diff --git a/core/.eslintrc.json b/core/.eslintrc.json
index 659807751b..a6f3d33a0b 100644
--- a/core/.eslintrc.json
+++ b/core/.eslintrc.json
@@ -13,6 +13,7 @@
     "domready": true,
     "jQuery": true,
     "_": true,
+    "loadjs": true,
     "matchMedia": true,
     "Backbone": true,
     "Modernizr": true,
diff --git a/core/assets/vendor/loadjs/loadjs.min.js b/core/assets/vendor/loadjs/loadjs.min.js
new file mode 100644
index 0000000000..bc398a6b08
--- /dev/null
+++ b/core/assets/vendor/loadjs/loadjs.min.js
@@ -0,0 +1,267 @@
+loadjs = (function () {
+  /**
+   * Global dependencies.
+   * @global {Object} document - DOM
+   */
+
+  var devnull = function() {},
+    bundleIdCache = {},
+    bundleResultCache = {},
+    bundleCallbackQueue = {};
+
+
+  /**
+   * Subscribe to bundle load event.
+   * @param {string[]} bundleIds - Bundle ids
+   * @param {Function} callbackFn - The callback function
+   */
+  function subscribe(bundleIds, callbackFn) {
+    // listify
+    bundleIds = bundleIds.push ? bundleIds : [bundleIds];
+
+    var depsNotFound = [],
+      i = bundleIds.length,
+      numWaiting = i,
+      fn,
+      bundleId,
+      r,
+      q;
+
+    // define callback function
+    fn = function (bundleId, pathsNotFound) {
+      if (pathsNotFound.length) depsNotFound.push(bundleId);
+
+      numWaiting--;
+      if (!numWaiting) callbackFn(depsNotFound);
+    };
+
+    // register callback
+    while (i--) {
+      bundleId = bundleIds[i];
+
+      // execute callback if in result cache
+      r = bundleResultCache[bundleId];
+      if (r) {
+        fn(bundleId, r);
+        continue;
+      }
+
+      // add to callback queue
+      q = bundleCallbackQueue[bundleId] = bundleCallbackQueue[bundleId] || [];
+      q.push(fn);
+    }
+  }
+
+
+  /**
+   * Publish bundle load event.
+   * @param {string} bundleId - Bundle id
+   * @param {string[]} pathsNotFound - List of files not found
+   */
+  function publish(bundleId, pathsNotFound) {
+    // exit if id isn't defined
+    if (!bundleId) return;
+
+    var q = bundleCallbackQueue[bundleId];
+
+    // cache result
+    bundleResultCache[bundleId] = pathsNotFound;
+
+    // exit if queue is empty
+    if (!q) return;
+
+    // empty callback queue
+    while (q.length) {
+      q[0](bundleId, pathsNotFound);
+      q.splice(0, 1);
+    }
+  }
+
+
+  /**
+   * Load individual file.
+   * @param {string} path - The file path
+   * @param {Function} callbackFn - The callback function
+   */
+  function loadFile(path, callbackFn, args, numTries) {
+    var doc = document,
+      async = args.async,
+      maxTries = (args.numRetries || 0) + 1,
+      beforeCallbackFn = args.before || devnull,
+      isCss,
+      e;
+
+    numTries = numTries || 0;
+
+    if (/(^css!|\.css$)/.test(path)) {
+      isCss = true;
+
+      // css
+      e = doc.createElement('link');
+      e.rel = 'stylesheet';
+      e.href = path.replace(/^css!/, '');  // remove "css!" prefix
+    } else {
+      // javascript
+      e = doc.createElement('script');
+      e.src = path;
+      e.async = async === undefined ? true : async;
+    }
+
+    e.onload = e.onerror = e.onbeforeload = function (ev) {
+      var result = ev.type[0];
+
+      // Note: The following code isolates IE using `hideFocus` and treats empty
+      // stylesheets as failures to get around lack of onerror support
+      if (isCss && 'hideFocus' in e) {
+        try {
+          if (!e.sheet.cssText.length) result = 'e';
+        } catch (x) {
+          // sheets objects created from load errors don't allow access to
+          // `cssText`
+          result = 'e';
+        }
+      }
+
+      // handle retries in case of load failure
+      if (result == 'e') {
+        // increment counter
+        numTries += 1;
+
