CommonJS vs ESM differences in Node.js - Quick Revision & Key Differences

Recall & Review

beginner

What is CommonJS in Node.js?

CommonJS is a module system used in Node.js that uses require() to load modules and module.exports to export them. It loads modules synchronously.

beginner

What does ESM stand for and what is its main feature?

ESM stands for ECMAScript Modules. It is the official JavaScript module system that uses import and export syntax and supports asynchronous loading.

intermediate

How do you export a function in CommonJS vs ESM?

In CommonJS: module.exports = functionName;
In ESM: export function functionName() {} or export default functionName;

intermediate

Can CommonJS and ESM modules be mixed in the same Node.js project?

Yes, but with some restrictions. ESM can import CommonJS modules, but CommonJS cannot directly import ESM without dynamic import or special handling.

beginner

What file extensions and package settings indicate ESM usage in Node.js?

Files with .mjs extension or .js files with "type": "module" in package.json are treated as ESM modules.

Which syntax is used to import modules in ESM?

A export default moduleName;

B import moduleName from 'module';

C module.exports = moduleName;

D const moduleName = require('module');

ESM uses the import statement like 'import moduleName from "module";' to load modules.

Which module system loads modules synchronously in Node.js?

A CommonJS

B Both ESM and CommonJS

C ESM

D Neither

CommonJS loads modules synchronously using require(), while ESM supports asynchronous loading.

How do you mark a Node.js project to use ESM for .js files?

A Add "type": "module" in package.json

B Rename files to .cjs

C Use require() instead of import

D Add "type": "commonjs" in package.json

Setting "type": "module" in package.json tells Node.js to treat .js files as ESM modules.

Which of these is NOT true about CommonJS and ESM?

A CommonJS uses module.exports to export

B ESM uses export keyword

C CommonJS supports top-level await

D ESM supports asynchronous loading

CommonJS does NOT support top-level await; this is a feature of ESM.

Can CommonJS modules import ESM modules directly using require()?

A Only in older Node.js versions

B Yes, always

C Only if using .mjs extension

D No, not directly

CommonJS cannot directly import ESM modules with require(); dynamic import() or other methods are needed.

Explain the main differences between CommonJS and ESM module systems in Node.js.

Think about how modules are loaded and how you write import/export code.

Describe how you can use both CommonJS and ESM modules together in a Node.js project.

Consider interoperability and loading methods.

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