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>