If you’re a web developer, I’m sure that you use bundlers (e.g., Webpack, Rollup, or Parcel) which transpile the JavaScript code of your application with Babel under the hood. No doubt that you also use various dependencies to cut off the development time.

However, I rarely see developers transpiling the code of their dependencies because everything seems to work fine without it, right? Wrong! Here’s why...

This is a part of the series of posts on best practices where I’m sharing bits of my experience contributing to open source. It’s both my hobby and job at Cube where we create and maintain open source tools for building data applications.

Adoption of ESM

Before browsers and Node.js got native support for ES modules, an npm package could contain several variants of source code:

  • CommonJS variant that doesn’t use modern features of JavaScript such as arrow functions. It’s compatible with most versions of Node.js and browsers. The source code location is specified in the main field of package.json.
  • module variant that has ES6 imports/exports but still doesn’t use modern features of JavaScript. Modules allow for tree-shaking, i.e., excluding unused code from a bundle. The source code location is specified in the module field of package.json (see this discussion for details).

Obviously, not every npm package follows this convention. It’s a choice that every author of a library makes on their own. However, it’s quite common for browser-side and universal libraries to be distributed in two variants.

Web browsers have natively supported ES modules for more than three years already, and Node.js supports them since version 12.20 (released in November 2020). That’s why authors of libraries now include one more variant of source code for execution environments that natively support ES modules, and many packages have completely removed the support for CommonJS.

Perks and features of ESM

It’s important to understand that native ES modules are very much different than modules that have ES6 imports/exports. Here are a few reasons:

  • The way we are used to using imports/exports will not work natively. Such code is intended for further processing by a bundler or a transpiler.

  • Native ES modules don’t support index file name and extension resolution, so you have to specify them explicitly in the import path:

    // Won't work:
    import _ from './utils'
    // Works:
    import _ from './utils/index.js'
  • If an npm package has ES modules, you have to add "type": "module" to package.json and specify the source code location in the exports field (see docs for details).

You can check this blog post by Axel Rauschmayer to learn more about ES modules.

Support for JavaScript features

Look! Since we know which versions of web browsers and Node.js support ES modules, we also know for sure which features of JavaScript we can use in the source code.

For instance, all web browsers that support ES modules natively also support arrow functions, so we don’t need to avoid using them or use Babel to transpile them to regular functions. Authors of libraries know that and ship the source code that leverages all modern features of JavaScript.

Transpilation of dependencies’ code

But wait! What can you do to make sure that your web application works in legacy browsers? Also, what to do if any of your application’s dependencies use modern features of JavaScript that aren’t supported by popular browsers?

In both cases, you need to transpile the code of the dependencies.

Manual transpilation

Let’s assume that we’re using webpack and babel-loader. Often, the config would look like this:

module: {
rules: [
test: /\.jsx?$/,
exclude: /node_modules/,
use: {
loader: 'babel-loader',
options: {
presets: [
['@babel/preset-env', { targets: 'defaults' }]

It’s suggested in the documentation and usage examples for Babel and babel-loader to exclude node_modules from transpilation (exclude: /node_modules/) to optimize the performance.

By removing the exclude rule, we’ll enable the transpilation of dependencies’ code in exchange for the increased bundling time. By providing a custom function as the exclude rule, we also can transpile just a subset of all dependencies:

exclude: _ => /node_modules/.test(_) && !/node_modules\/(nanostores|p-limit)/.test(_)

We can also select files by their filename extensions:

exclude: _ => /node_modules/.test(_) && !/(\.babel\.js|\.mjs|\.es)$/.test(_)

Manual transpilation — the benchmark

Let’s check how babel-loader configuration affects the bundle size and bundling time. Consider an application with three very different dependencies:

  • svelte (uses modern features of JavaScript such as arrow functions)
  • p-limit (uses bleeding-edge features of JavaScript such as private class fields)
  • axios (regular ES5 code)
ConfigTranspilationCompatibilityBundle sizeBundling time
BasicNo way to predict which web browsers will work with this bundle21 KB1.8 s
target: defaults and supports es6-moduleTo ES6 code. Private class fields will be downgraded, arrow functions and classes will remain as isModern browsers22 KB2.6 s
target: defaults with polyfillsTo ES5 codeAll browsers123 KB6.1 s

You can see that the total bundling time for modern browsers and all browsers with babel-loader is 8.7 s. Please also note that the basic, non-transpiled bundle won’t work with legacy browsers because of p-limit.

(By the way, I also have a blog post that explains in detail how to build several bundles for different browsers.)

Okay, but what if you don’t want to tinker with configs and specify files and packages to be transpiled manually? Actually, there’s a readily available tool for that!

Transpilation with optimize-plugin

Meet optimize-plugin for webpack by Jason Miller from Google (@_developit). It will take care of everything and even more:

  • It will transpile your application’s source code and the code of all dependencies.
  • If needed, it will generate two bundles (for modern and legacy browsers) using the module/nomodule pattern.
  • On top of that, it can also upgrade ES5 code to ES6 using babel-preset-modernize!

Let’s see what optimize-plugin will do to our example application with three dependencies:

ConfigTranspilationCompatibilityBundle sizeBundling time
BasicTo ES6 code. Also, to ES5 code with polyfillsAll browsers20 KB for modern browsers. 92 KB for legacy browsers (including 67 KB of polyfills)7.6 s

The total bundling time with optimize-plugin is 7.6 s. As you can see, optimize-plugin is not only faster than babel-loader, but it also produces a smaller bundle. You can check my results using the code from my optimize-plugin-demo repository.

Why optimize-plugin wins

The performance boost is possible because the code is analyzed and bundled only once. After that, optimize-plugin transpiles it for modern and legacy browsers.

Smaller bundle size is possible thanks to babel-preset-modernize. Chances are that you use ES6+ features in your application’s code but you never can predict which features are used in the code of the dependencies. Since optimize-plugin works with the bundle that already has the code of all dependencies, it can transpile it as a whole.

Here’s how babel-preset-modernize works. Consider this code snippet:

const items = [{
id: 0,
price: 400
}, {
id: 1,
price: 300
}, {
id: 2,
price: 550
const sum = items.reduce(function (sum, item) {
const price = item.price;
return sum + price;
}, 0);

After transpilation to ES6, we’ll get this code:

const items = [{
id: 0,
price: 400
}, {
id: 1,
price: 300
}, {
id: 2,
price: 550
const sum = items.reduce((sum, {
}) => sum + price, 0);

Here’s what has changed:

  • A regular anonymous function was upgraded to an arrow function.
  • item.price field access was replaced with the function argument destructuring.

Code size shrinked from 221 to 180 bytes. Note that we applied only two transformations here but babel-preset-modernize can do a lot more.

What’s next?

optimize-plugin works really great but it still has some room for improvement. Recently, I’ve contributed a few pull requests, including the support for webpack 5.

If optimize-plugin looks promising to you, I encourage you to give it a try in your projects and maybe contribute some improvements as well.

Anyway, starting today, please always transpile the code of the dependencies, whether with optimize-plugin or not, to make sure that you have full control over your application’s compatibility with modern and legacy browsers. Good luck!