typeof 操作符 可以准确检测基本类型数据
console.log(typeof num); //number
console.log(typeof str); //string
console.log(typeof boolean); //boolean
console.log(typeof arr); //object
console.log(typeof Array); //function
console.log(typeof obj); // object
console.log(typeof Object); // function
console.log(typeof testFunction); //function
console.log(typeof testClass); //function
console.log(typeof undefined); //undefined
console.log(typeof null); //object
console.log(typeof xxx); // undefined 未定义的值
