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How to use ESM on the web and in Node.js

December 14, 2023

ESM (or ECMAScript Modules) is a modern module format with many advantages over previous formats like CommonJS. It is supported natively in most web browsers, is very fast, and opens up new opportunities for tree shaking, among other features. However, it is a major shift from CommonJS/AMD/UMD, and it can be hard to use if you are used to one of those module formats.

What is ESM?

ECMAScript Modules is a module format created as part of the ECMAScript (read: JavaScript) standard. It was standardized in the ES6 ECMAScript version, which you might know for adding many syntax features. ES Modules aims to solve a significant problem in JavaScript: There is no built-in way to share code between scripts. You might be familiar with importing things using require(). That is using CommonJS, which is only supported in some bundlers and Node.js. Additionally, there are some problems with CommonJS, like its synchronous nature.

ESM aims to solve all of these issues while making one module format universal. Today, Chrome, Safari, and Firefox fully support ESM, so you should not have any problem running it in modern browsers. Additionally, Node v12+ supports ESM, although you have to tell it you are using ESM rather than CommonJS. We will talk more about how to do this later on.

ESM Syntax

The syntax used to import and export modules is slightly more complicated than in other module systems like CommonJS, but it is still fairly easy to pick up. The most basic example is this:

// script.js
import { example } from "./library.js";
example();

// library.js
export const example = function () {
	console.log("hello world");
};

Here we import a function called example from library.js. Notice the brackets around example. That shows that we are only importing example. So, if we wanted to import multiple things, we could do this:

import { example, example2 } from "./library.js";

As you can see, we put both example and example2 between the brackets to import both exported variables. Now, what if there was only one thing exported? Using export default, we can remove the brackets.

// script.js
import defaultExample from "./library.js";
defaultExample();

// library.js
export default function () {
	console.log("hello world");
}

Default exports also allow us to name the imported value whatever we want. We can also do this with named exports, but it is a little more complicated:

import { example as newExampleName } from "./library.js";
newExampleName();

Another thing you might want to do is dynamically import something. Dynamic imports allow you to import with a function rather than a statement that must be at the top of the file, which means you can import something conditionally or even import a module with a dynamic URL. In other words, dynamic imports are like asynchronous require() calls:

import("./library.js").then((library) => {
	// handle the promise here
	library.default();
});

Finally, you might want to import all functions in a module, which you can do using import * as:

// script.js
import * as library from "./library.js";
library.example();
library.example2();

// library.js
export const example = function () {
	console.log("hello world");
};
export const example2 = function () {
	console.log("hello world again");
};

Using ESM with Node

Make sure you are using Node v12 or higher

Per-file

The simplest way to use ESM in Node.js is to replace .js with .mjs as the file extension for the script you want to use ESM in. The new file extension tells Node to treat the file as ESM rather than CJS. This naming scheme also works the other way. You can use .cjs to designate a file as CommonJS if the default is ESM (more on that in the next section).

Package-wide

Often you want a whole package to be ESM by default. To do this, you must add "type": "module" to your package.json. Adding "type": "module" means files will be treated as ESM files unless they have a .cjs extension. However, dependencies can still use CommonJS.

Importing CJS Modules From ESM

To ease the transition to ESM, Node can load CommonJS modules from ESM. You can retrieve the module.exports object by importing it as a default export. For example:

// script.mjs
import example from "example-package";
example.helloworld();

// example-package index.cjs
module.exports = {
	helloworld: function () {
		console.log("hello world");
	},
};

This feature is very helpful as it allows you to use NPM packages built with CommonJS.

Importing ES Modules from CJS

Unfortunately, Node does not support importing ES Modules from CommonJS modules. Luckily, many compilers allow you to convert ESM to CommonJS, allowing you to write a library that offers a great experience for both groups. TypeScript is the most notable in this category. You can write code using ES Modules and target both ESM and CJS.

Migrating from CJS To ESM

There are tools to help make migrating code from CommonJS to ES Modules easy. One of the most popular tools is cjstoesm, a command line tool that automatically transforms almost all CommonJS code in Node to its ESM equivalent. However, some things are not converted. The most notable is the __dirname. __dirname is not part of Node.js, but it is one thing Node does not support in “ESM mode”. Luckily, there are replacements. A simple way to polyfill __dirname is to do this:

const __dirname = new URL(".", import.meta.url).pathname;

This snippet uses import.meta.url, which is available in all ESM contexts.

Another thing that is harder to migrate is conditional imports. You can use dynamic importing to replace running require() dynamically, but problems arise with that due to how import() is asynchronous and require() is not. Luckily, there is a simple solution to this. If you are using Node.js v14.8 or later, you can use top-level await to make import() synchronous. For example,

// CommonJS
const module = boolean ? require("module1") : require("module2");

// ES Modules
const module = await (boolean ? import("module1") : import("module2"));

With these tips, you should not have much trouble converting CommonJS code to ES Modules.

Using ESM on the Web

Native ESM

The simplest way to use ESM on the web is by utilizing the native support for it. For about over 95% of users (Caniuse), ESM is supported without polyfills. To load a script with ESM, add type="module" in the script tag.

<script src="./esmscript.js" type="module">

Then, all modules you import from that script will also load as ES modules.

To create a backup script for browsers that do not support ES Modules, you can use the nomodule attribute. nomodule tells any browser that supports ES Modules not to load the script, so only browsers without ESM support will load it.

Bundlers

While just using the browser’s native ESM support is simple, you will be missing out on many features and optimizations, like tree shaking, support for CommonJS dependencies, and automatic fallback generation. Luckily, many bundlers support ESM by default. The most notable modern ESM bundler for the web is Vite. Vite is a bundler created by the Vue team that is extremely fast and feature-rich. By default, Vite minifies and optimizes your code, and you can do a lot more with Vite/Rollup plugins.

To create a Vite project, run npm create vite@latest, which will help you set up a project with Vite using ES Modules. If you want legacy browser support using nomodule, you can use the plugin @vitejs/plugin-legacy. Another feature that Vite, along with most other bundlers, has is support for dependencies that use CommonJS. Obviously, CommonJS requires a compiler step to convert from ESM and can add some code weight, but it is better than nothing, and you can still use ESM along with it.

Import Maps

While bundlers are great, you might be trying to avoid using them due to the increased complexity of having a build process. However, you might also want to import things using Node-style short package specifiers (e.g. “react” rather than ”https://esm.run/[email protected]”). Or perhaps you want to use a library that imports via specifiers like these, but don’t want to have to modify its code. Luckily, modern browsers have a solution to this problem: import maps.

Import maps are scripts with type="importmap" that contain JSON objects routing certain specifiers to URLs. For example, lets say you wanted to be able to import things from "react". You can do this by creating the following import map:

<script type="importmap">
	{
    	"imports": {
			"react": "https://esm.run/[email protected]",
		}
	}
</script>

The above import map essentially turns this import:

import {useEffect} from "react";

Into this:

import {useEffect} from "https://esm.run/[email protected]"

This will affect all modules on that webpage. You can also map by a prefix:

<script type="importmap">
 	{
    	"imports": {
			"scripts/": "https://example.com/scripts/",
		}
	}
</script>

This will reroute all imports to scripts/... to https://example.com/scripts/.... For example scripts/test would turn into https://example.com/scripts/test.

Imports maps are currently supported by 90%+ of users, and this number is continually growing.

Conclusion

Now you are familiar with ES Modules’ syntax and how to implement it on the web and in Node.js. Make sure to sign up for the newsletter or RSS below. I hope this article has been helpful, and thanks for reading.

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