Drupal 8.0.0 beta 12. More info: https://www.drupal.org/node/2514176

core/modules/system/js/system.js

@@ -0,0 +1,74 @@
+/**
+ * @file
+ * System behaviors.
+ */
+
+(function ($, Drupal, drupalSettings) {
+
+  "use strict";
+
+  // Cache IDs in an array for ease of use.
+  var ids = [];
+
+  /**
+   * When a field is filled out, apply its value to other fields that will likely
+   * use the same value. In the installer this is used to populate the
+   * administrator email address with the same value as the site email address.
+   *
+   * @type {Drupal~behavior}
+   */
+  Drupal.behaviors.copyFieldValue = {
+    attach: function (context) {
+      // List of fields IDs on which to bind the event listener.
+      // Create an array of IDs to use with jQuery.
+      for (var sourceId in drupalSettings.copyFieldValue) {
+        if (drupalSettings.copyFieldValue.hasOwnProperty(sourceId)) {
+          ids.push(sourceId);
+        }
+      }
+      if (ids.length) {
+        // Listen to value:copy events on all dependent fields.
+        // We have to use body and not document because of the way jQuery events
+        // bubble up the DOM tree.
+        $('body').once('copy-field-values').on('value:copy', this.valueTargetCopyHandler);
+        // Listen on all source elements.
+        $('#' + ids.join(', #')).once('copy-field-values').on('blur', this.valueSourceBlurHandler);
+      }
+    },
+    detach: function (context, settings, trigger) {
+      if (trigger === 'unload' && ids.length) {
+        $('body').removeOnce('copy-field-values').off('value:copy');
+        $('#' + ids.join(', #')).removeOnce('copy-field-values').off('blur');
+      }
+    },
+
+    /**
+     * Event handler that fill the target element with the specified value.
+     *
+     * @param {jQuery.Event} e
+     *   Event object.
+     * @param {string} value
+     *   Custom value from jQuery trigger.
+     */
+    valueTargetCopyHandler: function (e, value) {
+      var $target = $(e.target);
+      if ($target.val() === '') {
+        $target.val(value);
+      }
+    },
+
+    /**
+     * Handler for a Blur event on a source field.
+     *
+     * This event handler will trigger a 'value:copy' event on all dependent fields.
+     *
+     * @param {jQuery.Event} e
+     */
+    valueSourceBlurHandler: function (e) {
+      var value = $(e.target).val();
+      var targetIds = drupalSettings.copyFieldValue[e.target.id];
+      $('#' + targetIds.join(', #')).trigger('value:copy', value);
+    }
+  };
+
+})(jQuery, Drupal, drupalSettings);