Promo Code Assessment

Using Nx to allow two projects to be run on the same repository

Installation

    npm install

Nx workspace with:

• web: React app
• api: Express API

Run application

npx nx serve web
npx nx serve api

Build

npx nx build web
npx nx build api

Manual Frontend Test Cases

Start both apps:

npx nx serve api
npx nx serve web

Open the frontend:

http://127.0.0.1:4200

Expected checks:

• The page shows the product picker, cart items, promo code input, discount breakdown, and promo stats sections.
• The frontend uses the Vite /api proxy and does not require hardcoded backend URLs in the browser.

Basic cart flow:

• Add TRT, Sermorelin, Semaglutide, or Ozempic from the product cards.
• Confirm cart rows appear with quantity controls and remove actions.
• Confirm the discount breakdown updates after a short debounce instead of on every keystroke or click.

Promo code flow:

• Apply valid promo codes such as SAVE20, FLAT10, FLASH30, and VIP15.
• Confirm applied discounts appear in the breakdown.
• Apply invalid or conflicting promo codes and confirm rejected reasons are shown in the UI.
• Apply duplicate or lowercase promo codes and confirm they normalize and dedupe correctly.

Persistence:

• Add items and promo codes, then refresh the page.
• Confirm the cart and applied promo codes restore from localStorage.

Promo stats:

• Confirm the promo stats section loads on page render.
• Click the manual refresh button and confirm stats refresh successfully.
• Apply promo codes successfully and confirm stats update after refresh / polling.

Rate limit behavior:

• Trigger repeated cart recalculations by rapidly changing cart contents or promo codes.
• Confirm the frontend shows the backend cooldown message when rate limited.
• Confirm cart edits remain possible during cooldown and recalculation resumes after the cooldown expires.

Backend Tests

npm run test:api

This runs a lightweight Node test suite covering the promo engine, calculate endpoint validation and rate limiting, and promo stats usage tracking.

Manual API Test Cases

Start the backend:

npx nx serve api

Health check:

curl -i http://localhost:3000/api/health

Get promo usage stats:

curl -i http://localhost:3000/api/promo/stats

Expected checks:

• Response status is 200.
• Response contains success: true.
• Response lists each promo code with uses, max_uses, remaining_uses, and available.

Valid cart with stacked promos:

curl -i -X POST http://localhost:3000/api/cart/calculate \
  -H 'Content-Type: application/json' \
  -d '{
    "user_id": "usr_123",
    "items": [
      { "sku": "PROD-001", "quantity": 2, "price": "49.99" },
      { "sku": "PROD-002", "quantity": 1, "price": "149.99" }
    ],
    "promo_codes": ["SAVE20", "FLAT10"]
  }'

Expected checks:

• Request prices are sent as fixed two-decimal strings.
• subtotal, total_discount, final_total, and each discount amount are returned as fixed two-decimal strings.
• SAVE20 applies before FLAT10.
• The API server logs a line with masked user_id, applied promo codes, total discount, and processing time.

Conflict case:

curl -i -X POST http://localhost:3000/api/cart/calculate \
  -H 'Content-Type: application/json' \
  -d '{
    "user_id": "usr_conflict",
    "items": [
      { "sku": "PROD-001", "quantity": 1, "price": "200.00" }
    ],
    "promo_codes": ["SAVE20", "FLASH30", "VIP15"]
  }'

Expected checks:

• One of the conflicting priority-1 codes is rejected with CONFLICT_WITH_<CODE>.
• The accepted priority-1 code is applied before VIP15.

Unknown promo code:

curl -i -X POST http://localhost:3000/api/cart/calculate \
  -H 'Content-Type: application/json' \
  -d '{
    "user_id": "usr_unknown",
    "items": [
      { "sku": "PROD-001", "quantity": 1, "price": "100.00" }
    ],
    "promo_codes": ["NOTREAL", "FLAT10"]
  }'

Expected checks:

• NOTREAL appears in rejected_codes with CODE_NOT_FOUND.
• FLAT10 still applies.

Duplicate and normalized promo codes:

curl -i -X POST http://localhost:3000/api/cart/calculate \
  -H 'Content-Type: application/json' \
  -d '{
    "user_id": "usr_dupe",
    "items": [
      { "sku": "PROD-001", "quantity": 1, "price": "100.00" }
    ],
    "promo_codes": ["SAVE20", "save20", " FLAT10 "]
  }'

Expected checks:

• SAVE20 is only applied once.
• Lowercase and padded promo codes are normalized successfully.

Invalid payload:

curl -i -X POST http://localhost:3000/api/cart/calculate \
  -H 'Content-Type: application/json' \
  -d '{
    "user_id": "",
    "items": [
      { "sku": "PROD-001", "quantity": 0, "price": "-10.00" }
    ],
    "promo_codes": ["SAVE20"]
  }'

Expected checks:

• Response status is 400.
• Response contains INVALID_CART_REQUEST.

Rate limit test:

for i in $(seq 1 11); do
  echo "Request $i"
  curl -s -i -X POST http://localhost:3000/api/cart/calculate \
    -H 'Content-Type: application/json' \
    -d '{
      "user_id": "rate_limit_user",
      "items": [
        { "sku": "PROD-001", "quantity": 1, "price": "10.00" }
      ],
      "promo_codes": []
    }'
  echo
done

Expected checks:

• Requests 1 through 10 succeed.
• Request 11 returns 429.
• The 429 body includes RATE_LIMITED and retry_after.