number,string,boolean,undefined,object,function
var num = [];—-object
var num = {};—-object
var num = null;—object 历史遗留问题
var num = undefined; —-undefined
number,string,boolean,undefined,object,function
var num = [];—-object
var num = {};—-object
var num = null;—object 历史遗留问题
var num = undefined; —-undefined