diff -u b/core/misc/ajax.js b/core/misc/ajax.js
--- b/core/misc/ajax.js
+++ b/core/misc/ajax.js
@@ -1031,7 +1031,7 @@
       // context object that includes top-level text nodes. Therefore text nodes
       // will be wrapped with a <span> element. This also gives themers the
       // possibility to style the response.
-      var $new_content;
+      var $newContent;
       var onlyElementNodes;
       var trimedData = response.data.trim();
 
@@ -1049,20 +1049,21 @@
         });
       }
 
-      // If only NODE_ELEMENT or COMMENT_NODE elements are returned skip wrap processing.
+      // If only NODE_ELEMENT or COMMENT_NODE elements are returned
+      // skip wrap processing.
       if (!onlyElementNodes) {
         // If response.data contains only one TEXT_ELEMENT if the target element
         // is not styled as a block, response.data will be wrapped with SPAN.
         if (responseDataNodes.length === 1 || ($wrapper.css('display') !== 'block')) {
-          $new_content = $('<span></span>');
+          $newContent = $('<span></span>');
         }
         else {
-          $new_content = $('<div></div>');
+          $newContent = $('<div></div>');
         }
-        $new_content.html(responseDataNodes);
+        $newContent.html(responseDataNodes);
       }
       else {
-        $new_content = $(responseDataNodes);
+        $newContent = $(responseDataNodes);
       }
 
       // If removing content from the wrapper, detach behaviors first.
@@ -1077,32 +1078,32 @@
       }
 
       // Add the new content to the page.
-      $wrapper[method]($new_content);
+      $wrapper[method]($newContent);
 
       // Immediately hide the new content if we're using any effects.
       if (effect.showEffect !== 'show') {
-        $new_content.hide();
+        $newContent.hide();
       }
 
       // Determine which effect to use and what content will receive the
       // effect, then show the new content.
-      if ($new_content.find('.ajax-new-content').length > 0) {
-        $new_content.find('.ajax-new-content').hide();
-        $new_content.show();
-        $new_content.find('.ajax-new-content')[effect.showEffect](effect.showSpeed);
+      if ($newContent.find('.ajax-new-content').length > 0) {
+        $newContent.find('.ajax-new-content').hide();
+        $newContent.show();
+        $newContent.find('.ajax-new-content')[effect.showEffect](effect.showSpeed);
       }
       else if (effect.showEffect !== 'show') {
-        $new_content[effect.showEffect](effect.showSpeed);
+        $newContent[effect.showEffect](effect.showSpeed);
       }
 
       // Attach all JavaScript behaviors to the new content, if it was
       // successfully added to the page, this if statement allows
       // `#ajax['wrapper']` to be optional.
-      if ($new_content.parents('html').length > 0) {
+      if ($newContent.parents('html').length > 0) {
         // Apply any settings from the returned JSON if available.
         settings = response.settings || ajax.settings || drupalSettings;
         // Attach behaviors to all element nodes.
-        $new_content.each(function () {
+        $newContent.each(function () {
           if (this.nodeType === Node.ELEMENT_NODE) {
             Drupal.attachBehaviors(this, settings);
           }
diff -u b/core/modules/system/tests/modules/ajax_test/js/insert-ajax.js b/core/modules/system/tests/modules/ajax_test/js/insert-ajax.js
--- b/core/modules/system/tests/modules/ajax_test/js/insert-ajax.js
+++ b/core/modules/system/tests/modules/ajax_test/js/insert-ajax.js
@@ -1,6 +1,7 @@
 /**
  * @file
- * Provides method to test ajax requests.
+ * Drupal behavior to attach click event handlers to ajax-insert and
+ * ajax-insert-inline links for testing ajax requests.
  */
 
 (function ($, window, Drupal, drupalSettings) {
diff -u b/core/misc/ajax.js b/core/misc/ajax.js
--- b/core/misc/ajax.js
+++ b/core/misc/ajax.js
@@ -1031,7 +1031,7 @@
       // context object that includes top-level text nodes. Therefore text nodes
       // will be wrapped with a <span> element. This also gives themers the
       // possibility to style the response.
-      var $new_content;
+      var $newContent;
       var onlyElementNodes;
       var trimedData = response.data.trim();
 
