diff --git a/core/modules/book/book.js b/core/modules/book/book.js
index e67c6a2..9a398db 100644
--- a/core/modules/book/book.js
+++ b/core/modules/book/book.js
@@ -8,6 +8,8 @@
   "use strict";
 
   /**
+   * Adds summaries to the book outline form.
+   *
    * @type {Drupal~behavior}
    */
   Drupal.behaviors.bookDetailsSummaries = {