core/lib/Drupal/Core/Ajax/AjaxResponse.php            | 18 ++++++++----------
 .../system/src/Tests/Common/JavaScriptTest.php        | 10 ++++++++--
 .../tests/modules/common_test/common_test.module      | 15 +++++++++++++++
 core/modules/system/theme.api.php                     | 19 ++++++++++++++++++-
 4 files changed, 49 insertions(+), 13 deletions(-)

diff --git a/core/lib/Drupal/Core/Ajax/AjaxResponse.php b/core/lib/Drupal/Core/Ajax/AjaxResponse.php
index 2ae1853..df8a776 100644
--- a/core/lib/Drupal/Core/Ajax/AjaxResponse.php
+++ b/core/lib/Drupal/Core/Ajax/AjaxResponse.php
@@ -129,8 +129,8 @@ protected function ajaxRender(Request $request) {
     // HTML in the page. We pass TRUE as the $skip_alter argument to prevent the
     // data from being altered again, as we already altered it above. Settings
     // are handled separately, afterwards.
-    if (isset($items['js']['settings'])) {
-      unset($items['js']['settings']);
+    if (isset($items['js']['drupalSettings'])) {
+      unset($items['js']['drupalSettings']);
     }
     $styles = drupal_get_css($items['css'], TRUE);
     $scripts_footer = drupal_get_js('footer', $items['js'], TRUE, TRUE);
@@ -153,17 +153,15 @@ protected function ajaxRender(Request $request) {
 
     // Prepend a command to merge changes and additions to drupalSettings.
     $scripts = _drupal_add_js();
-    if (!empty($scripts['settings'])) {
-      $settings = drupal_merge_js_settings($scripts['settings']['data']);
+    if (!empty($scripts['drupalSettings'])) {
+      $settings = drupal_merge_js_settings($scripts['drupalSettings']['data']);
       // During Ajax requests basic path-specific settings are excluded from
       // new drupalSettings values. The original page where this request comes
-      // from already has the right values for the keys below. An Ajax request
-      // would update them with values for the Ajax request and incorrectly
-      // override the page's values.
+      // from already has the right values. An Ajax request would update them
+      // with values for the Ajax request and incorrectly override the page's
+      // values.
       // @see _drupal_add_js()
-      foreach (array('basePath', 'currentPath', 'scriptPath', 'pathPrefix') as $item) {
-        unset($settings[$item]);
-      }
+      unset($settings['path']);
       $this->addCommand(new SettingsCommand($settings, TRUE), TRUE);
     }
 
diff --git a/core/modules/system/src/Tests/Common/JavaScriptTest.php b/core/modules/system/src/Tests/Common/JavaScriptTest.php
index a8a4874..e862496 100644
--- a/core/modules/system/src/Tests/Common/JavaScriptTest.php
+++ b/core/modules/system/src/Tests/Common/JavaScriptTest.php
@@ -77,11 +77,11 @@ function testAddSetting() {
     $attached['#attached']['library'][] = 'core/drupalSettings';
     $this->render($attached);
     $javascript = _drupal_add_js();
-    $last_settings = reset($javascript['settings']['data']);
+    $last_settings = reset($javascript['drupalSettings']['data']);
     $this->assertTrue(array_key_exists('currentPath', $last_settings['path']), 'The current path JavaScript setting is set correctly.');
 
     $javascript = _drupal_add_js(array('drupal' => 'rocks', 'dries' => 280342800), 'setting');
-    $last_settings = end($javascript['settings']['data']);
+    $last_settings = end($javascript['drupalSettings']['data']);
     $this->assertEqual(280342800, $last_settings['dries'], 'JavaScript setting is set correctly.');
     $this->assertEqual('rocks', $last_settings['drupal'], 'The other JavaScript setting is set correctly.');
   }
@@ -241,6 +241,12 @@ function testHeaderSetting() {
     $settings_two['moduleName']['thingiesOnPage']['id1'] = array();
     $this->assertIdentical($settings_one, $parsed_settings['commonTestRealWorldIdentical'], '_drupal_add_js handled real world test case 1 correctly.');
     $this->assertEqual($settings_two, $parsed_settings['commonTestRealWorldAlmostIdentical'], '_drupal_add_js handled real world test case 2 correctly.');
+
+    // Tests whether altering JavaScript settings via hook_js_settings_alter()
+    // is working as expected.
+    // @see common_test_js_settings_alter()
+    $this->assertIdentical($parsed_settings['locale']['pluralDelimiter'], '☃');
+    $this->assertIdentical($parsed_settings['foo'], 'bar');
   }
 
   /**
diff --git a/core/modules/system/tests/modules/common_test/common_test.module b/core/modules/system/tests/modules/common_test/common_test.module
index 9cf802d..1148810 100644
--- a/core/modules/system/tests/modules/common_test/common_test.module
+++ b/core/modules/system/tests/modules/common_test/common_test.module
@@ -289,3 +289,18 @@ function common_test_page_attachments_alter(array &$page) {
     ];
   }
 }
+
+/**
+ * Implements hook_js_settings_alter().
+ *
+ * @see \Drupal\system\Tests\Common\JavaScriptTest::testHeaderSetting()
+ */
+function common_test_js_settings_alter(&$settings) {
+  // Modify an existing setting.
+  if (isset($settings['locale']['pluralDelimiter'])) {
+    $settings['locale']['pluralDelimiter'] = '☃';
+  }
+
+  // Add a setting.
+  $settings['foo'] = 'bar';
+}
diff --git a/core/modules/system/theme.api.php b/core/modules/system/theme.api.php
index 5747ff9..85a6224 100644
--- a/core/modules/system/theme.api.php
+++ b/core/modules/system/theme.api.php
@@ -717,8 +717,25 @@ function hook_js_alter(&$javascript) {
   $javascript['core/assets/vendor/jquery/jquery.js']['data'] = drupal_get_path('module', 'jquery_update') . '/jquery.js';
 }
 
+/**
+ * Perform necessary alterations to the JavaScript settings (drupalSettings).
+ *
+ * @param array &$settings
+ *   An array of all JavaScript settings (drupalSettings) being presented on the
+ *   page.
+ *
+ * @see _drupal_add_js()
+ * @see drupal_get_js()
+ * @see drupal_js_defaults()
+ */
 function hook_js_settings_alter(array &$settings) {
-  // @todo document…
+  // Add settings.
+  $settings['user']['uid'] = \Drupal::currentUser();
+
+  // Manipulate settings.
+  if (isset($settings['dialog'])) {
+    $settings['dialog']['autoResize'] = FALSE;
+  }
 }
 
 /**