+        // exit function and try again
+        if (numTries < maxTries) {
+          return loadFile(path, callbackFn, args, numTries);
+        }
+      }
+
+      // execute callback
+      callbackFn(path, result, ev.defaultPrevented);
+    };
+
+    // add to document (unless callback returns `false`)
+    if (beforeCallbackFn(path, e) !== false) doc.head.appendChild(e);
+  }
+
+
+  /**
+   * Load multiple files.
+   * @param {string[]} paths - The file paths
+   * @param {Function} callbackFn - The callback function
+   */
+  function loadFiles(paths, callbackFn, args) {
+    // listify paths
+    paths = paths.push ? paths : [paths];
+
+    var numWaiting = paths.length,
+      x = numWaiting,
+      pathsNotFound = [],
+      fn,
+      i;
+
+    // define callback function
+    fn = function(path, result, defaultPrevented) {
+      // handle error
+      if (result == 'e') pathsNotFound.push(path);
+
+      // handle beforeload event. If defaultPrevented then that means the load
+      // will be blocked (ex. Ghostery/ABP on Safari)
+      if (result == 'b') {
+        if (defaultPrevented) pathsNotFound.push(path);
+        else return;
+      }
+
+      numWaiting--;
+      if (!numWaiting) callbackFn(pathsNotFound);
+    };
+
+    // load scripts
+    for (i=0; i < x; i++) loadFile(paths[i], fn, args);
+  }
+
+
+  /**
+   * Initiate script load and register bundle.
+   * @param {(string|string[])} paths - The file paths
+   * @param {(string|Function)} [arg1] - The bundleId or success callback
+   * @param {Function} [arg2] - The success or error callback
+   * @param {Function} [arg3] - The error callback
+   */
+  function loadjs(paths, arg1, arg2) {
+    var bundleId,
+      args;
+
+    // bundleId (if string)
+    if (arg1 && arg1.trim) bundleId = arg1;
+
+    // args (default is {})
+    args = (bundleId ? arg2 : arg1) || {};
+
+    // throw error if bundle is already defined
+    if (bundleId) {
+      if (bundleId in bundleIdCache) {
+        throw "LoadJS";
+      } else {
+        bundleIdCache[bundleId] = true;
+      }
+    }
+
+    // load scripts
+    loadFiles(paths, function (pathsNotFound) {
+      // success and error callbacks
+      if (pathsNotFound.length) (args.error || devnull)(pathsNotFound);
+      else (args.success || devnull)();
+
+      // publish bundle load event
+      publish(bundleId, pathsNotFound);
+    }, args);
+  }
+
+
+  /**
+   * Execute callbacks when dependencies have been satisfied.
+   * @param {(string|string[])} deps - List of bundle ids
+   * @param {Object} args - success/error arguments
+   */
+  loadjs.ready = function ready(deps, args) {
+    // subscribe to bundle load event
+    subscribe(deps, function (depsNotFound) {
+      // execute callbacks
+      if (depsNotFound.length) (args.error || devnull)(depsNotFound);
+      else (args.success || devnull)();
+    });
+
+    return loadjs;
+  };
+
+
+  /**
+   * Manually satisfy bundle dependencies.
+   * @param {string} bundleId - The bundle id
+   */
+  loadjs.done = function done(bundleId) {
+    publish(bundleId, []);
+  };
+
+
+  /**
+   * Reset loadjs dependencies statuses
+   */
+  loadjs.reset = function reset() {
+    bundleIdCache = {};
+    bundleResultCache = {};
+    bundleCallbackQueue = {};
+  };
+
+
+  /**
+   * Determine if bundle has already been defined
+   * @param String} bundleId - The bundle id
+   */
+  loadjs.isDefined = function isDefined(bundleId) {
+    return bundleId in bundleIdCache;
+  };
+
+
+// export
+  return loadjs;
+
+})();
diff --git a/core/core.libraries.yml b/core/core.libraries.yml
index 369130c0eb..25d25353f8 100644
--- a/core/core.libraries.yml
+++ b/core/core.libraries.yml
@@ -95,6 +95,7 @@ drupal.ajax:
     - core/drupalSettings
     - core/drupal.progress
     - core/jquery.once
+    - core/loadjs
 
