Difference between CommonJS and ESM modules

Intro

CommonJS (CJS) and ECMAScript Modules (ESM) are two different module systems in JavaScript used for organizing and structuring code. They have different syntax and behaviors, and they are often used in different contexts. Let's explore each format:

CommonJS (CJS):

1. Synchronous: CommonJS modules are loaded synchronously. When you require a module, it is loaded and executed immediately.
2. Server-Side: CommonJS modules were originally designed for server-side JavaScript (e.g., Node.js). They are commonly used in server-side applications and back-end development.
3. require and module.exports: CommonJS modules use require to import modules and module.exports to export values from a module. For example:

// Importing a module
   const someModule = require('./someModule');

   // Exporting a value
   module.exports = someValue;

1. No Support for Tree Shaking: CommonJS modules don't support tree shaking, which means all exports are bundled, even if they are not used in the consuming code. This can result in larger bundle sizes in front-end applications.
2. Dynamic Imports: CommonJS does not support dynamic imports. You cannot conditionally import modules based on runtime conditions.

ECMAScript Modules (ESM):

1. Asynchronous: ESM is asynchronous. Modules are loaded and executed asynchronously, which can lead to better performance in certain scenarios.
2. Front-End and Back-End: ESM modules are supported in both front-end (browsers) and back-end (Node.js) environments. They are becoming more widely used for front-end development.
3. import and export Statements: ESM modules use import to import modules and export to export values from a module. For example:

// Import a module

import { someValue } from './myModule';

// Export a value from a module

export default someValue;

Key Differences:

• Synchronous vs. Asynchronous Loading: CommonJS modules are loaded synchronously, which means they block the execution of code until the module is loaded. ESM modules, on the other hand, are loaded asynchronously, allowing for better parallelism.
• Static vs. Dynamic Imports: ESM supports static imports, meaning that the import statements are resolved at compile time. CommonJS supports dynamic imports, where module paths can be constructed dynamically at runtime.
• Browser Compatibility: ESM is more suitable for modern web browsers, while CommonJS is often used in server-side environments. However, with the growth of JavaScript bundlers and transpilers, ESM can also be used in web applications with broader compatibility.
• Syntax: ESM uses import and export statements, which are more concise and align with the language's evolution. CommonJS uses require() and module.exports, which have a different syntax.

Summary

CommonJS and ESM are two module systems in JavaScript, with CommonJS historically used in server-side environments like Node.js, and ESM emerging as the standard for modern web and server-side applications. Your choice of module system depends on your target environment and project requirements.