return leftObject - rightObject -- note: since compareKeys' return value can be any -ve/0/+ve number, a single subtraction operation is sufficient for numbers and dates. (Caveat: this doesn't take into account minor differences due to real imprecision. Currently this doesn't matter as quicksort isn't stable anyway so makes no guarantees about the order of effectively-if-not-exactly-equal real numbers; however, if quicksort ever gets replaced with a stable sorting algorithm then this method will need revised to use Number library's `cmp` instead.)