An XPathExpression is a compiled XPath query returned from document.createExpression(). It has a method evaluate() which can be used to execute the compiled XPath.
XMLHttpRequest makes sending HTTP requests very easy.  You simply create an instance of the object, open a URL, and send the request.  The HTTP status of the result, as well as the result's contents, are available in the request object when the transaction is completed. This page outlines some of the common, and even slightly obscure, use cases for this powerful JavaScript object.