Protected Inheritance

var Parent = function Parent(){
    
};

Parent.prototype = {
    a:function a(){
        console.log(333);
    }
};

// make sure that prototype of parent is protected and can't be changed by children
Object.freeze(Parent.prototype);

var Child = function Child(){
    
};

Child.prototype = Object.create(Parent.prototype);

Child.prototype.a = 5; // should not work
Child.prototype.z = 2;

var c = new Child();
console.log(c.a, c.z);