List.swap

Swap the item at $index1 with the item at $index2.

An index may be negative (counting from the end of the List, where -1 is the last item).

['a', 'B', 'Y', 'z'].swap(1, 2)
//= ['a', 'Y', 'B', 'z']

['A', 'b', 'c', 'y', 'Z'].swap(0, -1)
//= ['Z', 'b', 'c', 'y', 'A']