Set:類似于數組,但是成員的值都是唯一的
let a = new Set([1, 2, 3]);
let b = new Set([4, 3, 2]);// 并集
let union = new Set([...a, ...b]);// 交集
let intersect = new Set([...a].filter(x => b.has(x)));// 差級
let difference = new Set( [...a].filter(x => !b.has(x)));
參考《ES6標準入門》 P211