req.xhrA flag indicating whether the current request (req) appears to be an AJAX request (i.e. it was issued with its "X-Requested-With" header set to "XMLHttpRequest").
req.xhr;
if (req.xhr) {
  // Yup, it's AJAX alright.
}
- Whenever possible, you should prefer the
 req.wantsJSONflag. Avoid writing custom content negotiation logic into your app, as it makes your code more brittle and verbose.