并集(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}

交集(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}

差集(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}

参考：http://2ality.com/2015/01/es6-set-operations.html