Array#lastIndexOf()

Returns the index of the given object's last occurrence. If no startAt argument is given, the search starts from the last position. If it's negative, will count backward from the end of the array. Returns -1 if no match is found.

let arr = ['a', 'b', 'c', 'd', 'a'];

arr.lastIndexOf('a');       //  4
arr.lastIndexOf('z');       // -1
arr.lastIndexOf('a', 2);    //  0
arr.lastIndexOf('a', -1);   //  4
arr.lastIndexOf('b', 3);    //  1
arr.lastIndexOf('a', 100);  //  4