subbu.org

Ever wonder why the method getElementById returns null when applied to responseXML of XMLHttpRequest?

Thomas Rabaix discusses about this problem briefly in his Issues when developing AJAX Libraries. Here is a more detailed answer.

In the snippet below, the value of getElementById() would be null.

request.onreadystatechange = function()
{
if(request.readyState == 4) {
if(request.status == 200) {
var info = request.responseXML.getElementById("someid");
alert(info);
}
}
};

In XML, attribute with name id is not automatic. For an element to have an attribute named id, an attribute with name id and type ID must be defined in a schema or DTD underlying an XML document. For instance, XHTML defines such an attribute. The ID type is defined in XML 1.0 and further in XML Schema to be an attribute with a value which must be unique within a given document.

DOM Level 2 describes the behavior of getElementById().

Further, xml:id Version 1.0 encourages schemas to declare attributes named xml:id with the type xs:ID.

The bottomline is that, unless an attribute of type ID is known to the DOM implementation, this method with return null.

For responseXML.getElementById() to return an element with the matching id value, XMLHttpRequest implementations must be aware of the underlying schema/DTD that defines an id attribute of type ID. Currently browsers are not schema/DTD aware for XMLHttpRequests, although they support well-known DTDs like HTML and XHTML for documents.