[FuzzyDunlop]

You can do Rails to a fair extent without really knowing Ruby, and just fumbling through it with a bunch of gems and messy code. In some ways it can be the gateway to learning Ruby itself, if you were so inclined to do so.

For things like CoffeeScript, which are fairly thin layers over another language, I'd always advise to just learn JS first. Dart and Clojurescript though are so far removed from their compilation target that you don't really need to know the JS, as long as there's good library support for what you want to do. In the case of clojurescript the interop is so seamless that it puts the focus on learning the APIs, which is where the true complexity lies. Of course, and even more so with typing, you also have the compiler to back you up.