看了 railscasts 的一个视频 Endless Page,想自己实现一个。可是怎么也跑不起来。

#controller
  def index 
   @products = Product.paginate(page: params[:page],per_page: 15)
  end
#view /index.js.rjs
 page.insert_html :bottom, :products, :partial => 'products'
if @products.total_pages > @products.current_page
  page.call 'checkScroll'
else
  page[:loading].hide
end

#assets/javascript/endless_page.js
var currentPage = 1;
function checkScroll() {
  if (nearBottomOfPage()) {
    currentPage++;
    new Ajax.Request('/products.js?page=' + currentPage, {asynchronous:true, evalScripts:true, method:'get'});
  } else {
    setTimeout("checkScroll()", 250);
  }
}

function nearBottomOfPage() {
  return scrollDistanceFromBottom() < 150;
}

function scrollDistanceFromBottom(argument) {
  return pageHeight() - (window.pageYOffset + self.innerHeight);
}

function pageHeight() {
  return Math.max(document.body.scrollHeight, document.body.offsetHeight);
}

document.observe('dom:loaded', checkScroll);
这里还出现了js错误
##Uncaught TypeError: undefined is not a function 

关于这个加载更多的应该如何实现的吗？