1
var sumOfUnique = function (nums) {
2
  let obj = {};
3
  let sum = 0;
4
  // count frequency of each number
5
  for (let num of nums) {
6
    if (obj[num] === undefined) {
7
      sum += num;
8
      obj[num] = 1;
9
    } else if (obj[num] === 1) {
10
      sum -= num;
11
      obj[num] = -1;
12
    }
13
  }
14

15
  return sum;
16
};