（五十七）ES6新特征 - Set与Map
1：Set基本概念
类数组对象, 内部元素唯一
let set = new Set([1, 2, 3, 2, 1]);
console.log(set);
/* Set(3){ 1, 2, 3 } */
[...set];
/* [1, 2, 3] */接收数组或迭代器对象
let set = new Set(document.getElementsByName('div'));
set.size;
/* 10 */
let set = new Set([1, 2, 3, 2, 1]);不存在隐式转换
let set = new Set([5, '5']);
set;
/* Set(3){ 5, '5' } */
let set = new Set([NaN, NaN]);
set;
/* Set(3){ NaN } */
let set = new Set([{}, {}]);
set;
/* Set(3){ {...}, {...} } */
let set = new Set('abcdabcd');
set;
/* Set(3){ 'a', 'b', 'c', 'd' } */
