routes_js.rb

# Hi! I'am rack middleware!
# I was born for show right way to you JavaScript

# My author's name was Aleksandr Koss. Mail him at kossnocorp@gmail.com
# Nice to MIT you!

require(File.dirname(__FILE__) + '/../config/environment') unless defined?(Rails)

class RoutesJs
  def initialize app, options = {}
    @app = app
    @options = options
  end

  def call env
    if env['PATH_INFO'] =~ /^\/javascripts\/routes.js$/
      response = rails_route_function
      
      all_routes = ENV['CONTROLLER'] ? Rails.application.routes.routes.select { |route| route.defaults[:controller] == ENV['CONTROLLER'] } : Rails.application.routes.routes
      routes = all_routes.collect do |route|
        host = "#{env['rack.url_scheme']}://#{env['HTTP_HOST']}"

        path = route.path+"?"
        key_method = Hash.method_defined?('key') ? 'key' : 'index'

        #named_route = ActionController::Routing::Routes.named_routes.routes.index(route).to_s
        named_route = Rails.application.routes.named_routes.routes.send(key_method, route).to_s

        if named_route != ''
          response << route_function("#{named_route}_path", path)
          response << route_function("#{named_route}_url",  host + path)
        end
      end

      [200, {'Content-Type' => 'application/javascript'}, [response]]
    else
      @app.call env
    end
  end

private

  def rails_route_function
<<EOF
function rails_route(path, var_pairs) {
  var pair_index = 0,
      path_copy = path;
  for(; pair_index < var_pairs.length; pair_index++) {
    path_copy = path_copy.replace(
      ':' + var_pairs[pair_index][0],
      var_pairs[pair_index][1]
    );
  }
  return(
    path_copy
      .replace(/[(][.]:.+[)][?]/g, '')
      .replace(/[(]|[)][?]/g, '')
  );
}
EOF
  end

  def route_function name, path
<<EOF
function #{name}(variables) {
  var var_pairs = [];
  for(var key in variables) {
    var_pairs.push([key, variables[key]]);
  }
  return(rails_route('#{path}', var_pairs));
}
EOF
  end
end

With Rails 3 Support!