Calling JS functions on browser tab close

Question

I need to call a JS function when user closing the browser tab. the problem is i want t

Answer 1

JSFiddle uses the following to alert the user they are trying to close their modified fiddle without saving:

window.onbeforeunload = function(){
  if (window.editorsModified){
    return "You've modified your fiddle, reloading the page will reset all changes."
  }
};

This works reliably (http://jsfiddle.net/MaxPRafferty/b6uEF/show/ ,Latest Chrome, FF, IE10 ), but will always prompt the user. You can't force the page open by returning false;