(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.
   */
  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 e
     *   Event object.
     * @param 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.
     */
    valueSourceBlurHandler: function (e) {
      var value = $(e.target).val();
      var targetIds = drupalSettings.copyFieldValue[e.target.id];
      $('#' + targetIds.join(', #')).trigger('value:copy', value);
    }
  };

})(jQuery, Drupal, drupalSettings);