app.js

function _isEmpty = function(obj) {
    if ('undefined' !== Object.keys) {
        return (0 === Object.keys(obj).length);
    }
  for(var prop in obj) {
      if(obj.hasOwnProperty(prop)) {
                return false;
            }
  }
  return true;
};

// called whenever a checkin is requested to be displayed
function checkCheckinStatus( checkinID ) {
  var tS = Math.floor(now.getTime() / 1000),
      removeCheckinFromApp = function(cid) {
    // remove from data 
    delete app.data[cid];

    // update local cache
    localforage.setItem('flightData',app.data);

    // trigger event for updating UI elements
    var event = new CustomEvent(
      'updatedData',
      {detail: {modified: cid}}
    );
    document.dispatchEvent(event);
 };

  // no data or no checkin data? - return
  if(_isEmpty(app.data) || !app.data[checkinID]) {
    return false;
  }

  // remove only in case arrival time is min. 48 hours in past
  if((tS - app.data[checkinID].ticket.arrivalTimestamp) < (60 * 60 * 48) ) { return false;}

  if ('serviceWorker' in navigator) {
    // delete cache of flight in service worker...
    app.sendMessage( {
      command: 'deleteCheckin', 
      keyID: checkinID } 
    ).then(function(data) {
      // remove the checkin from app data...
      removeCheckinFromApp(checkinID);
    }).catch(e) {
      // could not remove checkin from service worker
    };
  } else {
    removeCheckinFromApp(checkinID);
  }
}

// send data to the service worker
app.sendMessage = function(message) {
  return new Promise(function(resolve, reject) {
    var messageChannel = new MessageChannel();
    // the onmessage handler
    messageChannel.port1.onmessage = function(event) {
      if (event.data.error) {
        reject(event.data.error);
      } else {
        resolve(event.data);
      }
    };
    
    if(!navigator.serviceWorker.controller){
      return;
    }

    // This sends the message data and port to the service worker.
    // The service worker can use the port to reply via postMessage(), which
    // will he onmessage handler on messageChannel.port1.
    navigator.serviceWorker
      .controller.postMessage(message,[messageChannel.port2]);
  });
}