Effective Node.js Modules - Part 2: ESM dynamic imports

One of the main differences between CJS and ESM modules is the way they handle dynamic imports. In CJS you can use require to import modules anywhere in your code, but in ESM you can only use import at the top level of your code. What if you want to import a module dynamically in ESM?

In this post, we will create a simple Hello world! application that uses dynamic imports to greet the user in different languages. If you don’t know how to enable ESM in Node.js, check the previous post.

Below is the directory structure of our application:

.
├── main.js
├── package.json
└── translations
    ├── ar.js
    ├── de.js
    └── en.js

The translations directory contains three files: en.js, de.js, and ar.js. Each file exports constants that represent the greeting message in a specific language. Here are the contents of the files:

// translations/en.js
export const GREETING = 'Hello world!';

// translations/ar.js
export const GREETING = 'مرحبا بالعالم!';

// translations/de.js
export const GREETING = 'Hallo Welt!';

The main.js file is the entry point of our application. It gets the user’s preferred language from the command line then imports the corresponding translation module dynamically if it exists.

Here is the content of the main.js file:

// main.js
const languages = ['en', 'de', 'ar'];
const defaultLanguage = 'en';

const language = process.argv[2] || defaultLanguage;

if (!languages.includes(language)) {
    console.error(`Unsupported language: ${language}`);
    process.exit(1);
}

// Get advantage of top-level await
const {GREETING} = await import(`./translations/${language}.js`);
console.log(GREETING);

To run the application, execute the following command:

node main.js de

In the main.js we used the import() expression, commonly called dynamic import. This expression will be evaluated at runtime, and the module will be loaded asynchronously. The await keyword is used to wait for the module to be loaded before continuing the execution.

That’s it! You have learned how to use dynamic imports in ESM modules.