Edit in JSFiddle

var subModel = Backbone.Model.extend({
    constructor:function(){ //extend this constructor, not Model
        this.foo = 'bar';

myModel = new subModel();

console.log(myModel.foo); //logs bar

/*cid is an instance property set by the Model constructor but logs undefined 
because we created our own constructor*/

/*the cid property is an instance property set by Model() and we didn't use Model, we used 
a custom constructor so myModel does not have a cid property*/