Retries with backoff in RxJS

rxjs-backoff.js

const backoff = (maxRetries, delayFunction) =>
  attempts => attempts
    .zip(Rx.Observable.range(1, maxRetries), (a, idx) => idx)
    .delayWhen(idx => Rx.Observable.timer(delayFunction(idx)));

// usage
const withIncrementalBackoff = observable.retryWhen(backoff(3, nRetry => nRetry * 1000));
const withExponentialBackoff = observable.retryWhen(backoff(3, nRetry => nRetry * nRetry * 1000));

The backoff retry here works, but if the final retry fails the observable still completes. Maybe more natural to fail the observable if the final retry fails.