I have a system where I want to check with the user if they're sure they want to leave the page once a dirty flag is set.

I'm using the following code - In FireFox, I can look at the page source through FireBug and the tag correctly has the onbeforeunload attribute inserted in it.

In Chrome and FireFox, this doesn't happen though and I'm able to navigate away from the page without any warning at all. The jQuery line to update the body tag is definitely being executed, it just isn't performing it.

if ($(body).attr('onbeforeunload') == null) {

    if (window.event) {

        // IE and Chrome use this

        $(body).attr('onbeforeunload', 'CatchLeavePage(event)');

    }

    else {

        // Firefox uses this

        $(body).attr('onbeforeunload', 'return false;CatchLeavePage(event)');

    }

}

Any ideas how to proceed from here?