dollarX.js

// extend version of $X
function $exp (exp, ownerDocument) {
	if (!ownerDocument)
		ownerDocument = document;
	return ownerDocument.createExpression(exp, function (prefix) {
		var o = document.createNSResolver(context)(prefix);
		if (o) return o;
		return (document.contentType == "application/xhtml+xml") ? "http://www.w3.org/1999/xhtml" : "";
	});
}

// $X(exp);
// $X(exp, context);
// $X(exp, type);
// $X(exp, context, type);
function $X (exp, context, type /* want type */) {
	if (context !== null && !(context instanceof Node)) {
		type    = context;
		context = null;
	}
	if (!context) context = document;

	if (typeof exp == 'string' || exp instanceof String)
		exp = $exp(exp, context.ownerDocument || context);

	switch (type) {
	case Number:  type = XPathResult.NUMBER_TYPE;  break;
	case String:  type = XPathResult.STRING_TYPE;  break;
	case Boolean: type = XPathResult.BOOLEAN_TYPE; break;
	case Array:   type = XPathResult.ORDERED_NODE_SNAPSHOT_TYPE; break;
	case Node:    type = XPathResult.FIRST_ORDERED_NODE_TYPE;    break;
	}

	var result = exp.evaluate(context, type, null), ret, i, len;
	switch (result.resultType) {
	case XPathResult.NUMBER_TYPE:  return result.numberValue;
	case XPathResult.STRING_TYPE:  return result.stringValue;
	case XPathResult.BOOLEAN_TYPE: return result.booleanValue;
	case XPathResult.UNORDERED_NODE_ITERATOR_TYPE:
	case XPathResult.ORDERED_NODE_ITERATOR_TYPE:
		ret = []; i = null;
		while ((i = result.iterateNext()) != null)
			ret.push(i);
		return ret;
	case XPathResult.UNORDERED_NODE_SNAPSHOT_TYPE:
	case XPathResult.ORDERED_NODE_SNAPSHOT_TYPE:
		ret = [];
		for (i = 0, len = result.snapshotLength; i < len; i++)
		ret.push(result.snapshotItem(i));
		return ret;
	case XPathResult.FIRST_ORDERED_NODE_TYPE: return result.singleNodeValue;
	}
	return null;
}