[JavaScript] 'window.onbeforeunload' fires twice.

It is a common practice to handle the 'unsaved data on page' scenario by means of handling 'window.onbeforeunload' event. A typical way of handling this event would be like this.

window.onbeforeunload = handleUnsavedData;

function handleUnsavedData() 
{
  if (pageUpdated) 
  {
    return 'You have unsaved data on the page. Are you sure you want to    navigate away from the page?';
  }
}

This works, but you may have noticed that the 'onbeforeunload' event is getting fired twice, effectively executing the function 'handleUnsavedData' twice!

Well, there is a work around for this. The solution is to disable the event handler. So the code would look like below:

function handleUnsavedData() 
{ 
    if(pageUpdated) 
    { 
        event.returnValue = 'You have unsaved data on the page. ' + 
          'Are you sure you wan to navigate away from the page?'; 
        window.onbeforeunload = null; 
    } 
}

Now this makes sure that the event is not fired twice - only to introduce a new problem!

Imagine that you have chosen not to exit the page when you were warned. Now you are back in your page (without a post-back of course). If you try to navigate away from the page again, will it warn you? Of course not, because you have nullified the event handler. 

Luckily there is a work around for that too :-) 

The idea is to temporarily disable the event handler and then enable it later (within a few milliseconds). The modified code would look like below.

function handleUnsavedData() 
{ 
    if(pageUpdated) 
    { 
        event.returnValue = 'You have unsaved data on the page. ' + 
          'Are you sure you wan to navigate away from the page?'; 
        window.onbeforeunload = null; 
        setTimeout("enableBeforeUnloadHandler()", "100"); 
    } 
} 

function enableBeforeUnloadHandler()  { window.onbeforeunload = confirmExit; }

In this case, the event handler would be reinstated shortly after returning to the page. You may want to play with the '100 ms' a little bit to suit your requirements.

Happy coding!