Because this is kumite. Here it's about showing off skills related to the knowledge of tricks to reduce the length of the code as much as possible, I think. Of course, in "normal" code readability is very important.

Why you dont return simple undefined instead od {}.a ?

I mean:
isEven=n=> n%1==0 ? !(n%2) : undefined

and this is more readability. When you put some whitespace beetween