 drupal.announce:
   version: VERSION
@@ -850,6 +851,16 @@ jquery.ui.widget:
   dependencies:
     - core/jquery.ui
 
+loadjs:
+  remote: https://github.com/muicss/loadjs
+  version: 3.5.1
+  license:
+    name: MIT
+    url: https://github.com/muicss/loadjs/blob/master/LICENSE.txt
+    gpl-compatible: true
+  js:
+    assets/vendor/loadjs/loadjs.min.js: {}
+
 matchmedia:
   remote: https://github.com/paulirish/matchMedia.js
   version: &matchmedia_version 0.2.0
diff --git a/core/lib/Drupal/Core/Ajax/AddJsCommand.php b/core/lib/Drupal/Core/Ajax/AddJsCommand.php
new file mode 100644
index 0000000000..ed4e8a5671
--- /dev/null
+++ b/core/lib/Drupal/Core/Ajax/AddJsCommand.php
@@ -0,0 +1,69 @@
+<?php
+
+namespace Drupal\Core\Ajax;
+
+/**
+ * An AJAX command for adding JS to the page via ajax.
+ *
+ * This command is implemented by Drupal.AjaxCommands.prototype.add_js()
+ * defined in misc/ajax.js.
+ *
+ * @see misc/ajax.js
+ *
+ * @ingroup ajax
+ */
+class AddJsCommand implements CommandInterface {
+
+  /**
+   * A CSS selector string.
+   *
+   * If the command is a response to a request from an #ajax form element then
+   * this value can be NULL.
+   *
+   * @var string|null
+   */
+  protected $selector;
+
+  /**
+   * An array containing the attributes of the scripts to be added to the page.
+   *
+   * @var string[]
+   */
+  protected $scripts;
+
+  /**
+   * The DOM manipulation method to be used.
+   *
+   * @var string[]
+   */
+  protected $method;
+
+  /**
+   * Constructs an AddJsCommand.
+   *
+   * @param string|null $selector
+   *   A CSS selector.
+   * @param array $scripts
+   *   An array containing the attributes of the scripts to be added to the page.
+   * @param string $method
+   *   The DOM manipulation method to be used.
+   */
+  public function __construct($selector, array $scripts, $method = 'appendChild') {
+    $this->selector = $selector;
+    $this->scripts = $scripts;
+    $this->method = $method;
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function render() {
+    return [
+      'command' => 'add_js',
+      'selector' => $this->selector,
+      'data' => $this->scripts,
+      'method' => $this->method,
+    ];
+  }
+
+}
diff --git a/core/lib/Drupal/Core/Ajax/AjaxResponseAttachmentsProcessor.php b/core/lib/Drupal/Core/Ajax/AjaxResponseAttachmentsProcessor.php
index ee5208b078..269e62ba9d 100644
--- a/core/lib/Drupal/Core/Ajax/AjaxResponseAttachmentsProcessor.php
+++ b/core/lib/Drupal/Core/Ajax/AjaxResponseAttachmentsProcessor.php
@@ -174,11 +174,13 @@ protected function buildAttachmentsCommands(AjaxResponse $response, Request $req
     }
     if ($js_assets_header) {
       $js_header_render_array = $this->jsCollectionRenderer->render($js_assets_header);
-      $resource_commands[] = new PrependCommand('head', $this->renderer->renderPlain($js_header_render_array));
+      $scripts_attributes = array_map(function ($render_array) { return $render_array['#attributes']; }, $js_header_render_array);
+      $resource_commands[] = new AddJsCommand('head', $scripts_attributes, 'insertBefore');
     }
     if ($js_assets_footer) {
       $js_footer_render_array = $this->jsCollectionRenderer->render($js_assets_footer);
-      $resource_commands[] = new AppendCommand('body', $this->renderer->renderPlain($js_footer_render_array));
+      $scripts_attributes = array_map(function ($render_array) { return $render_array['#attributes']; }, $js_footer_render_array);
+      $resource_commands[] = new AddJsCommand('body', $scripts_attributes, 'appendChild');
     }
     foreach (array_reverse($resource_commands) as $resource_command) {
       $response->addCommand($resource_command, TRUE);
diff --git a/core/misc/ajax.es6.js b/core/misc/ajax.es6.js
index c3633b4636..110410dfea 100644
--- a/core/misc/ajax.es6.js
+++ b/core/misc/ajax.es6.js
@@ -11,7 +11,8 @@
  * included to provide Ajax capabilities.
  */
 
