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

Create a Set by applying all fn to all values of a.
Create a Set from a and all elements in b whose value, after applying fn does not match a value in the previously created set.
Return the last set converted to an array.


const unionBy = (a, b, fn) => {
  const s = new Set(;
  return Array.from(new Set([...a, ...b.filter(x => !s.has(fn(x)))]));


unionBy([2.1], [1.2, 2.3], Math.floor); // [2.1, 1.2]