var letters = ["a", "b", "c", "d", "e", "f"];
function swap (arr, from, to) {
let temp = arr[from];
arr.splice(from, 1, arr[to]);
arr.splice(to, 1, temp);
}
swap(letters, 1, 4)
console.log(letters); // output [ 'a', 'e', 'c', 'd', 'b', 'f' ]