diff -u b/core/tests/Drupal/FunctionalJavascriptTests/JSWebAssert.php b/core/tests/Drupal/FunctionalJavascriptTests/JSWebAssert.php
--- b/core/tests/Drupal/FunctionalJavascriptTests/JSWebAssert.php
+++ b/core/tests/Drupal/FunctionalJavascriptTests/JSWebAssert.php
@@ -53,12 +53,14 @@
    *   (optional) A message for exception.
    *
    * @throws \RuntimeException
-   *   When the activity is not completed. If left blank, a default message will
-   *   be displayed.
+   *   When the activity is not completed.
    */
   public function waitForJavascriptActivity($timeout = 10000, $message = 'Unable to complete javascript activity.') {
     $condition = <<<JS
       (function() {
+        /**
+        * Helper method for asserting there are no Drupal ajax requests running.
+        */
         function IsNotAjaxing(instance, index, array) {
           "use strict";
           return instance ? !instance.ajaxing : true;
@@ -67,20 +69,24 @@
         return (
           (
             typeof jQuery === "undefined" || (
+              // Assert no ajax request is running through jQuery.
               jQuery.active === 0 &&
+              // Assert no element is animating.
               jQuery(':animated').length === 0
             )
           ) &&
           (
             typeof Drupal === 'undefined' || (
+              // Assert there is no debounced function waiting for execution.
               (typeof Drupal.debounceCount === 'undefined' || Drupal.debounceCount === 0) &&
-              (typeof Drupal.ajax === 'undefined' || Drupal.ajax.instances.every(IsNotAjaxing))              
+              // Assert Drupal.ajax is finished with all the ajax requests.
+              (typeof Drupal.ajax === 'undefined' || Drupal.ajax.instances.every(IsNotAjaxing))
             )
           )
         );
       }());
 JS;
-
+    
     $result = $this->session->wait($timeout, $condition);
     if (!$result) {
       throw new \RuntimeException($message);