Mmm... depends, I guess. We had a team of JavaScript programmers and we (me included) needed to maintain the Scala back-end our client consumed, that was built by contractors a year or two previously and not really been worked on since then. It used Spray for routing, which IMO was the worst part of it, but there were a lot of other quite "cerebral" Scala-specific concepts in there too like implicits and other stuff that I forget now. A lot of it I'd never really come across before in any other programming language.
Maybe if you're familiar with something like Haskell Scala is more straightforward to decode?