diff --git a/core/includes/common.inc b/core/includes/common.inc
index 27c9c54..2731dc1 100644
--- a/core/includes/common.inc
+++ b/core/includes/common.inc
@@ -20,6 +20,7 @@
 use Drupal\Component\Utility\String;
 use Drupal\Component\Utility\Tags;
 use Drupal\Component\Utility\UrlHelper;
+use Drupal\Core\Asset\AttachedAssets;
 use Drupal\Core\Cache\Cache;
 use Drupal\Core\Language\LanguageInterface;
 use Drupal\Core\Site\Settings;
@@ -889,15 +890,12 @@ function drupal_js_defaults($data = NULL) {
  *
  * @return array
  *   The merged #attached array.
+ *
+ * @deprecated To be removed in Drupal 8.0.x. Use
+ *   \Drupal\Core\Asset\AttachedAssets::mergeAttachments() instead.
  */
 function drupal_merge_attached(array $a, array $b) {
-  // If both #attached arrays contain drupalSettings, then merge them correctly;
-  // adding the same settings multiple times needs to behave idempotently.
-  if (!empty($a['drupalSettings']) && !empty($b['drupalSettings'])) {
-    $a['drupalSettings'] = NestedArray::mergeDeepArray([$a['drupalSettings'], $b['drupalSettings']], TRUE);
-    unset($b['drupalSettings']);
-  }
-  return NestedArray::mergeDeep($a, $b);
+  return AttachedAssets::mergeAttachments($a, $b);
 }
 
 /**
diff --git a/core/lib/Drupal/Core/Ajax/AjaxResponse.php b/core/lib/Drupal/Core/Ajax/AjaxResponse.php
index 343c813..766026d 100644
--- a/core/lib/Drupal/Core/Ajax/AjaxResponse.php
+++ b/core/lib/Drupal/Core/Ajax/AjaxResponse.php
@@ -77,7 +77,7 @@ public function addCommand(CommandInterface $command, $prepend = FALSE) {
         'library' => $assets->getLibraries(),
         'drupalSettings' => $assets->getSettings(),
       ];
-      $attachments = drupal_merge_attached($this->attachments, $attachments);
+      $attachments = AttachedAssets::mergeAttachments($this->attachments, $attachments);
       $this->setAttachments($attachments);
     }
 
diff --git a/core/lib/Drupal/Core/Asset/AttachedAssets.php b/core/lib/Drupal/Core/Asset/AttachedAssets.php
index d94618e..f0ef5ec 100644
--- a/core/lib/Drupal/Core/Asset/AttachedAssets.php
+++ b/core/lib/Drupal/Core/Asset/AttachedAssets.php
@@ -5,6 +5,7 @@
  */
 
 namespace Drupal\Core\Asset;
+use Drupal\Component\Utility\NestedArray;
 
 /**
  * The default attached assets collection.
@@ -51,6 +52,59 @@ public static function createFromRenderArray(array $render_array) {
   }
 
   /**
+   * Merges two #attached arrays.
+   *
+   * The values under the 'drupalSettings' key are merged in a special way, to
+   * match the behavior of
+   *
+   * @code
+   *   jQuery.extend(true, {}, $settings_items[0], $settings_items[1], ...)
+   * @endcode
+   *
+   * This means integer indices are preserved just like string indices are,
+   * rather than re-indexed as is common in PHP array merging.
+   *
+   * Example:
+   * @code
+   * function module1_page_attachments(&$page) {
+   *   $page['a']['#attached']['drupalSettings']['foo'] = ['a', 'b', 'c'];
+   * }
+   * function module2_page_attachments(&$page) {
+   *   $page['#attached']['drupalSettings']['foo'] = ['d'];
+   * }
+   * // When the page is rendered after the above code, and the browser runs the
+   * // resulting <SCRIPT> tags, the value of drupalSettings.foo is
+   * // ['d', 'b', 'c'], not ['a', 'b', 'c', 'd'].
+   * @endcode
+   *
+   * By following jQuery.extend() merge logic rather than common PHP array merge
+   * logic, the following are ensured:
+   * - Attaching JavaScript settings is idempotent: attaching the same settings
+   *   twice does not change the output sent to the browser.
+   * - If pieces of the page are rendered in separate PHP requests and the
+   *   returned settings are merged by JavaScript, the resulting settings are
+   *   the same as if rendered in one PHP request and merged by PHP.
+   *
+   * @param array $a
+   *   An #attached array.
+   * @param array $b
+   *   Another #attached array.
+   *
+   * @return array
+   *   The merged #attached array.
+   */
+  public static function mergeAttachments(array $a, array $b) {
+    // If both #attached arrays contain drupalSettings, then merge them
+    // correctly; adding the same settings multiple times needs to behave
+    // idempotently.
+    if (!empty($a['drupalSettings']) && !empty($b['drupalSettings'])) {
+      $a['drupalSettings'] = NestedArray::mergeDeepArray([$a['drupalSettings'], $b['drupalSettings']], TRUE);
+      unset($b['drupalSettings']);
+    }
+    return NestedArray::mergeDeep($a, $b);
+  }
+
+  /**
    * {@inheritdoc}
    */
   public function setLibraries(array $libraries) {