CoffeScript is endorsed by DHH and so it's more of a first class citizen than JavaScript in Rails. However it's not something you have to use. I never really liked using it and I easily managed to keep using JavaScript in all these years since Rails 3.1. I hope it's going to the deprecation bin soon but I believe that Basecamp is using it a lot. DHH won't have added it to Rails if that was not the case.