unionBy javascript Code Snippet in 2025

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

• Create a new Set() by applying all fn to all values of a.
• Create a new 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(a.map(fn));
  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]
unionBy([{ id: 1 }, { id: 2 }], [{ id: 2 }, { id: 3 }], x => x.id)
// [{ id: 1 }, { id: 2 }, { id: 3 }]