Array sort of undefined elements

[glemco]

At the moment it looks like undefined elements aren't correctly sorted in an array (they should always go on the bottom https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/sort), tried this in the ide:

>[1,undefined,5,undefined,2].sort((a,b)=>b-a)
=[ 5, 2, 1, undefined, undefined ]
>[1,undefined,5,undefined,2].sort((a,b)=>a-b)
=[ 1, undefined, 2, 5, undefined ] //this doesn't work!
>[1,undefined,5,undefined,2].sort()
=[ 1, 2, 5, undefined, undefined ]

Is that a desired behaviour/ something which would stay for some time? Should I make any assumption while doing this kind of sorting?

[gfwilliams]

Well, it's not desired behaviour if it's not in the spec - hopefully someone could do a PR for this as it's probably not a big deal to fix, based on:

If compareFn is supplied, all non-undefined array elements are sorted according to the return value of the compare function (all undefined elements are sorted to the end of the array, with no call to compareFn).

Having said that, the behaviour with sparse arrays may be harder to fix I guess.

To be honest this is another bit of JS I don't really like though - it means that actually the compare function can't (for instance) choose to sort some items before undefined and some after.

[glemco]

Mmh I'm not sure how exactly is it implemented, but bringing all undefined instances down the array could be as simple as adding a case for isNaN(compareFn(a, b)), in this case if the compareFn accidentally handles undefined input (giving back a number), anything can be done.
Although not 100% compliant, I guess that could make everyone happy