Javascript Prototypal inheritance

Question

I am reading Javascript Web Applications by Alex Maccaw. Below is a snippet where a Class constructor is built.

    var Class = function(parent){
        var klass = function(){
        this.init.apply(this, arguments);
     };

  // Change klass' prototype 
    if (parent) {
        var subclass = function() { };
        subclass.prototype = parent.prototype;
        klass.prototype = new subclass;
    };

  klass.prototype.init = function(){};

  // Shortcuts
  klass.fn = klass.prototype;
  klass.fn.parent = klass;
  klass._super = klass.__proto__;      

  /* include/extend code... */

  return klass;
};

The abit i'm interested in is

    var subclass = function() { };
    subclass.prototype = parent.prototype;
    klass.prototype = new subclass;

Why doesn't he go

    klass.prototype = parent.prototype;

He explains it: "This little dance around creating a temporary anonymous function prevents instances from being created when a class is inherited". This still doesn't make sense to me, how would

    klass.prototype = parent.prototype;

make instances be created?

Answer 1

Because then anything you put on klass.prototype would also go on parent.prototype since they're the same object.

What he's doing is making a new object that inherits from parent.prototype , and assigning that object to klass.prototype , so that any properties added to it, don't get put on parent.prototype .

---

FYI, a more modern way to do that would be to use Object.create()

klass.prototype = Object.create(parent.prototype);

This accomplishes basically the same thing.

---

This sentence you pointed out:

" "This little dance around creating a temporary anonymous function prevents instances from being created when a class is inherited""

...is an odd way to explain it, unless there's a greater context where it makes sense. Perhaps he means to say that you don't need to invoke the parent constructor in order to obtain an object that inherits from parent.prototype .

If you did this:

klass.prototype = new parent();

It would accomplish the same thing, except that the parent constructor would be invoked, and there could be undesired side effects in doing so.