typeof 操作符 可以準確檢測基本類型數據

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? 未定義的值