@@ -1049,20 +1049,21 @@
         });
       }
 
-      // If only NODE_ELEMENT or COMMENT_NODE elements are returned skip wrap processing.
+      // If only NODE_ELEMENT or COMMENT_NODE elements are returned
+      // skip wrap processing.
       if (!onlyElementNodes) {
         // If response.data contains only one TEXT_ELEMENT if the target element
         // is not styled as a block, response.data will be wrapped with SPAN.
         if (responseDataNodes.length === 1 || ($wrapper.css('display') !== 'block')) {
-          $new_content = $('<span></span>');
+          $newContent = $('<span></span>');
         }
         else {
-          $new_content = $('<div></div>');
+          $newContent = $('<div></div>');
         }
-        $new_content.html(responseDataNodes);
+        $newContent.html(responseDataNodes);
       }
       else {
-        $new_content = $(responseDataNodes);
+        $newContent = $(responseDataNodes);
       }
 
       // If removing content from the wrapper, detach behaviors first.
@@ -1077,32 +1078,32 @@
       }
 
       // Add the new content to the page.
-      $wrapper[method]($new_content);
+      $wrapper[method]($newContent);
 
       // Immediately hide the new content if we're using any effects.
       if (effect.showEffect !== 'show') {
-        $new_content.hide();
+        $newContent.hide();
       }
 
       // Determine which effect to use and what content will receive the
       // effect, then show the new content.
-      if ($new_content.find('.ajax-new-content').length > 0) {
-        $new_content.find('.ajax-new-content').hide();
-        $new_content.show();
-        $new_content.find('.ajax-new-content')[effect.showEffect](effect.showSpeed);
+      if ($newContent.find('.ajax-new-content').length > 0) {
+        $newContent.find('.ajax-new-content').hide();
+        $newContent.show();
+        $newContent.find('.ajax-new-content')[effect.showEffect](effect.showSpeed);
       }
       else if (effect.showEffect !== 'show') {
-        $new_content[effect.showEffect](effect.showSpeed);
+        $newContent[effect.showEffect](effect.showSpeed);
       }
 
       // Attach all JavaScript behaviors to the new content, if it was
       // successfully added to the page, this if statement allows
       // `#ajax['wrapper']` to be optional.
-      if ($new_content.parents('html').length > 0) {
+      if ($newContent.parents('html').length > 0) {
         // Apply any settings from the returned JSON if available.
         settings = response.settings || ajax.settings || drupalSettings;
         // Attach behaviors to all element nodes.
-        $new_content.each(function () {
+        $newContent.each(function () {
           if (this.nodeType === Node.ELEMENT_NODE) {
             Drupal.attachBehaviors(this, settings);
           }
diff -u b/core/modules/system/tests/modules/ajax_test/js/insert-ajax.js b/core/modules/system/tests/modules/ajax_test/js/insert-ajax.js
--- b/core/modules/system/tests/modules/ajax_test/js/insert-ajax.js
+++ b/core/modules/system/tests/modules/ajax_test/js/insert-ajax.js
@@ -1,6 +1,7 @@
 /**
  * @file
- * Provides method to test ajax requests.
+ * Drupal behavior to attach click event handlers to ajax-insert and
+ * ajax-insert-inline links for testing ajax requests.
  */
 
 (function ($, window, Drupal, drupalSettings) {
diff -u b/core/tests/Drupal/FunctionalJavascriptTests/Ajax/AjaxTest.php b/core/tests/Drupal/FunctionalJavascriptTests/Ajax/AjaxTest.php
--- b/core/tests/Drupal/FunctionalJavascriptTests/Ajax/AjaxTest.php
+++ b/core/tests/Drupal/FunctionalJavascriptTests/Ajax/AjaxTest.php
@@ -155,7 +155,7 @@
       $this->clickLink("Link html {$test_case['render_type']}");
       $assert->assertWaitOnAjaxRequest();
       // Extra span added by a second prepend command on the ajax requests.
-      $assert->responseContains($this->wrapAjaxTarget('<span class="processed"></span>' . $test_case['expected']));
+      $assert->responseContains($this->wrapAjaxTarget($test_case['expected']));
     }
 
     foreach ($test_cases as $test_case) {