30-seconds-of-code/snippets/js/s/mapped-array-union.md

title, type, language, tags, cover, excerpt, dateModified

title	type	language	tags	cover	excerpt	dateModified
Mapped array union	snippet	javascript	array	lake-church	Returns every element from both arrays that exists at least once after applying the provided function.	2020-10-22T20:24:44+03:00

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 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(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 }]