retryWhen(errors =>
   errors.pipe(
     // log error message
     tap(val => console.log(`Value ${val} was too high!`)),
     // restart in 5 seconds
     delayWhen(val => timer(val * 1000))
   )

 delayWhen1() {
   // emit value every second
   const message = interval(1000);
   // emit value after five seconds
   const delayForFiveSeconds = () => timer(5000);
   // after 5 seconds, start emitting delayed interval values
   const delayWhenExample = message.pipe(delayWhen(delayForFiveSeconds));
   // log values, delayed for 5 seconds
   // ex. output: 5s....1...2...3
   const subscribe = delayWhenExample.subscribe(val => console.log(val));
 }

  return (errors: Observable<any>) =>
    errors.pipe(
      scan((count, error) => {
        if (maxRetries >= 0 && count >= maxRetries) {
          throw error;
        }

        return count + 1;
      }, 0),
      delayWhen(val =>
        timer(
          Math.min(val * delay, maxDelay || Number.MAX_SAFE_INTEGER),
          undefined,
          scheduler
        )
      )
    );

 retryWhen(error =>
   error.pipe(
     tap(error => console.error(error)),
     tap(() => console.debug('Retrying in 3s...')),
     delayWhen(() => timer(3000))
   )

export function delayWhen<T>(this: Observable<T>, delayDurationSelector: (value: T) => Observable<any>,
                             subscriptionDelay?: Observable<any>): Observable<T> {
  return higherOrder(delayDurationSelector, subscriptionDelay)(this);
}

 observable1 = constructorZone1.run(() => {
   return of(1, 2, 3).pipe(delayWhen((v: any) => {
     return timer(v * 10);
   }));
 });

it('should support an empty notifier', () => {
  const o = of(1, 2, 3).pipe(delayWhen(() => NEVER)); // $ExpectType Observable<number>
});