Yes, like turbolinks it's a default dependency in the Gemfile.

But e.g. jquery-rails https://github.com/rails/jquery-ujs is written in JS not CS, so I can use all the jQuery integration in Rails without using CS.

With turbolinks being written in CS if I want to use the full power of Rails+JS I need to add CS to my project.

or, you could just compile the coffeescript into javascript once and return to your normal javascript flow

You can even write your Javascript in Javascript, it should still work.