It's common in other languages as well (at least Haskell) and a bit surprising at first. However, a `.sortBy(fn).groupBy(fn)` is easy and of similar efficiency and when you actually need the local-only `groupBy()` you're happy it's there.

A bit more expressive overall.

At least it is better than lodash' useless groupBy which creates this weird key value mapping, loses order and converts keys to string and what not.