Javascript Array mutation(arr)

/*Mutations/* w ww .ja v  a 2s.  c o  m*/
Return true if the string in the first element of the array contains all of the letters of the string in the second element of the array.

For example, ["hello", "Hello"], should return true because all of the letters in the second string are present in the first, ignoring case.

The arguments ["hello", "hey"] should return false because the string "hello" does not contain a "y".

Lastly, ["Alien", "line"], should return true because all of the letters in "line" are present in "Alien".

String.prototype.indexOf()
*/

function mutation(arr) {
    for (var i = 0; i < arr[1].length; i++) {
        if (arr[0].toLowerCase().indexOf(arr[1][i].toLowerCase()) < 0) {
            return false;
        }
    }
    return true;
}

mutation(["hello", "hey"]);