Javascript has some unique concepts, while Ruby or Pyton are just mix of features also present in Smalltalk or LISP. So learning Ruby has no added value if you already know those.
Maybe learning it just as a learning exercise is not so valuable, but Ruby is fantastically useful at getting shit done in the real world with a language that's pretty good. For instance, try and do this in standard Erlang:
stdout_str, stderr_str, status = Open3.capture3(command)
I'd love to see a language take its backtracking feature.