Quick tip: Non-standard npm module resolution

I’m putting the finishing touches on a publishing script for my blog and ran across an interesting situation. The scripts use native ES modules, but Eleventy, the JavaScript framework my blog uses, currently does not support them. I have to specify that my scripts use ES modules with the .mjs extension.

scripts/
└── my-publishing-script/
		└── index.mjs

I like to abstract the internals of a script as much as possible, so I like to utilize Node.js module resolution, which allows you to do stuff like this:

const library = require('library');

The assumption is that you’ve already run npm install library and library now lives in your node_modules directory.

If you’ve ever cracked open one of those libraries and looked at their package.json file, chances are they’re making use of the main field, which instructs Node.js as to where the entry file lives relative to the package.json. By default, Node.js looks for index.js, but it can literally be named anything.

Going back to my script, since it now has to use the .mjs extension, Node.js no longer automatically resolves it, but I can fix that by adding a package.json to my script’s directory with a single main field:

scripts/
└── my-publishing-script/
		├── index.mjs
		└── package.json

{
	"main": "index.mjs"
}

Now I don’t have to specify the entry file when calling the script:

node scripts/my-publishing-script