With iron-ajax, how to read the headers of a response?

If an HTTP request is made over AJAX in javascript, it is possible to get the response headers with the getAllResponseHeaders() method. It's part of the XMLHttpRequest API.

var req = new XMLHttpRequest();
req.open('GET', document.location, false);
req.send(null);
var headers = req.getAllResponseHeaders().toLowerCase();
alert(headers);

EDIT:

I just noticed the iron-ajax part of question.

<iron-ajax
    url="http://gdata.youtube.com/feeds/api/videos/"
    params='{"alt":"json", "q":"chrome"}'
    handle-as="json"
    on-response="handleResponse"
    debounce-duration="300">
</iron-ajax>
    ...
handleResponse: function(e, request) {
    var headers = request.xhr.getAllResponseHeaders();
    alert(headers)
  }

Hope it helps :)

The response event handlers gets passed the <iron-request> as the second argument. <iron-request> has an xhr property that is the XMLHttpRequest used to make the request. You should be able to get the response headers from that.

<iron-ajax on-response="ajaxResponse"></iron-ajax>
...
ajaxResponse: function(e, request) {
  var headers = request.xhr.getAllResponseHeaders();
}