CJS and ESM Modules in Node.js: A Comparison of Advantages and Disadvantages

Node.js is a popular runtime environment for building server-side applications in JavaScript. In the past, Node.js relied on the CommonJS (CJS) module system, which allowed developers to write modular code that could be easily shared between different files and projects. However, with the release of ECMAScript (ES) modules, Node.js developers now have the option to use a new module system that offers some distinct advantages over CJS. In this blog, we’ll explore the differences between CJS and ES modules in Node.js and discuss how to use them in your projects.

CommonJS (CJS) Modules

CommonJS (CJS) is the module system used in Node.js before the introduction of ES modules. CJS modules are loaded synchronously and wrapped in a function, which allows them to have their own local scope. This means that variables and functions defined in one module are not visible to other modules unless they are explicitly exported.

To export variables and functions from a CJS module, you can use the module.exports object. For example, suppose you have a module called utils.js that contains a function called sum:

// utils.js
function sum(a, b) {
return a + b;
}
module.exports = {
sum
};

You can then import the sum function into another module using the require() function

// index.js
const utils = require('./utils.js');
console.log(utils.sum(1, 2)); // output: 3

While CJS modules have been the de facto standard in Node.js for a long time, they do have some limitations. One of the biggest drawbacks of CJS modules is that they are loaded synchronously, which can be slow when dealing with large applications. Additionally, CJS modules can be difficult to tree-shake, or eliminate unused code during the build process.

ECMAScript (ES) Modules

ECMAScript (ES) modules are a new module system introduced in ECMAScript 6 (ES6). ES modules are loaded asynchronously and can be tree-shaken, making them faster and more efficient than CJS modules. Additionally, ES modules have a simpler syntax than CJS modules, making them easier to read and write.

To use ES modules in Node.js, you need to use the .mjs file extension and use the import and export keywords to import and export modules, respectively. For example, suppose you have a module called utils.mjs that contains a function called sum:

// utils.mjs
export function sum(a, b) {
return a + b;
}

You can then import the sum function into another module using the import keyword:

// index.mjs
import { sum } from './utils.mjs';
console.log(sum(1, 2)); // output: 3

One important thing to note when using ES modules in Node.js is that they are loaded asynchronously, which means that you cannot use them in the top-level code of your application. Instead, you need to use an asynchronous module loader like import() to load your ES modules at runtime. For example:

// index.js
async function main() {
const { sum } = await import('./utils.mjs');
console.log(sum(1, 2)); // output: 3
}
main();

Advantages and Disadvantages of CJS and ESM

Both CJS and ESM have their advantages and disadvantages.

CJS modules have been the standard in Node.js for many years and are widely used in the Node.js ecosystem. They are simple to use and provide a straightforward way to modularize code. However, CJS modules are synchronous and can slow down the application’s startup time, especially if there are many modules to load.

ESM modules, on the other hand, are asynchronous, which means that they load faster than CJS modules. ESM modules also provide a more standardized module system, which can help make code more portable between different environments. However, ESM modules are still relatively new and are not yet widely supported in the Node.js ecosystem. Additionally, ESM modules require a .mjs file extension, which can be confusing for developers who are used to using .js for JavaScript files.

Conclusion

In conclusion, both CJS and ESM modules have their advantages and disadvantages, and the choice between the two depends on the specific needs of your project. If you are working with a large codebase that relies heavily on CJS modules, it may be more efficient to stick with CJS. However, if you are starting a new project or want to take advantage of the faster startup times and standardized module system provided by ESM, it may be worth considering switching to ESM modules. It’s important to note that Node.js now supports both CJS and ESM modules, so you can use either or both in your project as needed.

To use ESM modules in Node.js, you will need to use the — experimental-modules flag when running Node.js and ensure that your files have the .mjs file extension. You can also use third-party tools like Babel to transpile your ESM modules into CJS modules if needed.

In summary, both CJS and ESM modules have their place in the Node.js ecosystem, and the choice between the two depends on the specific needs of your project. As Node.js continues to evolve, it’s possible that ESM modules will become more widely used and eventually replace CJS modules as the standard module system in Node.js.