expand with a limit

index.js

const { rxObserver } = require('api/v0.3');
const { of, timer, EMPTY } = require('rxjs');
const { expand, delay, flatMap, take } = require('rxjs/operators');

// PLEASE, OPEN THE CONSOLE

const items = [
  [1, 2, 3, 4, 5, 6, 7, 8, 9, 10],
  [11, 12, 13, 14, 15, 16, 17, 18, 19, 20],
  [21, 22, 23, 24, 25, 26, 27, 28, 29, 30],
];

const call = () => {
  console.log('#call');

  const batch = items.shift();

  if (batch) {
    return of(batch).pipe(delay(100));
  }

  return EMPTY;
};

const o$ = call().pipe(
  expand((values) => {
    console.log('expansion');
    return timer(0).pipe(map(call));
  })
  , flatMap((val) => val)
);

const log = (prefix) => (...args) => console.log(prefix, ...args);

o$.pipe(take(9))
  .subscribe(rxObserver());