https://www.prevost.eu/modules/contrib/field_group/formatters/fieldset/fieldset.js?tmj4jp

1/**
2 * @file
3 * Provides the processing logic for fieldsets.
4 */
5
6(($) => {
7  Drupal.FieldGroup = Drupal.FieldGroup || {};
8  Drupal.FieldGroup.Effects = Drupal.FieldGroup.Effects || {};
9
10  /**
11   * This script adds the required and error classes to the fieldset wrapper.
12   */
13  Drupal.behaviors.fieldGroupFieldset = {
14    attach(context) {
15      $(once('field-group-fieldset', '.field-group-fieldset', context)).each(
16        (index, element) => {
17          const $this = $(element);
18
19          if (
20            element.matches('.required-fields') &&
21            ($this.find('[required]').length > 0 ||
22              $this.find('.form-required').length > 0)
23          ) {
24            $('legend', $this).first().addClass('form-required');
25          }
26        },
27      );
28    },
29  };
30})(jQuery);