action.rb

# it's not critical for understanding
# but here is the associated server-side action
#

  def version
    @program = load_program!
    @version = @program.versions.find(params[:version_id])

    if stale?(:last_modified => @version.updated_at, :etag => @version.cache_key)
      render
    end
  end

quiz.js

// today's quiz is simple:
//
// fork this code and explain, in english, what it does.
//
// suggest simpler alternatives.
//

              if(state != 'complete'){

                var get = function(){
                  jq.ajax({
                    'type'    : 'GET',
                    'url'     : url,
                    'cache'   : false,

                    'success' : function(html){
                      version.replaceWith(html);
                    }
                  });
                };

                var head = function(callback){
                  jq.ajax({
                    'type'    : 'HEAD',
                    'url'     : url,

                    'success' : function(res, code, xhr){
                      var server_last_modified = xhr.getResponseHeader("Last-Modified");

                      if(Date.parse(server_last_modified) > Date.parse(last_modified)){
                        callback();
                      };
                    }
                  });
                };

                head(get);
              };