手写instanceof

- instanceOf原理，手动实现function isInstanceOf(child,parent)

• instanceof原理：查询parent的prototype是否在child的原型链上，所以在查找过程中，instanceof会遍历child的原型链，直到找到或查找失败。

判断一个实例是否属于某种类型：

let person = function(){};
let person1 = new person();
person1 instanceof person; //true

• 手动实现isInstanceOf

function isInstanceOf(child,parent){
    let parentProto = parent.prototype;
    child = child.__proto__;
    while(1){
        if(child === null){
            return false;
        }
        if(child === parentProto){
            return true;
        }
        child = child.__proto__;
    }
}

function isInstanceOf(child,parent){
    let parentProto = parent.prototype;
    child = child.__proto__;
    while(child){
        if(child === parentProto){
            return true;
        }
        child = child.__proto__;
    }
    return false;
}