Briefly: detecting changed form inputs with jQuery

I was working on a project a few months back that required me to detect whether the user had changed the values of HTML form inputs, and prompt them suitably. After a little poking into the defaultValue, defaultSelected and defaultChecked DOM properties, I knocked together a quick jQuery selector.

It’s since worked its way into a number of other projects—this site included—so I thought I’d post it in case anyone is interested or has suggestions for improvements. Share and enjoy :)

     * Selector that returns form input elements that have been changed by the
     * user since the page was uploaded.
     * Example use:
     *    $('form :input').filter(':changed').each(function() { … })
     * @param   oEl     the element itself
     * @param   index   the current loop index (unused)
     * @param   meta    metadata about the selector (unused)
     * @param   stack   a stack of the elements to loop (unused)
    $.expr[':'].changed = function(oEl, index, meta, stack)
        // 0. drop out
        var sN = oEl.nodeName.toLowerCase();
        if ((sN !== 'input') && (sN !== 'textarea') && (sN !== 'select'))
            return false;
        if (oEl.getAttribute('data-no-check') != null)
            return false;
        // 1. custom hook for SELECT boxes
        if (sN == 'select')
            // a) iterate through all options. If one has changed, we've changed
            for (var i = 0; i < oEl.options.length; i++)
                if (oEl.options[i].defaultSelected != oEl.options[i].selected)
                    return true;
            // b. nothing has changed
            return false;
        // 2. otherwise, for radios and checkboxen, use (default)checked
        var sType = oEl.getAttribute('type');
        if ((sType !== null) && 
            ((sType.toLowerCase() == 'radio') || (sType.toLowerCase() == 'checkbox')))
            return (oEl.checked != oEl.defaultChecked);
        // 3. everything else uses (default)value
        return (oEl.defaultValue != oEl.value);

At some point, I’ll get around to making it play nice with Zepto as well: it’s not being picked up quite right at the moment…