1function moveArrayItemToNewIndex(arr, old_index, new_index) {
2    if (new_index >= arr.length) {
3        var k = new_index - arr.length + 1;
4        while (k--) {
5            arr.push(undefined);
6        }
7    }
8    arr.splice(new_index, 0, arr.splice(old_index, 1)[0]);
9    return arr; 
10};
11
12//move index 1(b) to index 2(c)
13console.log(moveArrayItemToNewIndex(["a","b","c","d"], 1, 2)); // returns ["a", "c", "b", "d"]