Union

let a = new Set([1,2,3]);
let b = new Set([4,3,2]);
let union = new Set([...a, ...b]);
    // {1,2,3,4}

Intersection

let a = new Set([1,2,3]);
let b = new Set([4,3,2]);
let intersection = new Set(
    [...a].filter(x => b.has(x)));
    // {2,3}

Difference

let a = new Set([1,2,3]);
let b = new Set([4,3,2]);
let difference = new Set(
    [...a].filter(x => !b.has(x)));
    // {1}

References

1. MDN JavaScript Reference – Sets