Index: lib/notificationHelper.js
===================================================================
--- a/lib/notificationHelper.js
+++ b/lib/notificationHelper.js
@@ -82,17 +82,17 @@
     {
       buttons = [
         {
           type: "open-all",
           title: ext.i18n.getMessage("notification_open_all")
         }
       ];
     }
-    if (["critical", "relentless"].indexOf(notificationType) == -1)
+    if (!["critical", "relentless"].includes(notificationType))
     {
       buttons.push({
         type: "configure",
         title: ext.i18n.getMessage("notification_configure")
       });
     }
   }
 
@@ -265,17 +265,17 @@
  *
  * @param {string} method Display method: icon, notification or popup
  * @param {string} notificationType
  * @return {boolean}
  */
 exports.shouldDisplay = (method, notificationType) =>
 {
   let methods = displayMethods[notificationType] || defaultDisplayMethods;
-  return methods.indexOf(method) > -1;
+  return methods.includes(method);
 };
 
 ext.pages.onLoading.addListener(page =>
 {
   NotificationStorage.showNext(stringifyURL(page.url));
 });
 
 NotificationStorage.addShowListener(showNotification);