Accessing the web page’s HTTP Headers in JavaScript

How do I access a page’s HTTP response headers via JavaScript?

Related to this question, which was modified to ask about accessing two specific HTTP headers.

Related:
How do I access the HTTP request header fields via JavaScript?

It’s not possible to read the current headers. You could make another request to the same URL and read its headers, but there is no guarantee that the headers are exactly equal to the current.

Use the following JavaScript code to get all the HTTP headers by performing a get request:

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