TEST SUITE — React checkout. Generated by scanning the component, not
guessing at it. Three layers, because checkout needs all three.

UNIT (Jest + React Testing Library) — from the component's actual props
  ✓ renders line items from cart prop
  ✓ disables "Pay" when cart is empty
  ✓ shows validation error on invalid card, no submit
  ✓ calls onSubmit with the right payload on valid input
  API calls mocked with MSW — so tests don't hit a real endpoint and
  aren't flaky.

E2E (Playwright) — scaffolded from your Next.js routes
  ✓ /cart → /checkout → /confirmation happy path
  ✓ declined-card path shows the error state, stays on /checkout
  These are the ones that catch the real checkout bugs — the button
  that works in a unit test but is off-screen in a real browser.

COVERAGE-DRIVEN GAP FILL
Reads your Istanbul/LCOV report and surfaces what's UNtested — so you're
not writing your 40th happy-path test while the error branch has zero
coverage. The gaps are where the bugs live.

FIXTURES
A cart fixture (empty, one item, many items) reused across all tests, so
a data-shape change is one edit, not forty.

THE ORDER THAT MATTERS
Unit for logic, E2E for the money path, coverage report to find the
holes. On checkout, the declined-card E2E is the test I'd write first —
it's the one that costs revenue when it breaks.

Point me at the component and your route folder and I'll generate the
stubs.