What happens when there's a problem at the interface between your favorite compiles-to-javascript language and javascript? If you don't know javascript, how ar

What happens when there's a problem at the interface between your favorite compiles-to-javascript language and javascript? If you don't know javascript, how are you going to debug and/or fix that? I've never used coffeescript or clojurescript, so maybe they're so thin that this never happens?