The horror that arises in js is due to a combination of unfortunate things like function arity, and coercion. The one I was thinking of is the famous

[1,2,10].map(parseInt)

It’s several things that on their own aren’t mad which taken together produces a crazy result. 10 as the default for an optional second argument of parseInt is fine. But map should only use a single argument closure by default. So map(parseInt) must be equivalent to map(x -> parseInt(x)).

It’s not the stability that bothers me but the fact that the comparison function is implicit and basically is

(a, b) -> a.toString().compareTo(b.toString())

Unlike say int.parse() where 10 is a reasonable default for radix, doing string comparison as default is almost never what the caller wants, so is a bad default. Functions where there is no obvious (99% of the time the desired value) should not have a default value when the argument is omitted. I’d much rather have things.sort() just fail and force me to specify the comparison, than to see it sort alphabetically.

As a side note: a second optional argument specifying stability true/false would be reasonable. True could be the default too, since unstable sorting is effectively an optimization with a tradeoff.