fourth_example.ts

import { from, of } from "rxjs";
import { bufferCount, concatMap } 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((dividedArray: number[]) => of(dividedArray)),
    ).subscribe(console.log)
  }
}

const test = new Test();