ES6提供了数据结构Set。类似于数组，但是没有重复值。
Set本身是一个构造函数，用来生成Set数据结构
let a = new Set([1, 2, 3])
let b = new Set([4, 3, 2])
// 并集
let union = new Set([...a, ...b])
console.log(union) // Set(4) {1, 2, 3, 4}
// 交集
let intersect = new Set([...a].filter(x => b.has(x)))
console.log(intersect) // Set(2) {2, 3}
// 差集
let difference = new Set([...a].filter(x => !b.has(x)))
console.log(difference) // Set(1) {1}
数组的map()和filter()可用于Set
let set1 = new Set([1, 2, 3])
set1 = new Set([...set1].map(x => x * 2))
console.log(set1) // Set(3) {2, 4, 6}
let set2 = new Set([1, 2, 3, 4, 5])
set2 = new Set([...set2].filter(x => (x % 2) === 0))
console.log(set2) // Set(2) {2, 4}
