drupal/core/modules/book/book.es6.js

/**
 * @file
 * Javascript behaviors for the Book module.
 */

(function ($, Drupal) {
  /**
   * Adds summaries to the book outline form.
   *
   * @type {Drupal~behavior}
   *
   * @prop {Drupal~behaviorAttach} attach
   *   Attaches summary behavior to book outline forms.
   */
  Drupal.behaviors.bookDetailsSummaries = {
    attach(context) {
      $(context).find('.book-outline-form').drupalSetSummary((context) => {
        const $select = $(context).find('.book-title-select');
        const val = $select.val();

        if (val === '0') {
          return Drupal.t('Not in book');
        }
        else if (val === 'new') {
          return Drupal.t('New book');
        }

        return Drupal.checkPlain($select.find(':selected').text());
      });
    },
  };
}(jQuery, Drupal));