How to detect if URL has changed after hash in JavaScript

How can I check if a URL has changed in JavaScript? For example, websites like GitHub, which use AJAX, will append page information after a # symbol to create a unique URL without reloading the page. What is the best way to detect if this URL changes?

• Is the onload event called again?
• Is there an event handler for the URL?
• Or must the URL be checked every second to detect a change?

In modern browsers (IE8+, FF3.6+, Chrome), you can just listen to the hashchange event on window.

In some old browsers, you need a timer that continually checks location.hash. If you're using jQuery, there is a plugin that does exactly that.

Example

Below I undo any URL change, to keep just the scrolling:

<script type="text/javascript">
  if (window.history) {
    var myOldUrl = window.location.href;
    window.addEventListener('hashchange', function(){
      window.history.pushState({}, null, myOldUrl);
    });
  }
</script>

Note that above used history-API is available in Chrome, Safari, Firefox 4+, and Internet Explorer 10pp4+