JSFiddle - React, Tailwind, and code Playground

HTML

<body class="logged-in">
  <li class="menu-item menu-item-type-post_type menu-item-object-page logged-out-button"><a href="http://www.savingaddicts.com/wp-login.php?action=register"><span>Register</span></a>
  </li>
  <li class="menu-item menu-item-type-post_type menu-item-object-page logged-out-button"><a href="http://www.savingaddicts.com/wp-login.php?action=login"><span>Login</span></a>
  </li>
  <li class="menu-item menu-item-type-post_type menu-item-object-page logged-in-button"><a href="http://www.savingaddicts.com/wp-login.php?action=logout"><span>Logout</span></a>
  </li>
</body>

JavaScript

jQuery(document).ready(function($) {
  if ($("body").hasClass("logged-in")) {
    $(".logged-out-button").css("cssText", "display:none !important");
    $(".logged-in-button").css("cssText", "display:inline !important");
  }
});