var largestUniqueNumber = function (A) {
  var map = {};
  A.map(function (item) {
    if (item in map) {
      map[item] = map[item]   1;
    } else {
      map[item] = 1;
    }
  });
  var array = [];
  for (var key in map) {
    if (map[key] == 1) {
      array.push(key);
    }
  }
  if (array.length == 0) {
    return -1;
  }
  array.sort(function (a, b) { return b - a });
  return array[0];
};

console.log(largestUniqueNumber([9,9,8,8]));