手写instanceof，并牢记这句话

核心要点：原型链的向上查找。

function myInstanceof(left, right) {
    let proto = Object.getPrototypeOf(left);
    while(true) {
        if(proto == null) return false;
        if(proto == right.prototype) return true;
        proto = Object.getPrototypeof(proto);
    }
}

instanceof 检测一个对象A是不是另一个对象B的实例的原理是：查看对象B的prototype指向的对象是否在对象A的[[prototype]]链上。如果在，则返回true,如果不在则返回false。不过有一个特殊的情况，当对象B的prototype为null将会报错(类似于空指针异常)。

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