Shouldn't you not be accessing private variables in prototype methods? I'm curious as to how you formatted your first solution since locally scoped variables aren't accessible in methods defined in the prototype.

In my opinion it is not just preference. At least not when you try to write code which most clearly conveys what it is supposed to do. In this case we want to know if every element in an array is an integer. Afterwards, it's just micro optimization I'd say. (I.e. if you have an every implementation that continues after the predicate returns false, than fix the implementation in stead of working around).

I'll move it to a 6 kyu...

I agree, though it's a consequence of the age of this kata. I don't think I have any way of changing its rating.