diff -u b/core/modules/views/tests/modules/views_test_exposed_filter/views_test_exposed_filter.module b/core/modules/views/tests/modules/views_test_exposed_filter/views_test_exposed_filter.module
--- b/core/modules/views/tests/modules/views_test_exposed_filter/views_test_exposed_filter.module
+++ b/core/modules/views/tests/modules/views_test_exposed_filter/views_test_exposed_filter.module
@@ -10,11 +10,20 @@
+    // Attach an AJAX callback to all exposed filters for 'title'.
     $form['title']['#ajax']['callback'] = 'views_test_exposed_filter_ajax_callback';
     $form['title']['#ajax']['wrapper'] = 'views-test-exposed-filter-test';
     $form['title']['#prefix'] = '<span id="views-test-exposed-filter-test">Default prefix</span>';
   }
 }
 
+/**
+ * Returns test markup to indicate a AJAX call has been made.
+ *
+ * @param array $form
+ * @param \Drupal\Core\Form\FormStateInterface $form_state
+ *
+ * @return array
+ */
 function views_test_exposed_filter_ajax_callback(array &$form, FormStateInterface $form_state) {
   return [
     '#markup' => 'Callback called.',
   ];
-}
\ No newline at end of file
+}