Edit in JSFiddle

var ContactModel = Backbone.Model.extend({ //extend Backbone.Model
    defaults: function(){
        this.firstName = 'no first name yet';
        this.lastName = 'no last name yet';
        return this;
    }
});

var contact1Model = new ContactModel( //instantiate model instance
    {firstName:'John'}
);
// firstName is set to John while lastname remains the default value
console.log(contact1Model.get('firstName')); //logs John
console.log(contact1Model.get('lastName')); //logs 'no last name yet'