ActionController::UnknownFormat when using respond_to do |format|

I am using jquery slider ui and rails 4. I am trying to implement a price slider for my product page. However, i have run into a problem. I have tried to search around but i cant seems to find a solution that helps. Hope someone could help me as I am quite new to ruby on rails. Thanks.

Here are my code

products_controller.rb

 respond_to do |format|
  format.js do
    render :update do |page|
      page.replace_html 'x_product_list', :partial => 'products/products_list', :locals => { :product => @product }

with this error:

You have not really given us enough information. Show us the contents
of the log for the request.

Colin