An RxJS poller with incremental back-off as a let-function

rxjs-poller-with-backoff.js

import { TimeoutError, timer } from 'rxjs'
import { AjaxTimeoutError } from 'rxjs/ajax'
import {
  delay,
  filter,
  flatMap,
  repeatWhen,
  retryWhen,
  scan,
  timeout,
} from 'rxjs/operators'

/**
Repeat an observable on a regular interval.
Wait for one poll to end before starting another.
Gradually increase the poll interval if a poll times out.

Usage:

    // Repeat SomeObservable; must complete to trigger a repeat.
    SomeObservable.pipe(
        makePoller(),
        // Repeats indefinitely without end condition.
        takeUntil(endCondition),
    )
**/
export const makePoller = ({
  intervalms = 10000,
  timeoutms = 30000,
  backoffIncrementms = 5000,
  backoffMax = 5,
  scheduler,
} = {}) => o =>
  o.pipe(
    repeatWhen(no => no.pipe(delay(intervalms, scheduler))),
    timeout(timeoutms, scheduler),
    retryWhen(no =>
      no.pipe(
        filter(x => x instanceof TimeoutError || x instanceof AjaxTimeoutError),
        scan(count => count + 1, 0),
        flatMap(i => {
          const backoff = i < backoffMax ? i * backoffIncrementms : backoffMax * backoffIncrementms
          return timer(backoff, scheduler)
        }),
      ),
    ),
  )