-(function ($, window, Drupal, drupalSettings) {
+(function ($, window, Drupal, drupalSettings, loadjs) {
+
   /**
    * Attaches the Ajax behavior to each Ajax form element.
    *
@@ -357,6 +358,11 @@
 
     $.extend(this, defaults, elementSettings);
 
+    /**
+     * @type {null|Promise}
+     */
+    this.ajaxDeferred = null;
+
     /**
      * @type {Drupal.AjaxCommands}
      */
@@ -440,7 +446,6 @@
     // Set the options for the ajaxSubmit function.
     // The 'this' variable will not persist inside of the options object.
     const ajax = this;
-
     /**
      * Options for the jQuery.ajax function.
      *
@@ -513,7 +518,16 @@
         return ajax.success(response, status);
       },
       complete(xmlhttprequest, status) {
-        ajax.ajaxing = false;
+        if ((ajax.ajaxDeferred !== null && ajax.ajaxDeferred.then !== null) &&
+          (typeof ajax.ajaxDeferred === 'object' && typeof ajax.ajaxDeferred.then === 'function')) {
+          ajax.ajaxDeferred.then(() => {
+            ajax.ajaxing = false;
+          });
+        }
+        else {
+          ajax.ajaxing = false;
+        }
+
         if (status === 'error' || status === 'parsererror') {
           return ajax.error(xmlhttprequest, ajax.url);
         }
@@ -855,6 +869,8 @@
    *   XMLHttpRequest status.
    */
   Drupal.Ajax.prototype.success = function (response, status) {
+    this.ajaxDeferred = $.Deferred();
+
     // Remove the progress element.
     if (this.progress.element) {
       $(this.progress.element).remove();
@@ -873,14 +889,30 @@
     // Track if any command is altering the focus so we can avoid changing the
     // focus set by the Ajax command.
     let focusChanged = false;
-    Object.keys(response || {}).forEach((i) => {
-      if (response[i].command && this.commands[response[i].command]) {
-        this.commands[response[i].command](this, response[i], status);
-        if (response[i].command === 'invoke' && response[i].method === 'focus') {
+    const responseKeys = Object.keys(response);
+    responseKeys.reduce((deferredCommand, key, currentIndex) => deferredCommand.then(() => {
+      const command = response[key].command;
+      if (command && this.commands[command]) {
+        if (command === 'invoke' && response[key].method === 'focus') {
           focusChanged = true;
         }
+
+        const result = this.commands[command](this, response[key], status);
+        if (typeof result === 'object' && typeof result.then === 'function') {
+          // Handle a promise.
+          result.done(() => {
+            if (currentIndex + 1 === responseKeys.length) {
+              this.ajaxDeferred.resolve();
+            }
+          });
+          return result;
+        }
       }
-    });
+
+      if (currentIndex + 1 === responseKeys.length) {
+        this.ajaxDeferred.resolve();
+      }
+    }), $.Deferred().resolve());
 
