# 1.2.6.2.「typeof」、「instanceof」與搞清楚這是什麼

* typeof
  * 告訴你型別是甚麼

    ```
    var a = 3;
    //印出Number
    console.log(typeof a);

    var d = [];
    //印出Object
    console.log(typeof d);
    ```
* 用陣列在[原型鍊](https://jenhsuan.gitbooks.io/javascript-node-js/content/chapter1/124javascript-de-wu-jian-dao-xiang-yu-yuan-xing-ji-cheng/1242liao-jie-yuan-xing.html)上呼叫toString
  * [call](https://jenhsuan.gitbooks.io/javascript-node-js/content/chapter1/12321callapply-yu-bind.html): 將this重新指向d並傳入其他參數, 並執行函數
  * toString會用this變數輸出他的值

    ```
    //印出[object Array]
    console.log(Object.prototype.toString.call(d));
    ```
* instanceof
  * 告訴你型別是甚麼

    ```
    var e = new Person('Jane');
    //true
    console.log(a instanceof Person);
    ```
* undefined, null
  * bug: typeof null是object

    ```
    //印出undefined
    console.log(typeof undefined);
    //印出object
    console.log(typeof null);
    ```
* empty function

  ```
    var z = function(){}
    //印出function
    console.log(typeof z);
  ```


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