Javascript Array peek()

Array.prototype.peek = function() {
  return this[this.length - 1];
}

Array.prototype.peek = function() { return this[this.length - 1] };

// https://leetcode.com/problems/remove-duplicates-from-sorted-list-ii/

/**/*from   w  ww .  j a v a2s.  c  om*/
 * Definition for singly-linked list.
 * function ListNode(val) {
 *     this.val = val;
 *     this.next = null;
 * }
 */

Array.prototype.peek = function () {
 return this.length > 0 ? this[this.length - 1] : undefined;
};

/**
 * @param {ListNode} head
 * @return {ListNode}
 */
var deleteDuplicates = function (head) {
 var r = [];
 var a = new Set();

 while (head) {
  if (a.has(head.val)) {
   // found duplicate
   while (r.peek() == head.val) {
    r.pop();
   }
  } else {
   r.push(head.val);
  }

  a.add(head.val);
  head = head.next;
 }

 return r;
};