One order.
A changing audience.

YOUR BRIEF

One display works. A log is next.

A customer display shows the order's latest status. Now an activity log needs every update too—and the customer can close the display while the order keeps moving. Make listeners independent of the publisher.

One immutable OrderUpdate carries an order ID and status. Every registered listener receives the same event through update(event). Removing one subscription must leave the others working.

The working starting point

let customerStatus = "Packed";
function publish(status: string): void {
  customerStatus = status;
}

One output is easy. The new requirement is a changing set of listeners, each with its own reaction.