Error handling in javascript

See more:

Javascript

Below is the Javascript for a page I am working as a demo of how to do AJAX style background tasks. The onchange event of a select control calls the AjaxRequest function.

If the code being executed at the address in the url variable returns a 500 status, it ends up executing the line after catch(err). I thought it would have executed the alert and not called AjaxResults at all. In Chrome with the console showing, it shows that the send results in a 500.

The code works great when there isn't an error. I need to know why the error handling isn't working.

What I have tried:

function removeOptions(selectbox)
{
   var i;
   for(i = selectbox.options.length - 1 ; i >= 0 ; i--) {
      selectbox.remove(i);
   }
}

function AjaxResults(xhr, funct) {
   var xmldoc = xhr.responseXML;
   // process the returned data
   switch (funct) {
      case 1:
         try {
            ddlbProducts = document.getElementById("products");
            removeOptions(ddlbProducts);
            var products = xmldoc.getElementsByTagName("d_products_row");
            for (var i = 0; i < products.length; i++) {   
               var prodname  = xmldoc.getElementsByTagName('productname').item(i).firstChild.data;
               var productid = xmldoc.getElementsByTagName('productid').item(i).firstChild.data;
               var option = document.createElement("option");
               option.innerHTML = prodname;
               option.value = productid;
               ddlbProducts.options.add(option);
            }
         }
         catch(err) {
            document.documentElement.innerHTML = xhr.responseText;
         }
         break;
      default:
         alert('Unsupported function: ' + funct);
   }
}

function AjaxRequest(url, funct) {
   xhr = new XMLHttpRequest();

   xhr.onload = function () {
      if (xhr.status = 200) {
         AjaxResults(xhr, funct);
      } else {
         alert('Error: ' + xhr.statusText);
      }
   };

   xhr.open('GET', url, true);
   xhr.send();
}