The file login_link_helper_CLASS.js needs to be added to the HTML folder (and referenced appropriately in the HTML, e.g. <script src="login_link_helper.js"></script>), when you are using multiple instances of the login/logout class selectors, e.g. class="login_link".
The file login_link_helper_ID.js can be used where only one instance of the login/logout button is required and therefore the HTML uses an ID selector, e.g. id="login_link", rather than a CLASS selector.