CHAPTER 1 OF 3
One order. Has it been paid?
A tiny checkout only records payment. A boolean prevents a second payment. For this requirement, a small check is clear and sufficient.
STATE / THE WALKTHROUGH
Follow the order.
Find a home for its rules.
Start with one payment check. Add lifecycle rules, then see what changes when each stage owns its behavior.
A checkout with one responsibility
This first version only records whether payment happened. It has no shipping or cancellation rules yet.
No payment recorded yet.
A boolean is enough here · TypeScript
export class CheckoutOrder {
private paid = false;
pay(): boolean {
if (this.paid) return false;
this.paid = true;
return true;
}
}Chapter changes start a fresh example. Theme and language preserve the current step.