Extending the defaults of a Model superclass in Backbone.js

Question

I would like to pose this as a question to this answer but I can't seem to do so, I apologize.

Extending the defaults for the subclass are reflected in the superclass. This seems to defeat the purpose and I'm more apt to explicitly list the superclass' defaults in the subclass to get the structure I'm looking for.

var Inventory = Backbone.Model.extend({
    defaults: {
        cat: 3,
        dog: 5
    }
});

var ExtendedInventory = Inventory.extend({
});

_.extend(ExtendedInventory.prototype.defaults, {rabbit: 25});

var i = new Inventory();
var ei = new ExtendedInventory();
console.log(i.attributes);
console.log(ei.attributes);

This outputs:

{cat: 3, dog: 5, rabbit: 25}
{cat: 3, dog: 5, rabbit: 25}

Not what I (nor, I assume, the op) want:

{cat: 3, dog: 5}
{cat: 3, dog: 5, rabbit: 25}

Answer 1

The problem is that Inventory.prototype.defaults and Extended.prototype.defaults has the same reference, because you have not override the reference.

So you can do this in 2 ways, maybe more but i only found this 2:

Edit: The first example is incorrect (see comments); please refer to the second.

var ExtendedInventory = Inventory.extend({
    defaults: {
        rabit:25
    }
});

_.extend(ExtendedInventory.prototype.defaults, Inventory.prototype.defaults);

or

var ExtendedInventory = Inventory.extend({
    defaults: _.extend({},Inventory.prototype.defaults,
         {rabit:25}
    )
});

I think the first looks cleaner.