Master JavaScript Modules vs. CommonJS: The Ultimate 2024 Guide

JavaScript modules and CommonJS are two key concepts for organizing code in JavaScript applications. Understanding their differences and when to use each is essential for both beginners and experienced developers. This blog will explain these concepts in detail, cover their differences, and provide interview questions with detailed answers to help you prepare.

What is a JavaScript Module?

A JavaScript module is a reusable piece of code that can be imported and exported across different parts of an application. Modules help keep code organized, maintainable, and avoid namespace collisions by encapsulating functionality.

Types of JavaScript Modules

ES Modules (ECMAScript Modules – ESM):

Introduced in ES6 (ECMAScript 2015), the ESM syntax is the modern way of writing modules in JavaScript.
Uses import and export keywords to manage dependencies. Example:

   // math.js - Module File
   export const add = (a, b) => a + b;
   export const multiply = (a, b) => a * b;

   // main.js - Importing Module
   import { add, multiply } from './math.js';
   console.log(add(2, 3)); // Outputs: 5
   console.log(multiply(2, 3)); // Outputs: 6

CommonJS Modules (CJS):

A module system used primarily in Node.js applications.
Uses require to import and module.exports or exports to export modules. Example:

   // math.js - Module File
   const add = (a, b) => a + b;
   const multiply = (a, b) => a * b;
   module.exports = { add, multiply };

   // main.js - Importing Module
   const { add, multiply } = require('./math.js');
   console.log(add(2, 3)); // Outputs: 5
   console.log(multiply(2, 3)); // Outputs: 6

Differences Between JavaScript Modules and CommonJS

Feature	JavaScript Modules (ESM)	CommonJS (CJS)
Syntax	`import` / `export`	`require` / `module.exports`
Environment	Browser and Node.js (with support)	Node.js primarily
Loading	Asynchronous (using native browser loaders)	Synchronous
File Extension	`.mjs` (Node.js) or `.js`	`.js`
Scope	Strict mode by default	Non-strict mode
Tree Shaking	Supported (allows dead code elimination)	Not supported
Exports	Supports named exports and default exports	Supports only single `module.exports`

When to Use JavaScript Modules vs. CommonJS

Use JavaScript Modules (ESM) when:

Building modern web applications.
Working with tools like Webpack, Parcel, or Rollup that support ES Modules.
Writing code that needs to be tree-shakable for better optimization.

Use CommonJS (CJS) when:

Developing server-side Node.js applications.
Working on projects that use older tools not compatible with ES Modules.
Need synchronous loading (typically in smaller Node.js scripts).

Advanced Interview Questions on JavaScript Modules and CommonJS

Q1: Explain the concept of “Tree Shaking” in ES Modules.

Answer:
Tree shaking is a process that eliminates unused or dead code from a JavaScript bundle during the build process. It’s commonly used in ES Modules because they support static analysis. Tree shaking helps reduce the file size, resulting in faster loading times for web applications.

Example:

// math.js
export const add = (a, b) => a + b;
export const multiply = (a, b) => a * b;

// main.js
import { add } from './math.js'; // Only 'add' is imported
console.log(add(2, 3));

In this example, if tree shaking is applied, the multiply function will be removed from the final bundle since it’s not used in main.js.

Q2: How does the loading mechanism differ between CommonJS and ES Modules?

Answer:

CommonJS (CJS): Loads modules synchronously, meaning the entire module is loaded before execution continues. This is suitable for server-side applications but can cause blocking in larger modules.
ES Modules (ESM): Loads modules asynchronously, allowing parallel loading of modules. This is beneficial for web applications where non-blocking execution improves performance.

Example:

In CJS: const module = require('./module'); loads synchronously.
In ESM: import module from './module.js'; can load asynchronously.

Q3: Can you mix CommonJS and ES Modules in a single project? How?

Answer:
Yes, you can mix CommonJS and ES Modules in a single project, but there are limitations:

In Node.js, you can use require() to import CommonJS modules within an ESM file.
For ESM syntax in a CommonJS file, you’d need tools like Babel or Webpack to transpile the code.
Node.js version 12+ supports dual use with .mjs (for ES Modules) and .cjs (for CommonJS).

Example:

// ESM file (main.mjs)
import { myFunction } from './commonjsModule.cjs';
myFunction();

Q4: What is the difference between “default export” and “named export” in ES Modules?

Answer:

Default Export: Allows exporting a single value or object as the module’s default. Importing is done without curly braces.
Named Export: Allows exporting multiple values from a module, requiring curly braces for importing.

Example:

// default export
export default function() { console.log('Default Export'); }

// named export
export const namedFunc = () => console.log('Named Export');

Q5: Why does CommonJS execute in a non-strict mode while ES Modules use strict mode by default?

Answer:
CommonJS was designed before ECMAScript 5’s introduction of strict mode, so it runs in non-strict mode by default. On the other hand, ES Modules were introduced in ES6, which follows strict mode to enforce better coding practices and prevent common JavaScript pitfalls.

Q6: How does scope handling differ between CommonJS and ES Modules?

Answer:

CommonJS: Each file/module has its own scope but shares a single global scope for all modules within an application.
ES Modules: Always operate in strict mode and each module maintains its own scope, preventing global pollution.

Interview Questions Recap

What is the difference between ES Modules and CommonJS?
How does tree shaking work in JavaScript modules?
Can you use import and require together in a project?
What is the purpose of default and named exports?
Why does ES Module use strict mode by default, and how does it benefit?

By mastering these concepts, you’ll be well-prepared for JavaScript module-related interview questions.

Additional Essential JavaScript Interview Questions on Various Topics

React Js Interview questions:

Conclusion

Understanding the differences between JavaScript modules and CommonJS is crucial for modern web and server-side development. While ES Modules offer advanced features like tree shaking and asynchronous loading, CommonJS remains popular in Node.js environments due to its simplicity and legacy support. Knowing when to use each will make you a more versatile and effective developer.

Contents hide

What is a JavaScript Module?

Types of JavaScript Modules

Differences Between JavaScript Modules and CommonJS

When to Use JavaScript Modules vs. CommonJS

Advanced Interview Questions on JavaScript Modules and CommonJS

Q1: Explain the concept of “Tree Shaking” in ES Modules.

Q2: How does the loading mechanism differ between CommonJS and ES Modules?

Q3: Can you mix CommonJS and ES Modules in a single project? How?

Q4: What is the difference between “default export” and “named export” in ES Modules?

Q5: Why does CommonJS execute in a non-strict mode while ES Modules use strict mode by default?

Q6: How does scope handling differ between CommonJS and ES Modules?