var age = 20;
var name = "sdhff"
var flag = true;
var user = {};

// typeof:判断基本数据类型

console.log(typeof age);//nember
console.log(typeof name);//string
console.log(typeof flag);//boolean
console.log(typeof user)//object

//不能判断
console.log(typeof []);
console.log(typeof null);
console.log(typeof undefined);

//null 对象没有
//undefined 数值没有