How to get header value in AJAX request?

Question

JavaScript

Connecting to the server via https.
It gives me headers:
Content-Type: application/json; charset=utf-8
and
X-Powered-By: ASP.NET
Which the fidler confirms.
Ajax request via XMLHttpRequest

var xmlhttp = new XMLHttpRequest();
xmlhttp.open("GET", url, true);
xmlhttp.onreadystatechange = function(){
 if (xmlhttp.readyState === 4) {
  if(this.status >= 200 && this.status < 400) {
   var contentType = xmlhttp.getResponseHeader('Content-Type');
   var poweredBy = xmlhttp.getResponseHeader('X-Powered-By');
  }
 }
};
xmlhttp.send(null);

But for some reason, I'm only getting the value from the Content-Type header, and I'm getting null from the X-Powered-By header.
Why is that? And how to make it so that through XMLHttpRequest to get the value of the X-Powered-By header?
==============================
As it turned out later, the problem lies in the cross-domain request. On the server, all headers seem to be allowed. Is there anything else that needs to be specified?

Answer 1

Helped Access-Control-Expose-Headers
Ie . registered on the server
And everything worked

Answer 2

Should work. Try looking through xmlhttp.getAllResponseHeaders().