Understanding ES Module and CommonJS: Key Differences and Compatibility

Module systems are essential in software development, as they allow code to be divided into reusable and maintainable components. In the world of JavaScript, two of the most popular module systems are ES Module (ESM) and CommonJS (CJS). Although both have the same goal of modularization, there are important differences between them and the way they interact can be a cause of confusion for developers. Let's break down these differences and compatibilities by offering practical examples and meaningful considerations.

What is CommonJS?

CommonJS emerged as a convention for modularizing code in non-browser environments such as the server, particularly used in Node.js. It became known for its syntax require to import modules and module.exports to export them. For example:

// Exporting a module with CommonJS module.exports = { add: function(a, b) { return a + b; } }; // Importing the module into another file const { add } = require('./math'); console.log(sum(2, 3)); // Output: 5

CommonJS loads modules synchronously and is well suited for server-side applications where modules are available locally.

What is ES Module?

ES Module is the standard module system in ECMAScript 6 (ES6) and later, implemented in both modern browsers and the Node.js platform (as of version 13.2 as stable). ESM uses the syntax import y export. Let's see an example:

// Exporting a module with ES Module export function add(a, b) { return a + b; } // Importing the module into another file import { add } from './math.js'; console.log(sum(2, 3)); // Output: 5

ES Module is designed for asynchronous loading of modules, which is a great advantage for in-browser applications, allowing for faster and more efficient startup of code.

Key Differences between ES Module and CommonJS

Syntax

The first big difference is the syntax. While CommonJS uses require y module.exports, ESM uses import y export. This is a clear distinction that can affect how code is structured and read.

Module Loading

ES Module loads modules asynchronously, allowing script execution to not block while waiting for imports, while CommonJS performs synchronous loading which can slow down code execution until all modules have been loaded.

Static vs. Dynamic

ES Module is static, meaning that imports and exports must be at the top level of your JavaScript file and cannot be changed at runtime. In contrast, CommonJS is dynamic and allows modules to be imported anywhere in the code.

Interoperability between Modules

While ESM is a more modern specification and has features that favor it for web use, the reality is that many existing libraries and code are still written in CommonJS. This has generated the need for interoperability mechanisms between both module systems.

Compatibility and the Transition from CommonJS to ES Module

Interoperability in Node.js

Node.js introduced support for ESM since version 8.5.0 with the possibility of working with ESM-type modules, but it was in the most recent versions where better interoperability was established. An important point to note is that Node.js distinguishes ESM module files with the extension .mjs or with configuration "type": "module" in it package.json for .js.

We can import CommonJS modules into ESM, but we cannot use import in CommonJS files. Likewise, the export of modules between both systems follows certain rules. For example, to use the functions of a CommonJS module in ESM:

// Importing a CommonJS module into an ES file Module import math from './math.cjs'; console.log(math.sum(4, 5)); // Output: 9

and to use an ESM module in CommonJS, use is made import():

// Importing an ES Module into a CommonJS file const math = import('./math.mjs'); math.then(module => { console.log(module.sum(4, 5)); // Output: 9 });

Construction and Transpilation Tools

For projects that need to support both modules, tools such as Babel or Webpack can be used that transpile the ES Module code to a CommonJS-compatible format for environments that do not natively support ESM. Additionally, these tools often provide additional features such as minification and support for newer JavaScript features.

Final Considerations for Developers

When working with modules in JavaScript, developers should be aware of the differences between ESM and CommonJS, as well as the particularities of the environment in which they are working. It is essential to have a clear understanding of how both specifications handle the import and export of modules, and how additional tools can be used to ensure compatibility between them.

For more technical details and assistance on your projects involving ES Module and CommonJS, feel free to visit nelkodev.com and if you require specific advice or have any questions, contact us through nelkodev.com/contact.

Over time, ESM is expected to become the default module system in JavaScript, but until then, understanding these differences and interoperability will play a crucial role in developing robust, modern applications.