How Does Stack Overflow Replace State? [closed]

Question

I'm a mac user and never use IE. But yesterday at work I went onto stack overflow and typed this into the browser using IE 9...

http://stackoverflow.com/questions/824349

And they replaced the URL to this without refreshing the page...

http://stackoverflow.com/questions/824349/modify-the-url-without-reloading-the-page

I could not believe what I saw. Any ideas how stack overflow is able to utilize a functionality that mimics the history API's replace state on a browser that does not support it?

Answer 1

They actually redirect the user with a 301 redirection. Look at the headers:

GET /questions/824349 HTTP/1.1
Host: stackoverflow.com
User-Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:21.0) Gecko/20100101 Firefox/21.0
[...]

HTTP/1.1 301 Moved Permanently
Cache-Control: public, max-age=60
Content-Type: text/html
Expires: Sat, 08 Jun 2013 19:00:05 GMT
Last-Modified: Sat, 08 Jun 2013 18:59:05 GMT
Location: /questions/824349/modify-the-url-without-reloading-the-page
Vary: *
X-Frame-Options: SAMEORIGIN
Date: Sat, 08 Jun 2013 18:59:05 GMT
Content-Length: 0

Answer 2

It's a 301 Moved Permanently redirect, i.e. it's done server side. You don't see a refresh because the browser doesn't open the first URL, it redirects right away to the second.

Here's the result on chrome's console.