The JavaScript Set was introduced to the language in the ES2015 spec, but it has always seemed incomplete. That's about to change with the addition of functions like intersection, union and difference.
If you need to be able to do unions and you are doing it in javascript, you are being absurdly inefficient compared to setting up a postgres db, but i wont be able to convince you of this because of basically nonsense brainwashing from your corporate conditioning.
EDIT: Note to self, do not use lemmy while hangry.