Returns every element that exists in any of the two lists once, after applying the provided function to each element of both.

Use Iterable.toSet() and to get the unique values in a after applying fn to them.
Use to apply fn to all the values of b, Iterable.toSet(), Iterable.where() and Iterable.contains() to keep only the values in the resulting list contained in the unique mapped values of a, Iterable.toList() to return the appropriate result.
Finally, use the plus operator (+) to concatenate the two lists and return the result.


List<Y> unionBy<T, Y>(Iterable<T> a, Iterable<T> b, Y Function(T) fn) {
  final sA =;
  final sB = => !sA.contains(x)).toList();
  return sA + sB;


unionBy([{ 'x': 2 }, { 'x': 1 }], [{ 'x': 1 }], (v) => v['x']); // [2, 1]