It's in fact a dialect of ecmascript(just like actionscript and javascript).

Also while i agree with Sid on javascript, it's kinda getting better lately. JS on Node 8 and JS on browser are currently in waay different leagues. Also Typescript is kinda making things better in browser side of the thingd(albeit sloooowly).

Also yes, use C# version of unity. Unityscript will not be helpful when you actually try to move on to actual javascript(be it nodejs or browsers) because even though they are both ecmascript based, due to differences in nature of game engines and places where JS is used, it will not be much of a use to you.(In fact language does not matter on this point, what you actually need to learn is the way to program things and programming practices, which differ greatly when programming games vs other types of programs/apps, but i will not go in to detail in this thread) On the other hand, C# you learn will probably be more useful outside of unity, at least syntax and datastructures will be similar. Plus unityscript is deprecated and support will be removed in the future.