optimization Tips

1Use ESM for asynchronous, non-blocking module loading to improve startup speed.

2ESM supports tree shaking to reduce bundle size by excluding unused code.

3Avoid CommonJS in new projects to prevent blocking the event loop during module loading.

Performance Quiz - 3 Questions

Which module system in Node.js loads modules asynchronously, improving startup performance?

A CommonJS

B AMD

C ESM (ECMAScript Modules)

D UMD

DevTools: Performance

How to check: Record startup performance in Node.js with --trace-startup or use Chrome DevTools to profile module loading time.

What to look for: Look for blocking time during module loading and total startup duration to compare CommonJS vs ESM.

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