as far as I know it's possible to inject Ruby code into javascript code if the javascript code is included in a view/page.
But is this also possible when the javascript code is in a separate .js file that is located in the folder "public/javascripts"?
No. The files in public/javascripts never go through Rails. You could always create a controller/view to generate JS files though and then treat them similarly.
Or consider setting some initial JS objects or JSON strings that your JS methods can look at when doing their thing.