     // If the focus hasn't be changed by the ajax commands, try to refocus the
     // triggering element or one of its parents if that element does not exist
@@ -1357,5 +1389,44 @@
         } while (match);
       }
     },
+    add_js(ajax, response) {
+      const deferred = $.Deferred();
+      const scriptsSrc = response.data.map((script) => {
+        // loadjs requires a unique ID, AJAX instances' `instanceIndex` are
+        // guaranteed to be unique.
+        // @see Drupal.behaviors.AJAX.detach
+        const uniqueBundleID = script.src + ajax.instanceIndex;
+        loadjs(script.src, uniqueBundleID, {
+          async: !!script.async,
+          before(path, scriptEl) {
+            let selector = 'body';
+            if (response.selector) {
+              selector = response.selector;
+            }
+            if (script.defer) {
+              scriptEl.defer = true;
+            }
+            // To avoid synchronous XMLHttpRequest on the main thread and break
+            // load dependency, it should not use jQuery.
+            const parentEl = document.querySelector(selector);
+            if (response.method === 'insertBefore') {
+              parentEl.insertBefore(scriptEl, parentEl.firstChild);
+            }
+            else {
+              parentEl[response.method](scriptEl);
+            }
+            // Return `false` to bypass loadjs' default DOM insertion mechanism.
+            return false;
+          },
+        });
+        return uniqueBundleID;
+      });
+      loadjs.ready(scriptsSrc, {
+        success() {
+          deferred.resolve();
+        },
+      });
+      return deferred.promise();
+    },
   };
-}(jQuery, window, Drupal, drupalSettings));
+})(jQuery, window, Drupal, drupalSettings, loadjs);
diff --git a/core/misc/ajax.js b/core/misc/ajax.js
index 58759ac7a0..d3783c3b74 100644
--- a/core/misc/ajax.js
+++ b/core/misc/ajax.js
@@ -4,9 +4,11 @@
 * https://www.drupal.org/node/2815083
 * @preserve
 **/
+var _typeof = typeof Symbol === "function" && typeof Symbol.iterator === "symbol" ? function (obj) { return typeof obj; } : function (obj) { return obj && typeof Symbol === "function" && obj.constructor === Symbol && obj !== Symbol.prototype ? "symbol" : typeof obj; };
+
 function _toConsumableArray(arr) { if (Array.isArray(arr)) { for (var i = 0, arr2 = Array(arr.length); i < arr.length; i++) { arr2[i] = arr[i]; } return arr2; } else { return Array.from(arr); } }
 
-(function ($, window, Drupal, drupalSettings) {
+(function ($, window, Drupal, drupalSettings, loadjs) {
   Drupal.behaviors.AJAX = {
     attach: function attach(context, settings) {
       function loadAjaxBehavior(base) {
@@ -160,6 +162,8 @@ function _toConsumableArray(arr) { if (Array.isArray(arr)) { for (var i = 0, arr
 
     $.extend(this, defaults, elementSettings);
 
+    this.ajaxDeferred = null;
+
     this.commands = new Drupal.AjaxCommands();
 
     this.instanceIndex = false;
@@ -226,7 +230,14 @@ function _toConsumableArray(arr) { if (Array.isArray(arr)) { for (var i = 0, arr
         return ajax.success(response, status);
       },
       complete: function complete(xmlhttprequest, status) {
-        ajax.ajaxing = false;
+        if (ajax.ajaxDeferred !== null && ajax.ajaxDeferred.then !== null && _typeof(ajax.ajaxDeferred) === 'object' && typeof ajax.ajaxDeferred.then === 'function') {
+          ajax.ajaxDeferred.then(function () {
+            ajax.ajaxing = false;
+          });
+        } else {
+          ajax.ajaxing = false;
+        }
+
         if (status === 'error' || status === 'parsererror') {
           return ajax.error(xmlhttprequest, ajax.url);
         }
@@ -397,6 +408,8 @@ function _toConsumableArray(arr) { if (Array.isArray(arr)) { for (var i = 0, arr
   Drupal.Ajax.prototype.success = function (response, status) {
     var _this = this;
 
+    this.ajaxDeferred = $.Deferred();
+
     if (this.progress.element) {
       $(this.progress.element).remove();
     }
@@ -408,14 +421,31 @@ function _toConsumableArray(arr) { if (Array.isArray(arr)) { for (var i = 0, arr
     var elementParents = $(this.element).parents('[data-drupal-selector]').addBack().toArray();
 
