Servlet SetHeader and Access From Javascript

Question

I am trying have some custom header value set from a sevlet and access from a page which redirects to.

On the Sevlet side, I have

            response.setHeader("value", "ABC");
            response.sendRedirect("page.html");

On the page, I have js code

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

but the headers object does not contain any custom header.

Is there anything missing?

Kind Regards