Full Stack Ecommerce Checkout Web App: PromptGolf compares visible app completion against hidden product-engineering checks.
Use a spec with states, acceptance tests, promo validation, quantity limits, cents-safe totals, and clear mobile/error behavior.
Cart table is visible and scannable.
Increment and decrement controls are present.
Order summary includes expected rows.
Promo input and apply action are present.
Checkout reaches a success state.
Avoids floating-point totals and tax drift.
Trims codes and handles case-insensitive matches.
Bad codes produce clear, recoverable feedback.
Discounts cannot push payable total below zero.
Free shipping uses the specified subtotal-before-discount rule.
Unavailable line items prevent checkout.
Repeated clicks cannot create duplicate orders.
Quantities cannot go negative, zero accidentally, or above stock.
Async states are visible and buttons disable while pending.
Core controls work on small screens with labels and keyboard affordances.
Structured requirements catch most bugs, but misses a few domain-specific checkout rules.