var confirmOnPageExit = function (e)
        {
            // If we haven't been passed the event get the window.event
            e = e || window.event;

            var message = 'Any text will block the navigation and display a prompt';

            // For IE6-8 and Firefox prior to version 4
            if
 
 (e)
            {
                e.returnValue = message;
            }

            // For Chrome, Safari, IE8+ and Opera 12+
            return message;
        };
        window.onbeforeunload = confirmOnPageExit;

// Turn it on - assign the function that returns the string
window.onbeforeunload = confirmOnPageExit;
 


// Turn it off - remove the function entirely
window.onbeforeunload = null;

http://codingdict.com/questions/8647