Unfortunately I would say Javascript. Unlike Ruby, you will learn about compilers, static analysis, linting (eslint is more useful than rubocop), type systems, function composition, and have a good alternative to OO based programing built into the language (as opposed to your `UnboundMethod` lambda block proc whateverthehell design flaw Ruby has now). Javascript also gives you decent jumping off points into languages you will learn more from (Elm, Purescript).
I don’t know anything about Elm and Purescript but I’ll look into them. They aren’t hugely popular (as far as I know, feel free to correct me) so I’m not sure that’s a major win for portable skills versus Ruby engineers learning Scala or Python. If you don’t get too wild with the symbols, Scala can actually look quite familiar to Ruby programmers. Twitter had people use a subset of Scala when they transitioned many services from Ruby to Scala so that it would be an extra easy transition.
JavaScript has come a long way though and is certainly the lingua franca of the Internet.
Even assuming you implemented it their way, their code isn't very concise. In the snippet you pasted, "self.method" could be replaced with "method" for example without changing anything