code: function uniqueBy(arr, fun) {
    const seen = {};
    return arr.filter(el => {
      const e = fun(el);
      return !(e in seen) && (seen[e] = 1);
    })
  }
docs: Dedupes array based on criterion returned from iteratee function.
  Ex: uniqueBy(
  [{ id: 1 }, { id: 1 }, { id: 2 }],
  val => val.id
  ) = [{ id: 1 }, { id: 2 }]