import { pipe, range, timer, zip } from 'rxjs';
import { ajax } from 'rxjs/ajax';
import { retryWhen, map, mergeMap } from 'rxjs/operators';

function backoff(maxTries, ms) {
 return pipe(
   retryWhen(attempts => zip(range(1, maxTries), attempts)
     .pipe(
       map(([i]) => i * i),
       mergeMap(i =>  timer(i * ms))
     )
   )
 );
}

ajax('/api/endpoint')
  .pipe(backoff(3, 250))
  .subscribe(data => handleData(data));

function handleData(data) {
  // ...
}