Broken extends in the browser, stick this in your console.

broken.js

var Model = function(db) {
    this.db = db;
}

Model.prototype.extend = function(properties) {
    var Child = Model;
    Child.prototype = Model.prototype;
    for(var key in properties) {
        Child.prototype[key] = properties[key];
    }
    return Child;
};

var model = new Model();

var BlogModel = model.extend({
    results: 10,
    getContent: function() {
        return "blog content, results: " + this.results;
    }
});

var blogModel = new BlogModel();

var ContactsModel = model.extend({
    results: 50,
    getContent: function() {
        return "contacts content, results: " + this.results;
    }
});

var contactsModel = new ContactsModel();

console.log(blogModel.getContent());       // contacts content, results: 50
console.log(contactsModel.getContent());   // contacts content, results: 50