const arr = [4,1,2,1,2];

const singleNumberSet = (nums) => {
  let mySet = new Set();
  for (let i = 0; i < nums.length; i++) mySet.has(nums[i]) ? mySet.delete(nums[i]) : mySet.add(nums[i])
  return Array.from(mySet)[0];
};

const singleNumberXOR = (nums) => {
  let result = 0;
  nums.forEach(it => result ^= it);
  return result;
};

var singleNumberObj = (nums) => {
  const occurencies = {};

  nums.forEach(it => occurencies[it] ? occurencies[it] += 1 : (occurencies[it] = 1));
  for (let it in occurencies) if (occurencies[it] === 1) return it;
  return -1;
};