     var focusChanged = false;
-    Object.keys(response || {}).forEach(function (i) {
-      if (response[i].command && _this.commands[response[i].command]) {
-        _this.commands[response[i].command](_this, response[i], status);
-        if (response[i].command === 'invoke' && response[i].method === 'focus') {
-          focusChanged = true;
+    var responseKeys = Object.keys(response);
+    responseKeys.reduce(function (deferredCommand, key, currentIndex) {
+      return deferredCommand.then(function () {
+        var command = response[key].command;
+        if (command && _this.commands[command]) {
+          if (command === 'invoke' && response[key].method === 'focus') {
+            focusChanged = true;
+          }
+
+          var result = _this.commands[command](_this, response[key], status);
+          if ((typeof result === 'undefined' ? 'undefined' : _typeof(result)) === 'object' && typeof result.then === 'function') {
+            result.done(function () {
+              if (currentIndex + 1 === responseKeys.length) {
+                _this.ajaxDeferred.resolve();
+              }
+            });
+            return result;
+          }
         }
-      }
-    });
+
+        if (currentIndex + 1 === responseKeys.length) {
+          _this.ajaxDeferred.resolve();
+        }
+      });
+    }, $.Deferred().resolve());
 
     if (!focusChanged && this.element && !$(this.element).data('disable-refocus')) {
       var target = false;
@@ -592,6 +622,40 @@ function _toConsumableArray(arr) { if (Array.isArray(arr)) { for (var i = 0, arr
           document.styleSheets[0].addImport(match[1]);
         } while (match);
       }
+    },
+    add_js: function add_js(ajax, response) {
+      var deferred = $.Deferred();
+      var scriptsSrc = response.data.map(function (script) {
+        var uniqueBundleID = script.src + ajax.instanceIndex;
+        loadjs(script.src, uniqueBundleID, {
+          async: !!script.async,
+          before: function before(path, scriptEl) {
+            var selector = 'body';
+            if (response.selector) {
+              selector = response.selector;
+            }
+            if (script.defer) {
+              scriptEl.defer = true;
+            }
+
+            var parentEl = document.querySelector(selector);
+            if (response.method === 'insertBefore') {
+              parentEl.insertBefore(scriptEl, parentEl.firstChild);
+            } else {
+              parentEl[response.method](scriptEl);
+            }
+
+            return false;
+          }
+        });
+        return uniqueBundleID;
+      });
+      loadjs.ready(scriptsSrc, {
+        success: function success() {
+          deferred.resolve();
+        }
+      });
+      return deferred.promise();
     }
   };
-})(jQuery, window, Drupal, drupalSettings);
\ No newline at end of file
+})(jQuery, window, Drupal, drupalSettings, loadjs);
\ No newline at end of file
diff --git a/core/modules/big_pipe/tests/src/FunctionalJavascript/BigPipeRegressionTest.php b/core/modules/big_pipe/tests/src/FunctionalJavascript/BigPipeRegressionTest.php
index b4da7b0a9b..653f32f8a5 100644
--- a/core/modules/big_pipe/tests/src/FunctionalJavascript/BigPipeRegressionTest.php
+++ b/core/modules/big_pipe/tests/src/FunctionalJavascript/BigPipeRegressionTest.php
@@ -105,10 +105,11 @@ public function testCommentForm_2698811() {
       $comment->save();
     }
     $this->drupalGet($node->toUrl()->toString());
+    $this->assertSession()->assertWaitOnAjaxRequest();
     // Confirm that CKEditor loaded.
     $javascript = <<<JS
     (function(){
-      return Object.keys(CKEDITOR.instances).length > 0;
+      return typeof CKEDITOR !== 'undefined' && typeof CKEDITOR.instances !== 'undefined' && Object.keys(CKEDITOR.instances).length > 0;
     }());
 JS;
     $this->assertJsCondition($javascript);
diff --git a/core/modules/quickedit/js/quickedit.es6.js b/core/modules/quickedit/js/quickedit.es6.js
index eaacc3b6c5..0df75d1558 100644
--- a/core/modules/quickedit/js/quickedit.es6.js
+++ b/core/modules/quickedit/js/quickedit.es6.js
@@ -172,12 +172,12 @@
       url: Drupal.url('quickedit/attachments'),
       submit: { 'editors[]': missingEditors },
     });
-    // Implement a scoped insert AJAX command: calls the callback after all AJAX
+    // Implement a scoped add_js AJAX command: calls the callback after all AJAX
     // command functions have been executed (hence the deferred calling).
-    const realInsert = Drupal.AjaxCommands.prototype.insert;
-    loadEditorsAjax.commands.insert = function (ajax, response, status) {
+    const realAddJsCommand = Drupal.AjaxCommands.prototype.add_js;
+    loadEditorsAjax.commands.add_js = function (ajax, response, status) {
       _.defer(callback);
-      realInsert(ajax, response, status);
+      realAddJsCommand(ajax, response, status);
     };
     // Trigger the AJAX request, which will should return AJAX commands to
     // insert any missing attachments.
diff --git a/core/modules/quickedit/js/quickedit.js b/core/modules/quickedit/js/quickedit.js
index 1c677aa1e9..67c4df9261 100644
--- a/core/modules/quickedit/js/quickedit.js
+++ b/core/modules/quickedit/js/quickedit.js
@@ -86,10 +86,10 @@
       submit: { 'editors[]': missingEditors }
     });
 
