JS alter a rails form or have rails form variants conditionally on JS?

I have a form with a JavaScript requirement because the page loads a JS file that creates a form input field with a certain Facebook- autocomplete kind of style:

<%= javascript_include_tag "form_prettify_library" %>

<div class="box">   <ul id="first_form" class="tagit"><li class="prettify-new"></li></ul> <%= form_for @brand do |f| %>   <%= f.hidden_field :tag_list, :value => @brand.tags_from(current_user).join(", "),              :id => "mytaglist" %>     <%= f.submit "Add tags" %>     <%= f.error_messages %> <% end %> </div>

It works well, however without JavaScript the form doesn't work because the form above only has a hidden_field and no visible entry field, and I need this to work without JavaScript. Is there any way to show the code above if JavaScript is loaded, otherwise the code below can display?

<div class="box"> <%= form_for @brand do |f| %>

  <%= f.label :tag_list, "Your tags" %>         <%= f.text_field :tag_list, :value => @brand.all_tags_list %>         <%= f.submit "Add tags" %> <% end %> </div>