# new 背后的原理 ```js var cat = new Animal("cat"); ``` 背后執行如下操作 ```js new Animal('cat') = { var obj = {}; obj.__proto__ = Animal.prototype; var result = Animal.call(obj,"cat"); return typeof result === 'object'? result : obj; } ``` 判斷cat是否是Animal示例對象 ```js cat instanceof Aniaml // true ```