final_rxjs_example.ts

import { forkJoin, from, Observable } from 'rxjs';
import { bufferCount, concatMap, last, scan } from "rxjs/operators";

class Test {
  someArray = [1,2,3,4,5,6,7,8,9,10,11,12];

  constructor() {
    from(this.someArray).pipe(
      bufferCount(3),
      concatMap(this.controlChunk),
      scan((acc: number[], curr: number[]) => [...acc, ...curr], []),
      last()
    ).subscribe(console.log)
  }

  controlChunk = (chunk: number[]): Observable<number[]> =>
    forkJoin(chunk.map((n: number) => from(this.delayedPromise(n))))

  delayedPromise = (n: number): Promise<number> => new Promise((resolve => {
    setTimeout(() => {
      resolve(n+1)
    }, this.getRandomArbitrary(100, 1000))
  }));

  getRandomArbitrary = (min: number, max: number): number => Math.random() * (max - min) + min;
}

const test = new Test();