I'm trying to use a custom extension rather than using a template override (template_name.extra.tpl) to implement some jQuery on an event registration form (essentially to hide some form fields unless the right radio button is selected). Jackrabbithanna's useful guide here (http://www.jackrabbithanna.com/articles/easy-jquery-modificaiton-civicrm-forms) has been most helpful in getting me much of the way.

That guidance talks about targeting a specific contribution page by using:

    function tctaextend_civicrm_buildForm($formName, &$form) {
     if($form->_id==2 ){
      CRM_Core_Resources::singleton()->addScriptFile('com.skvare.testextend', 'my_alter_contribform.js');

In my case I want to target a particular event. The form name is CRM_Event_Form_Registration_Register, so I thought I could do a straight replacement, e.g.:

    function myextension_civicrm_buildForm($formName, &$form) {
     if($form->_id==1 ){
      CRM_Core_Resources::singleton()->addScriptFile('com.example.myextension', 'my_alter_eventform.js');

But that generates a php notice:

Notice: Undefined property: CRM_Event_Form_Registration_Register::$_id in myextension_civicrm_buildForm()

And also throws a fatal error if I try to create a new contribution page, even though the hook function is aiming to impact only event registration forms.

If I remove the line

 if($form->_id==1 ){

Then no errors, but my injected jQuery will presumably impact every event registration form rather than just the event I want to target? Where am I going wrong?

2 Answers 2


Digging into the hook reference documentation at https://docs.civicrm.org/dev/en/master/hooks/hook_civicrm_buildForm/ I can see that the correct approach is

if ($form->_eventId == 1) {

As Graham mentioned this is the correct approach:

if ($form->_eventId == 1) {

It is possible that _eventId is protected, so you can not access it directly. In that case use

$eventId = $form->getVar('_eventId');
if ($eventId == 1) {

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