Use classList#toggle instead of classList[whatever ? 'add' : 'remove'].

chrome/browser/resources/chromeos/login/screen_locally_managed_user_creation.js

 // Copyright (c) 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 /**
  * @fileoverview Locally managed user creation flow screen.
  */
 
 cr.define('login', function() {
 
@@ skipping 448 matching lines @@
 
       this.setButtonDisabledStatus('next', true);
     },
 
     /**
      * True if user name error should be displayed.
      * @type {boolean}
      */
     set nameErrorVisible(value) {
       $('managed-user-creation-flow-name-error').
-          classList[value ? 'add' : 'remove']('error');
+          classList.toggle('error', value);
       $('managed-user-creation-flow-name').
-          classList[value ? 'add' : 'remove']('duplicate-name');
+          classList.toggle('duplicate-name', value);
       if (!value)
         $('managed-user-creation-flow-name-error').textContent = '';
     },
 
     /**
      * True if user name error should be displayed.
      * @type {boolean}
      */
     set passwordErrorVisible(value) {
       $('managed-user-creation-flow-password-error').
-          classList[value ? 'add' : 'remove']('error');
+          classList.toggle('error', value);
       if (!value)
         $('managed-user-creation-flow-password-error').textContent = '';
     },
 
     /**
      * Updates state of Continue button after minimal checks.
      * @return {boolean} true, if form seems to be valid.
      * @private
      */
     updateNextButtonForManager_: function() {
@@ skipping 295 matching lines @@
 
   LocallyManagedUserCreationScreen.loadManagers = function(userList) {
     var screen = $('managed-user-creation-flow');
     screen.loadManagers(userList);
   };
 
   return {
     LocallyManagedUserCreationScreen: LocallyManagedUserCreationScreen
   };
 });