In project electrum, we need to ship not only the ES2015 source code, but also its transpiled version.

The way to go is to add a compile and a prebuild step for npm:

"scripts": {
  "compile": "babel -d lib/ src/",
  "prepublish": "npm run compile"
},

• The compile step invokes Babel and outputs the transpiled files found in folder src to folder lib. Invoking the compile step is done with npm run compile.
• The prepublish step launches compile. It will be automatically started as a result of doing an npm publish.

In order for the consumers of the electrum package to get the transpiled version of the code, the main property must be updated to point to lib:

"main": "lib/index.js"

I also modified my .gitignore file to exclude lib from being part of the git repository.

lib/

Having done that, I published an updated version of electrum using:

npm publish

…and voilà, here it is: https://www.npmjs.com/package/electrum

So npm install - where’s my lib?

Trying to consume the new electrum package did not succeed. I kept getting error messages from WebPack:

Module not found: Error: Cannot resolve module ‘electrum’ in S:\git\foo\bar

Why? Because node_modules/electrum does not contain a lib folder and lib/index.js cannot be found.

Here is the folder structure (use npm install electrum@1.1.2 if you want to have a look at the misbehaving package):

src
test
.babelrc
.npmignore
...

Do you see that .npmignore file? I did not define any in my package source, but there it is. And it contains:

lib/
node_modules/

This .npmignore is being synthesized by npm when running npm publish.

Why does lib/ get ignored?

Here is what the documentation says about .npmignore:

Use a .npmignore file to keep stuff out of your package. If there’s no .npmignore file, but there is a .gitignore file, then npm will ignore the stuff matched by the .gitignore file.

So npm is trying to be smart here: it sees that I don’t want lib/ to get included in my git repository, and it assumes that I don’t want it to be included it in the resulting package either.

The solution is simple:

If you want to include something that is excluded by your .gitignore file, you can create an empty .npmignore file to override it.

Version 1.1.4 of the Electrum package finally got it right…