Detecting Location Redirects from JavaScript

My scenario is basically the following. I have an ISA server which intercepts all of the requests and verifies the user's authentication. In case when the user isn't logged in it responds with a 302 and adds the location header to my login page.

The Initial Approach

In the case of a rich JavaScript client you would then have something like

$(document).ajaxComplete(function(e, xhr, settings){
    if(xhr.status === 302){
        //check for location header and redirect...
    }
});

But unfortunately the browser handles location redirects completely himself, wherefore the ajaxComplete callback won't fire with the 302 status code, but instead it will fire on the location redirect target (when that redirect already happened successfully). As a result, the approach before obviously won't work.

The Solution

The solution is a bit hacky but it works and I didn't found a better one so far (comment if I'm wrong). Basically what you do is to inject a custom header in your Login page for which you then listen on your client-side. The trick is to inject a header like

LoginPage: http://www.mydomain.com/Login

where the url is the one of the invoked Login page due to the location redirect. On the client side you can now check for the presence of the header like

if(xhr.status === 200){
    var loginPageHeader = xhr.getResponseHeader("LoginPage");
    if(loginPageHeader && loginPageHeader !== ""){
        window.location.replace(loginPageHeader);
    }
}

You might wonder why I included the url directly in the header value. The problem is that you have no way of detecting the URL of the redirected request in the ajaxComplete callback. You just get the initial one you started, that is, when you execute a GET /person/1 and you're no more logged in (because the session expired) and the server sends you a 302 redirect to /login, then you'll see the /person/1 as the url in your ajaxComplete callback although the content of the response is the one of your login page. That's why I directly included the url in my custom header.

Here's my explanation on SO: http://stackoverflow.com/a/10717647/50109