Real-life: concatMap() invoice queue

Generate invoices strictly in the order 1, 2, 3.

1. Solve the task in the editor — no long theory.
2. Run the check and compare your output with the expected result.
3. Stuck? Open the solution and carry the approach into your own code.

const { from, of, concatMap } = Rx;

const makeInvoice = id => of('invoice-' + id);

const result$ = from([1, 2, 3]).pipe(
  concatMap(id => makeInvoice(id))
);

result$.subscribe(value => console.log(value));