key会自动转化为字符串字符串
js对象的key只能是字符串, 如果不是字符串, 会自动调用toString()
"100" 跟 "2"比较, "2">"1"
var obj = {'a':'123','b':'345'};
console.log(Object.keys(obj)); //['a','b']
var obj1 = { 100: "a", 2: "b", 7: "c"};
console.log(Object.keys(obj1)); // console: ["2", "7", "100"]
