I have a module that creates an AJAX Form.

I need to copy some values from the DOM to a form field when (or before) the form is submitted.

In my module JS, I tried this:

    (function($, Drupal) {

      Drupal.behaviors.beforeAjaxFormSubmit = {
        attach: function(context, settings) {

          Drupal.Ajax.prototype.beforeSubmit = function(form_values, element, options) {

            console.log('Will alter form here ...')
          };
        }
      };
    })(jQuery, Drupal);

The console log is shown but right after comes an AJAX Error:

>An AJAX HTTP error occurred.<br>
>HTTP Result Code: 200<br>
>Debugging information follows.<br>
>Path: /intervention/create?ajax_form=1<br>
>StatusText: OK<br>
>ResponseText: TypeError: Argument 1 passed to Drupal\Core\Render\MainContent\AjaxRenderer::renderResponse() must be of the type array, null given, called in /var/www/html/web/core/lib/Drupal/Core/Form/FormAjaxResponseBuilder.php on line 89 in Drupal\Core\Render\MainContent\AjaxRenderer-&gt;renderResponse() (line 49 of /var/www/html/web/core/lib/Drupal/Core/Render/MainContent/AjaxRenderer.php).<br>
>[[Prototype]]: Error at http://d9.localhost:8000/core/misc/ajax.js?v=9.4.2:110:32

Does anyone know what I am doing wrong or if this is not the right approach?

### Edit:

    (function($, Drupal) {

      Drupal.behaviors.beforeAjaxFormSubmit = {
        attach: function(context, settings) {

          // Add submit handler to form.beforeSend.
          // Update Drupal.Ajax.prototype.beforeSend only once.
          if (typeof Drupal.Ajax !== 'undefined' && typeof Drupal.Ajax.prototype.beforeSubmitInterventionForm === 'undefined') {
            Drupal.Ajax.prototype.beforeSubmitInterventionForm = Drupal.Ajax.prototype.beforeSubmit;
            Drupal.Ajax.prototype.beforeSubmit = function(form_values, element_settings, options) {

			  // Things will happen here
              
              return this.beforeSubmitInterventionForm.apply(this, arguments);
            };
          }
        }
      };
    })(jQuery, Drupal);

This is what I have now but I am still getting the same AJAX Error.