gistfile1.txt

let starterChanges$ = of(null);
    const starter = this.frm.get('starter');
    if (starter) {
      starterChanges$ = starter.valueChanges;
    }
    let spreadChanges$ = of(null);
    const spread = this.frm.get('spread');
    if (spread) {
      spreadChanges$ = spread.valueChanges;
    }

    this.subscription.add(
      combineLatest([starterChanges$, spreadChanges$]).subscribe(
        ([st = 0, sp = 0]) => {
          let current = 0;
          if (typeof st === 'number' && typeof sp === 'number') {
            for (const control of this.orders.controls) {
              if (!current) {
                current = current + st + sp;
              } else {
                current = current + sp;
              }
              control.patchValue({ spread: current });
              control.disable();
            }
          }
        }
      )
    );