Confirm bandcamp window close if playing

somejs.js

window.onbeforeunload = function () {
  var shouldConfirmClose = ($('.playing').length > 0); //get boolen value
  if (shouldConfirmClose) {
    //this will alert user
    return 'Are you sure?';
  }
  else {
    //this wont
    window.onbeforeunload = undefined;
  }
};