third_example.ts

import { from } from "rxjs";
import { 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(
      concatMap(this.delayedPromise),
      scan((acc: number[], curr: number) => [...acc, curr], []),
      last()
    ).subscribe(console.log)
  }

  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();