// 判斷 foo 是否是 Foo 類的實例 , 并且是否是其父類型的實例function Aoo(){}
function Foo(){}
Foo.prototype = new Aoo();//JavaScript 原型繼承
var foo = new Foo();
console.log(foo instanceof Foo)//true
console.log(foo instanceof Aoo)//true

function Cat(){}
Cat.prototype = {}

function Dog(){}
Dog.prototype ={}

var dog1 = new Dog();
alert(dog1 instanceof Dog);//true
alert(dog1 instanceof Object);//true

Dog.prototype = Cat.prototype;
alert(dog1 instanceof Dog);//false
alert(dog1 instanceof Cat);//false
alert(dog1 instanceof Object);//true;

var dog2= new Dog();
alert(dog2 instanceof Dog);//true
alert(dog2 instanceof Cat);//true
alert(dog2 instanceof Object);//true

Dog.prototype = null;
var dog3 = new Dog();
alert(dog3 instanceof Cat);//false
alert(dog3 instanceof Object);//true
alert(dog3 instanceof Dog);//error