Parsing XML response with namespace in javascript

In my app using JavaScript i am calling a web-service and getting the following response

<?xml version='1.0' encoding='UTF-8'?>
<soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/">
<soapenv:Body><ns:createNewUserResponse xmlns:ns="http://ITAHTA.org.com">
<ns:return>2341</ns:return>
</ns:createNewUserResponse>
</soapenv:Body>
</soapenv:Envelope>

Now i want to parse this response , i want to get the value in the ns:return tag and store it in a variable in javascript.

I tried a below

xhr.onreadystatechange = function() {
    if (xhr.readyState == 4) {
        alert(xhr.responseText.getElementsByTagName("return"));

    }
}

But i am not getting any value. Please let me know the correct way to parse it.