responseXML always null

Try to open the value of url directly in the browser. You should get some error information.
If you see a parsing error, chances are your encoding is wrong and you have a special character in your XML that makes it invalid.

To avoid that, you need to be sure that all the chain is properly encoded.

If it is a static XML file, you need to set correctly your editor encoding when saving it. The encoding that does it all(almost) is UTF-8, it is usually a property you can choose in your editor settings or in the save dialog.

If it is dynamically generated. Your data, the page and the server response must be properly encoded too. And your XML starting with <?xml version="1.0" encoding="UTF-8"?>

You can try first with a very basic and static XML:

<?xml version="1.0" encoding="UTF-8"?><root>hi</root>

And then add the steps, one by one to make it like yours, without breaking it.

I bet you are violating the same origin policy.

For XHRs, you must have the same protocol, domain, port, etc. So if you are running an app on localhost:8080/app, you CANNOT ajax to www.cnn.com.

Different browsers handle this differently; I have seen FF do what you describe, which is the request appears to return normally but there is no data...

Besides the cross-domain issues already mentioned, responseXML requires completely valid XML and probably the correct Content-Type in the response headers sent from the server. It is very unlikely that either of these requirements would be met by the average website.

For the latter issue, you can use

xmlhttp.overrideMimeType('application/xml');

before you send the request to force the response to be interperted as XML. Still if the response is not valid XML, you will only get null.

If i recall correctly , this is a known problem with firefox ( i have had the same problem before ).

The fix is to parse the responseText back to an XML document , and then use this.

Something like this :

var parser = new DOMParser();
var xmlDoc = parser.parseFromString(xmlhttp.responseText, "application/xml");