The description is very clear, just like Insisted has just said, it's probably you're not comfortable with Javascript's prototypes. In a word, when you don't know something, don't raise an issue: STFG.

The kata's description is enough if you know how to use Array.prototype.filter(). As stated in the description, You'd probably want to have a look at this. And also when you are stuck on any kata please use the Question tag and not the Issue one, as the purpose of those tags are expained here

the question and instructions weren't very clear.