Simplest way to merge ES6 Maps/Sets?

Question

Is there a simple way to merge ES6 Maps together (like Object.assign)? And while we\'re at it, what about ES6 Sets (like Array.concat)?

Answer 1

Transform the sets into arrays, flatten them and finally the constructor will uniqify.

const union = (...sets) => new Set(sets.map(s => [...s]).flat());