Ah, didn't pay attention to the time being, I am new to the site.

This code was written before that syntax existed.

if doing the "single liners" you might as well use arrow functions, so

.map((v) => v.split(' ')...)

and so on.

Me too, I don't get it either :(

Agreed. And I'll add that it would also aim to resemble the stated requirements so that the poor maintenance developer need not infer the relationship between the two.

really nicer and more readable, yes. :)

Still prefer the one-liner, especially the new version with fat arrows: http://www.codewars.com/kata/reviews/54be2a15518b8dcff90001f2/groups/56fc240e4504004e510009dd

It's less readable because you put the function decs on new lines, which I've never seen before and don't think anybody actually does. The code itself is still not great as using arbitrary names such as "v" and "w" don't give you any hints about what the code actually does.

Also, best practice should definitely include how the code is presented. Code that can be passed around from colleage to colleage and be quickly understood by everyone should be something you're looking to achieve, i.e. it should be part of the best practice.