billing.save(); shipping.save(); functions override in Magento

These prototype js objects use in checkout. They defines in skin/frontend/base/default/js/opcheckout.js file. and adds to resulting html in onepage.html file using script html tag:

<script type="text/javascript" src="<?php echo $this->getSkinUrl('js/opcheckout.js') ?>"></script>

To override save functions' logic you can add your own billing.phtml and shipping.phtml files in own theme and there add new logic for save method. For example to override billing.save() logic you need to add new javascript tag with following code : if ('Billing' in window){ Object.extend(Billing, { save: function(){ //put your changes here } }) }

or, also you can just override any function after object will be created, example put this code after var billing = new Billing('co-billing-form', '<?php echo $this->getUrl('checkout/onepage/getAddress') ?>address/', '<?php echo $this->getUrl('checkout/onepage/saveBilling') ?>');:

billing.save = function() {
    //your code here
}