const PING_INTERVAL = 1000;
const PING_URL = 'http://localhost:4200';
const online$ = new BehaviorSubject<boolean>(navigator.onLine);
const ping$ = online$.pipe(
  mergeMap((online) => {
    if (online) {
      return interval(PING_INTERVAL).pipe(
        takeWhile(() => !!online$.value),
        mergeMap(() => {
          return fromPromise(fetch(PING_URL, {
            method: 'head',
            mode: 'cors',
          }).then());
        }),
        pluck('status'),
        map(status => status === 200),
      );
    }
    return of(online);
  }),
);

window.addEventListener('online', () => {
  online$.next(navigator.onLine);
});

window.addEventListener('offline', () => {
  online$.next(navigator.onLine);
});


online$.subscribe((yeah) => console.log('online is', yeah));

ping$.subscribe((status) => console.log('ping', status));