Webpacker 4.0 issue with Babel and CSS

[db0sch]

I've been trying to install Mapbox on a new rails app generated by Le Wagon template.

First, by default, webpack won't extract CSS anymore. We need to enable it manually:

// config/webpacker.yml
[...]
# Extract and emit a css file
extract_css: true
[...]

Second, i encounter a weird bug on my browser console saying that variable _typeof was not defined. Map was not displayed. Only a grey area. Turns out that webpack was transpiling all node_modules packages through Babel, which causes an issue with mapbox-gl.

To avoid that, we need to add this line: environment.loaders.delete('nodeModules'); to the file config/webpack/environment.js.
It will bypass node_modules packages from the transpiler.
But if we want to bypass only a specific package, another gist of code is available there: rails/webpacker#1903 (comment)

Will do a PR when I have the time next week