I've attached the following js to an exposed views form field and it works on first page load but when the form is changed (by ajax) it doesn't fire.

I can't quite work out where to add an event listener for onchange of the select list.

The following doesn't use jQuery but uses the core once library so I would like to avoid jQuery.

 * @file
 * Landing page adjustments.

 (function (Drupal, once) {
    'use strict';
     * Attached behavior for Custom DataLayer.
     * @type {{attach: Drupal.behaviors.CurrentSelectOption.attach}}
    Drupal.behaviors.CurrentSelectOption = {
      attach: function attach(context) {
        // First page load
        let elements = once('CurrentSelectOption', 'select#edit-field-event-country.form-select', context);
    function processingCallback(value, index) {
      let option = value.options[value.selectedIndex]
      value.remove (option);
      value.add (option, 0);

})(Drupal, once);
  • Why are you avoiding jQuery? It comes bundled with core. It's been almost 2 decades since I wrote raw JS without jQuery, so it's hard for me to provide the solution, though I'd have one without that requirement.
    – Jaypan
    Commented Jun 3, 2022 at 14:52
  • I didn't want to add another dependency to the script if I didn't have to and it was working apart from not being triggered when the form was changed. I've got nothing against jQuery :-) Commented Jun 4, 2022 at 16:17

1 Answer 1


When an AJAX call is made through the Drupal API, behaviours are re-attached. So if the API is in use, the answer is simply to put the code inside the attach method in your behavior, where you already have it.

The twist is: don’t use IDs to target elements. This is a general best practice rule for CSS/JS DOM selection, but especially important here, as Drupal will generate a new ID for the form element it replaces. If you inspect the select and its wrapper after the AJAX call, you’ll see -2 appended to the original ID.

Other than that (without a test) your code looks fine.

  • I had forgotten that :-) using a class solved the issue. Commented Jun 4, 2022 at 16:12

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