I think that's great, I already have an implementation in one of my projects, but I do have a problem with some of the semantics of the xor i'd like to get feedback on.

In lodash it does an implicit unique filtering of the input array, even when it's performed on a single array (xor([1,1,1]) => [1]). I think this is a mistake. Arrays are not sets, and there's no technical reason to do the uniquness, it's just semantics.

I think we should use multi-set semantics for arrays, basically meaning that multiple items can exist for the same value. Defining multisetDifference as the result of removing from A any items that exist in B (so for example: multisetDifference([1,1,1,2,2,3], [1,2]) => [1,1,2,3]) then xor would be defined as concat(multisetDifference(A, B), multisetDifference(B, A)) (so for example: xor([1,1,1,2,2,3], [1,2,4]) => concat([1,1,2,3], [4]) => [1,1,2,3,4]).

WDYT?

I agree regarding uniqueness - if one wants uniqueness they can explicitly do that before calling.