# Array union based on function

JavaScript, Array · Oct 22, 2020

Returns every element that exists in any of the two arrays at least 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, b, comp) => 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]