Say you have a document <root xmlns="https://weblogs.mozillazine.org/weirdal/namespaces/root/"/>. Your default namespace has no prefix for DOM XPath to look up. So if you try to do this:

  var resolver = document.createNSResolver();
var result = document.evaluate("//root", document, resolver,
Components.interfaces.nsIDOMXPathResult.ANY_UNORDERED_NODE_TYPE, null);
alert("Node: " + result.singleNodeValue + "\n");

It’s not going to work. You can’t get to it using Mozilla’s default namespace resolver. Instead, you have to create your own:

  var resolver = {
lookupNamespaceURI: function lookup(aPrefix) {
if (aPrefix == "default") {
return document.documentElement.namespaceURI;
}
}
}
var result = document.evaluate("//default:root", document, resolver,
Components.interfaces.nsIDOMXPathResult.ANY_UNORDERED_NODE_TYPE, null);
alert("Node: " + result.singleNodeValue + "\n");

That will alert a node instead of throwing an exception. It’d really, really be nice if Mozilla’s native XPathNSResolver was smart enough to actually give you a default prefix. Of course, to do that in any sane way, you’d probably have to add a matching lookupPrefix() method, so that if you fed lookupPrefix() a default namespace URI, it would spit back a prefix you could use with your xpath evaluation.

It took me several hours to figure this out, and then only with the help of a devmo-linked tutorial on XPath basics. I want to file a bug on this, but I’m not sure the bug would be valid. If it were valid, I’d fix it myself in a matter of minutes. (It’s worth noting the DOM 3 XPath spec is a Working Group Note, so we’re not under any obligations whatsoever to not do anything to the native resolver.)

UPDATE: Err, I spoke too soon about a matter of minutes, before writing any code on it. If I have <root xmlns="https://weblogs.mozillazine.org/weirdal/namespaces/root/"><real xmlns=https://weblogs.mozillazine.org/weirdal/namespaces/real/"/></root>, then I’m a wee bit hosed at real. I need to work out a JS-based solution first.

UPDATE 2 Hm. “//xmlns:root” seems to work just fine

. I remembered something about default namespaces. An attribute xmlns=”foo” has a namespaceURI of “http://www.w3.org/2000/xmlns/”, not null. So that works, and actually fits my needs perfectly. Nothing to see here, move along.

UPDATE 3 I realized about an hour ago that the xmlns: prefix in XPath only covers the namespace URI of the resolver’s initializing node; other nodes with different default namespaces are still lost. Plus, XPath is the wrong solution for the problem I had in mind anyway. Back to the drawing board…