Note this is a fairly old plugin, js_named_routes is a more up to date and fully featured take on the same problem.
Include '/javascripts/js_routes.js' in your HTML and it makes your routes available in javascript.
<%= javascript_include_tag :js_routes %>
In rails:
message_url(@message, :format => 'js')
Javascript:
var message_id = 1;
message_url(message_id, {format: 'js'})
These are built on the fly, to cache them for production run the task:
$rake js_routes:cache