准确判断js对象类型

Object.prototype.toString.call(obj) 输出字符串 "[object 对象类型]"
Object.prototype.toString.call(1) => '[object Number]'

Object.prototype.toString.call("1") => '[object String]'

Object.prototype.toString.call(new Object()) => '[object Object]'
Object.prototype.toString.call([]) => '[object Array]'

Object.prototype.toString.call(null) => '[object Null]'

Object.prototype.toString.call(undefined) => '[object Undefined]'

Object.prototype.toString.call(function(){}) => '[object Function]'