Detecting Back Button/Hash Change in URL

The answers here are all quite old.

In the HTML5 world, you should the use onpopstate event.

window.onpopstate = function(event)
{
    alert("location: " + document.location + ", state: " + JSON.stringify(event.state));
};

Or:

window.addEventListener('popstate', function(event)
{
    alert("location: " + document.location + ", state: " + JSON.stringify(event.state));
});

The latter snippet allows multiple event handlers to exist, whereas the former will replace any existing handler which may cause hard-to-find bugs.