-    var realInsert = Drupal.AjaxCommands.prototype.insert;
-    loadEditorsAjax.commands.insert = function (ajax, response, status) {
+    var realAddJsCommand = Drupal.AjaxCommands.prototype.add_js;
+    loadEditorsAjax.commands.add_js = function (ajax, response, status) {
       _.defer(callback);
-      realInsert(ajax, response, status);
+      realAddJsCommand(ajax, response, status);
     };
 
     loadEditorsAjax.execute();
diff --git a/core/modules/quickedit/src/Tests/QuickEditLoadingTest.php b/core/modules/quickedit/src/Tests/QuickEditLoadingTest.php
index 6945610db1..321ff6b87e 100644
--- a/core/modules/quickedit/src/Tests/QuickEditLoadingTest.php
+++ b/core/modules/quickedit/src/Tests/QuickEditLoadingTest.php
@@ -204,8 +204,8 @@ public function testUserWithPermission() {
     $this->assertIdentical(2, count($ajax_commands), 'The attachments HTTP request results in two AJAX commands.');
     // First command: settings.
     $this->assertIdentical('settings', $ajax_commands[0]['command'], 'The first AJAX command is a settings command.');
-    // Second command: insert libraries into DOM.
-    $this->assertIdentical('insert', $ajax_commands[1]['command'], 'The second AJAX command is an append command.');
+    // Second command: add asset libraries' JS to DOM.
+    $this->assertIdentical('add_js', $ajax_commands[1]['command'], 'The second AJAX command is an append command.');
     $this->assertTrue(in_array('quickedit/quickedit.inPlaceEditor.form', explode(',', $ajax_commands[0]['settings']['ajaxPageState']['libraries'])), 'The quickedit.inPlaceEditor.form library is loaded.');
 
     // Retrieving the form for this field should result in a 200 response,
diff --git a/core/modules/system/src/Tests/Ajax/DialogTest.php b/core/modules/system/src/Tests/Ajax/DialogTest.php
index 90c548b630..c3cf6144ef 100644
--- a/core/modules/system/src/Tests/Ajax/DialogTest.php
+++ b/core/modules/system/src/Tests/Ajax/DialogTest.php
@@ -148,9 +148,9 @@ public function testDialog() {
     $this->assertTrue(in_array('core/drupal.dialog.ajax', explode(',', $ajax_result[0]['settings']['ajaxPageState']['libraries'])), 'core/drupal.dialog.ajax library is added to the page.');
     $dialog_css_exists = strpos($ajax_result[1]['data'], 'dialog.css') !== FALSE;
     $this->assertTrue($dialog_css_exists, 'jQuery UI dialog CSS added to the page.');
-    $dialog_js_exists = strpos($ajax_result[2]['data'], 'dialog-min.js') !== FALSE;
+    $dialog_js_exists = strpos(json_encode($ajax_result[2]['data']), 'dialog-min.js') !== FALSE;
     $this->assertTrue($dialog_js_exists, 'jQuery UI dialog JS added to the page.');
-    $dialog_js_exists = strpos($ajax_result[2]['data'], 'dialog.ajax.js') !== FALSE;
+    $dialog_js_exists = strpos(json_encode($ajax_result[2]['data']), 'dialog.ajax.js') !== FALSE;
     $this->assertTrue($dialog_js_exists, 'Drupal dialog JS added to the page.');
 
