Edit in JSFiddle

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

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'