How can I detect an address bar change with JavaScript?

I have a Ajax heavy application that may have a URL such as

http://example.com/myApp/#page=1

When a user manipulates the site, the address bar can change to something like

http://example.com/myApp/#page=5

without reloading the page.

My problem is the following sequence:

1. A user bookmarks the first URL.
2. The user manipulates the application such that the second URL is the current state.
3. The user clicks on the bookmark created in step 1.
4. The URL in the address bar changes from http://example.com/myApp/#page=5 to http://example.com/myApp/#page=1, but I don't know of a way to detect the change happened.

If I detect a change some JavaScript would act on it.

HTML5 introduces a hashchange event which allows you to register for notifications of url hash changes without polling for them with a timer.

It it supported by all major browsers (Firefox 3.6, IE8, Chrome, other Webkit-based browsers), but I'd still highly suggest to use a library which handles the event for you - i.e. by using a timer in browsers not supporting the HTML5 event and using the event otherwise.

window.onhashchange = function() {
    alert("hashtag changed");
};

For further information on the event, see https://developer.mozilla.org/en/dom/window.onhashchange and http://msdn.microsoft.com/en-us/library/cc288209%28VS.85%29.aspx.