Javascript Array contains(index, obj)

/**//from  w w  w. j  ava  2  s . c  o m
 * @param {number[]} nums
 * @param {number} target
 * @return {number[]}
 */
var twoSum = function(nums, target) {
 for (var i = nums.length - 1; i >= 0; i--) {
  let val = target - nums[i];
  if (nums.contains(i, val) === true) {
   return [nums.indexOf(val), i];
  }
 }
 return [];
};

Array.prototype.contains = function(index, obj) {
 var i = this.length;
 while (i--) {
  if (i != index && this[i] === obj) {
   return true;
  }
 }
 return false;
};

console.log(twoSum([3,2,4], 6));