title: unionWith tags: array,function,intermediate

Returns every element that exists in any of the two arrays once, using a provided comparator function.

Create a Set with all values of a and values in b for which the comparator finds no matches in a, using Array.prototype.findIndex().

const unionWith = (a: any[], b: any[], comp: Predicate) =>
  Array.from(
    new Set([...a, ...b.filter((x) => a.findIndex((y) => comp(x, y)) === -1)])
  );

unionWith(
  [1, 1.2, 1.5, 3, 0],
  [1.9, 3, 0, 3.9],
  (a, b) => Math.round(a) === Math.round(b)
); // [1, 1.2, 1.5, 3, 0, 3.9]