docs: function windowCloseHandler(event) {
      if(!options.windowCloseWarning) {
        return;
      }
  
      if(sync.state !== sync.SYNC_SYNCING) {
        return;
      }
  
      var confirmationMessage = "Sync currently underway, are you sure you want to close?";
      (event || global.event).returnValue = confirmationMessage;
  
      return confirmationMessage;
    }