getIntersection doesn't work for arrays with repeated values

[brooswajne]

Since your check for an element being in the intersection is for it's count in the flattened array of arrays to be equal to the number of total arrays, this doesn't work correctly if one of the arrays has repeated values.

e.g. getIntersection([ 1, 1 ], [ 2, 3 ]) returns [ 1 ]

[brooswajne]

Something like this would work I believe

const getIntersect = (first, ...arrs) => [...arrs.reduce((insect, arr) => arr.reduce((next, elt) => insect.has(elt) ? next.add(elt) : next, new Set()), new Set(first))]

Not very pretty, but still actually shorter than the current solution. Haven't given it a huge amount of thought, but I think it should have slightly better time complexity (goes over each array once rather than twice, still effectively O(n), could be improved further but not sure how you'd do it in one line), just a shame that it creates a new Set each iteration rather than mutating the initial one... Not sure how you'd do that elegantly in one line though.