How does facebook rewrite the source URL of a page in the browser address bar?

Question

Go to http://www.facebook.com/facebook?v=wall, then click on the info tab. The content will be loaded, and the address bar now becomes http://www.facebook.com/facebook?v=info but the webpage didn't reload.

At first I think it is Ajax, but my question is, how do you change the address bar without reloading? I know I can change anchor (#wall) using JS but querystring (?v=wall), how?

Answer 1

It's using HTML5's new history.pushState() feature to allow the page to masquerade as being at a different URL to that from which it was originally fetched.

This seems only to be supported by WebKit at the moment, which is why the rest of us are seeing ?v=wall#!/facebook?v=info instead of ?v=info.

The feature allows dynamically-loaded pages to be properly bookmarked, exchanged etc between JS-supporting and non-JS-supporting user agents. Because if you as a JS user linked someone to ?v=wall#!/facebook?v=info and their browser didn't support JS and XMLHttpRequest, the page wouldn't work for them. The #! is also used as a tip to search engines to download the non-AJAX version.