How to do server side validation using observer?

Question

Observer Server Side Validation

Plan: Use observer to see if a checkbox is correctly set.. but how do i do this?

Does anyone know how I can call the session singleton in Javascript? I have a validation to a form like this which, everytime it validates the checkbox element, I want to to update a value in the session model.

Validation.add(
    'validate-tax', 'This is only applicable to certain states.', function(v) {
        var dropDown = document.getElementById("billing:region_id").querySelector('option:checked').text;
        if ((dropDown != 'New York' && document.getElementById("billing:tax_exempt").checked == false) || (dropDown == 'New York')) {
            Mage::getSingleton('core/session')->setYourNameSession($session_value);
            return true;
        }
    }
);

But I don't know if this is possible.

Additional Information I want to do a server-side validation on a checkbox. To do this my plan is to see if the value in the session is true or false. Alternatively, I was thinking to check the HTML elements directly from an observer, but I do not know how to do it.

Answer 1

Too long for a comment, but I think this is what you mean to do:

Validation.add(
    'validate-tax', 'This is only applicable to certain states.', function(v) {
        var dropDown = document.getElementById("billing:region_id").querySelector('option:checked').text;
        if ((dropDown != 'New York' && document.getElementById("billing:tax_exempt").checked == false) || (dropDown == 'New York')) {
            <?php Mage::getSingleton('core/session')->setYourNameSession($session_value); ?>
            return true;
        }
    }
);

Notice that Mage::getSingleton('core/session')->setYourNameSession($session_value); is php code, so above is wrapped with <?php ?>

Keep in mind, your file should be saved with the .php extension