CommonJS vs ESM differences in Node.js - Practice Questions

Challenge - 5 Problems

🧠 Conceptual

intermediate

How does CommonJS handle module exports compared to ESM?

Which statement correctly describes how CommonJS and ESM export modules in Node.js?

A CommonJS uses module.exports to export a single object, while ESM uses export statements to export multiple bindings.

B CommonJS uses export statements like ESM but requires a special flag to work.

C ESM uses module.exports to export a single object, while CommonJS uses export statements.

D Both CommonJS and ESM use the same syntax: export default and export named.

💡 Hint

Think about how you write exports in a CommonJS file versus an ESM file.

CommonJS modules export a single object using module.exports. ESM allows multiple named exports and a default export using export syntax.

❓ component_behavior

intermediate

What happens when importing a CommonJS module in ESM?

Given an ESM file importing a CommonJS module, what is the shape of the imported object?

Node.js

import pkg from './commonjsModule.js';
console.log(typeof pkg);

A "string" because CommonJS exports are converted to strings in ESM.

B "function" because CommonJS exports are always functions in ESM.

C "undefined" because ESM cannot import CommonJS modules.

D "object" because the entire CommonJS export is wrapped as the default export in ESM.

💡 Hint

Think about how ESM treats the CommonJS module.exports object when imported.

When importing a CommonJS module in ESM, the entire module.exports object is available as the default export, so its type is "object".

📝 Syntax

advanced

Identify the syntax error in mixing CommonJS and ESM imports

Which option contains a syntax error when mixing CommonJS require and ESM import in the same file?

Node.js

import fs from 'fs';
const path = require('path');

A Both import and require can be used without errors in the same file.

B Using import fs from 'fs' in a CommonJS file causes a syntax error.

C Using require('path') in an ESM file causes a syntax error.

D Using import fs from 'fs' requires a .cjs file extension.

💡 Hint

Consider the file type and module system when mixing import and require.

In an ESM file, using require() causes a syntax error because require is not defined. You must use import statements only.

🔧 Debug

advanced

Why does this ESM import fail when importing a CommonJS module?

Given this ESM code importing a CommonJS module, why does accessing a named export fail? commonjsModule.js: module.exports = { greet: () => 'hi' }; esmFile.mjs: import { greet } from './commonjsModule.js'; console.log(greet());

A Because the CommonJS module does not export greet as a default export.

B Because CommonJS exports are treated as a default export in ESM, named imports like { greet } are undefined.

C Because ESM cannot import CommonJS modules at all.

D Because the import path is missing the file extension.

💡 Hint

Think about how named imports work with CommonJS default exports.

CommonJS modules export a single object as default in ESM. Named imports expect named exports, so { greet } is undefined.

❓ state_output

expert

What is the output of this mixed CommonJS and ESM code?

Consider these two files: commonjsModule.cjs: let count = 0; module.exports.increment = () => ++count; module.exports.getCount = () => count; esmFile.mjs: import { increment, getCount } from './commonjsModule.cjs'; console.log(increment()); console.log(getCount()); What will be the output when running esmFile.mjs with Node.js configured for ESM?

A Throws a TypeError because named imports from CommonJS are undefined.

B 0 and 0 printed on separate lines.

C 1 and 1 printed on separate lines.

D Throws a SyntaxError due to mixing module types.

💡 Hint

Recall how named imports work with CommonJS default exports in ESM.

Named imports from a CommonJS module fail because the entire export is treated as a default export object. So increment and getCount are undefined, causing a TypeError.