SOURCE

// myInstanceof(child, parent)

function myInstanceof(child, parent) {
    if (!child || typeof child !== 'object') {
        return false;
    }

    let temp = child;

    while(temp.__proto__) {
        if (temp.__proto__ === parent.prototype) {
            return true;
        }
        else {
            temp = temp.__proto__;
        }
    }

    return false;
}

class Animal {

}

class Dog {

}

class Cat extends Animal {

}

const dog = new Dog();

console.log(myInstanceof(dog, Dog));

const cat = new Cat();

console.log(myInstanceof(cat, Animal));