非标准库对象
1.Map
使用方法：
var map = new Map();
map.set(window, "the global");
map.set(document, "the document");
for (var [key, value] of map) {
console.log(key + " is " + value);
}
就像一个普通对象,通过key值来设置value。它与Object最大的区别是：
- 对象含有原型对象属性prototype，但是Map没有
- 普通对象中，key只能是string；Map的key值，可以是string或者Symbols
- 可以通过map.size()取得长度;普通对象没有这个方法
但是并不是所有情况都适应用Map，大部分情况还是用简单对象。
如果出现下面的情况，可以考虑使用Map:
- Are keys usually unknown until run time, do you need to look them up dynamically?
- Do all values have the same type, and can be used interchangeably?
- Do you need keys that aren't strings?
- Are key-value pairs often added or removed?
- Do you have an arbitrary (easily changing) amount of key-value pairs?
- Is the collection iterated?
更多例子：