     // Check that the response matches the expected value.
diff --git a/core/modules/system/src/Tests/Ajax/FrameworkTest.php b/core/modules/system/src/Tests/Ajax/FrameworkTest.php
index 14bae06442..66821a3aa8 100644
--- a/core/modules/system/src/Tests/Ajax/FrameworkTest.php
+++ b/core/modules/system/src/Tests/Ajax/FrameworkTest.php
@@ -4,9 +4,8 @@
 
 use Drupal\Core\Ajax\AddCssCommand;
 use Drupal\Core\Ajax\AlertCommand;
-use Drupal\Core\Ajax\AppendCommand;
+use Drupal\Core\Ajax\AddJsCommand;
 use Drupal\Core\Ajax\HtmlCommand;
-use Drupal\Core\Ajax\PrependCommand;
 use Drupal\Core\Ajax\SettingsCommand;
 use Drupal\Core\Asset\AttachedAssets;
 
@@ -48,8 +47,8 @@ public function testOrder() {
     list($js_assets_header, $js_assets_footer) = $asset_resolver->getJsAssets($assets, FALSE);
     $js_header_render_array = $js_collection_renderer->render($js_assets_header);
     $js_footer_render_array = $js_collection_renderer->render($js_assets_footer);
-    $expected_commands[2] = new PrependCommand('head', $js_header_render_array);
-    $expected_commands[3] = new AppendCommand('body', $js_footer_render_array);
+    $expected_commands[2] = new AddJsCommand('head', $js_header_render_array, 'insertBefore');
+    $expected_commands[3] = new AddJsCommand('body', $js_footer_render_array);
     $expected_commands[4] = new HtmlCommand('body', 'Hello, world!');
 
     // Load any page with at least one CSS file, at least one JavaScript file
@@ -68,8 +67,8 @@ public function testOrder() {
     $commands = $this->drupalPostAjaxForm(NULL, [], NULL, 'ajax-test/order', [], [], NULL, []);
     $this->assertCommand(array_slice($commands, 0, 1), $expected_commands[0]->render(), 'Settings command is first.');
     $this->assertCommand(array_slice($commands, 1, 1), $expected_commands[1]->render(), 'CSS command is second (and CSS files are ordered correctly).');
-    $this->assertCommand(array_slice($commands, 2, 1), $expected_commands[2]->render(), 'Header JS command is third.');
-    $this->assertCommand(array_slice($commands, 3, 1), $expected_commands[3]->render(), 'Footer JS command is fourth.');
+    $this->assertEqual(array_slice($commands, 2, 1)[0]['data'][0]['src'], $expected_commands[2]->render()['data'][0]['#attributes']['src'], 'Header JS command is third.');
+    $this->assertEqual(array_slice($commands, 3, 1)[0]['data'][0]['src'], $expected_commands[3]->render()['data'][0]['#attributes']['src'], 'Footer JS command is fourth.');
     $this->assertCommand(array_slice($commands, 4, 1), $expected_commands[4]->render(), 'HTML command is fifth.');
   }
 
@@ -173,8 +172,8 @@ public function testLazyLoad() {
     // unexpected JavaScript code, such as a jQuery.extend() that would
     // potentially clobber rather than properly merge settings, didn't
     // accidentally get added.
-    $this->assertTrue(in_array($expected['library_2'], $new_libraries), format_string('Page state now has the %library library.', ['%library' => $expected['library_2']]));
-    $this->assertCommand(array_slice($commands, 2, 1), ['data' => $expected_js_html], format_string('Page now has the %library library.', ['%library' => $expected['library_2']]));
+    $system_js_exists = strpos($commands[2]['data'][0]['src'], 'modules/system/js/system.js') !== FALSE;
+    $this->assertTrue($system_js_exists, format_string('Page now has the %library library.', ['%library' => $expected['library_2']]));
   }
 
   /**