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Okiki Ojo
Okiki Ojo

Posted on • Originally published at on

Mastering the Art of ESM and CJS Package Handling

Greetings, fellow devs and bundlejs aficionados! 🚀

I was closing out some long lived issues over on bundlejs, when issue #50 reminded me of the ongoing debate about how bundlejs should handle the ESM and CJS packages.

Lightbulbs flickered, coffee was consumed (I don't drink coffee, but you get the point), and I'm pretty sure I've cracked a solution. But there are a few slight behavior changes you need to be aware of. So buckle up. If anything looks off or confusing, please let me know in either the comments below, on GitHub Discussions or issue #50 directly.

The Issue at Hand 🧐

The root of the problem lies in how bundlejs was handling CommonJS files.

"CommonJS files only export their default, which created a conundrum. How can we tell if a module is CommonJS if we don't fetch it? And if we fetch it first, it's too late for esbuild to deal with it properly."

But never fear! I've worked out a solution that is impartial to whether the module is ESM or CommonJS.

The Solution 🧩

Let's walk through this step by step, using our handy API for demonstration.

  1. Single CJS File: Only export the default (no renaming involved). Check out this example using postcss-easings:

  2. Multiple CJS Files: Export the default but use the file URL as the import name. Also, append ...Default to the end of it like so: reactDefault & reactDomDefault. Take a peek here:,react-dom&minify=false

  3. Single ESM File: Export everything, including the default. No renaming here, folks!

  4. Multiple ESM Files: Export all methods and variables from all exports, but remember to use the naming rules for all default exports. For instance, after exporting the other module exports, rename the default export to springEasingDefault and codepointIteratorDefault:,codepoint-iterator&minify=false

  5. Treeshaking: Here, we assume you want the driver's seat if you add the treeshake query param to the URL. So, all of the above rules are null and void, and you now have complete control over what is exported, including the default exports not being automatic.,react&treeshake=[*],[*]&minify=false

🚨 Note: You might run into issues with CJS modules if you don't export default properly. Part of this is because tree-shaking is somewhat of a no-show for CJS packages, so tread lightly here!

There you have it! A quick and dirty rundown of the latest updates to the way handles CJS and ESM packages.

So, go ahead and take the new system for a spin. Let me know what you think. Take it for a ride 🚗

Photo by Marcin Jozwiak

Originally posted on

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