Add X-Request-URL

README.md

@@ -162,6 +162,23 @@ fetch('/users')
   })
 ```
 
+### Response URL caveat
+
+The `Response` object has a URL attribute for the final responded resource.
+Usually this is the same as the `Request` url, but in the case of a redirect,
+its all transparent. Newer versions of XHR include a `responseURL` attribute
+that returns this value. But not every browser supports this. The compromise
+requires setting a special server side header to tell the browser what URL it
+just requested (yeah, I know browsers).
+
+``` ruby
+response.headers['X-Request-URL'] = request.url
+```
+
+If you want `response.url` to be reliable, you'll want to set this header. The
+day that you ditch this polyfill and use native fetch only, you can remove the
+header hack.
+
 ## Browser Support
 
 ![Chrome](https://raw.github.com/alrra/browser-logos/master/chrome/chrome_48x48.png) | ![Firefox](https://raw.github.com/alrra/browser-logos/master/firefox/firefox_48x48.png) | ![IE](https://raw.github.com/alrra/browser-logos/master/internet-explorer/internet-explorer_48x48.png) | ![Opera](https://raw.github.com/alrra/browser-logos/master/opera/opera_48x48.png) | ![Safari](https://raw.github.com/alrra/browser-logos/master/safari/safari_48x48.png)

fetch.js

@@ -181,7 +181,7 @@
           status: status,
           statusText: xhr.statusText,
           headers: headers(xhr),
-          url: xhr.responseURL
+          url: xhr.responseURL || xhr.getResponseHeader('X-Request-URL')
         }
         resolve(new Response(xhr.responseText, options))
       }

test/server.js

@@ -20,8 +20,11 @@ var routes = {
       }));
     })
   },
-  '/hello': function(res) {
-    res.writeHead(200, {'Content-Type': 'text/plain'});
+  '/hello': function(res, req) {
+    res.writeHead(200, {
+      'Content-Type': 'text/plain',
+      'X-Request-URL': 'http://' + req.headers.host + req.url
+    });
     res.end('hi');
   },
   '/redirect/301': function(res) {