# Mapped array union

JavaScript, Array · Oct 22, 2020

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 }]