luoling8192/ai-coding-principles/ai-coding-discipline

// FORBIDDEN — hides upstream bugs
const price = product?.price ?? 0;
const userName = user?.name || "Unknown";
// CORRECT — fail fast when data contract is violated
if (product.price == null) {
  throw new Error(`Product ${product.id} is missing price`);
}
const price = product.price;

// FORBIDDEN — swallows all errors into a useless null
async function createOrder(data: OrderInput) {
  try {
    const user = await getUser(data.userId);
    const coupon = await validateCoupon(data.couponCode);
    const order = await saveOrder({ ...data, discount: coupon.value });
    return order;
  } catch (error) {
    console.log('Error creating order:', error);
    return null; // caller gets null, has no idea what failed
  }
}
// CORRECT — let errors bubble up, catch at the boundary
async function createOrder(data: OrderInput) {
  const user = await getUser(data.userId);
  const coupon = await validateCoupon(data.couponCode);
  const order = await saveOrder({ ...data, discount: coupon.value });
  return order;
}
// Catch ONLY at the API/controller boundary
app.post('/orders', async (req, res) => {
  try {
    const order = await createOrder(req.body);
    res.json(order);
  } catch (error) {
    logger.error('Order creation failed', { error, body: req.body });
    res.status(500).json({ error: 'Order creation failed' });
  }
});

// FORBIDDEN — passes even if processOrder returns garbage
test('should process order', async () => {
  const result = await processOrder(mockOrder);
  expect(result).toBeDefined();
});
// CORRECT — verifies exact business behavior
test('should calculate total with 10% discount', async () => {
  const result = await processOrder({
    ...mockOrder,
    discount: 0.1,
  });
  expect(result.totalAmount).toBe(900);       // 1000 * 0.9
  expect(result.discountAmount).toBe(100);
  expect(result.status).toBe('confirmed');
});

// FORBIDDEN — fake implementation that "fits" the tests
function calculateDiscount(amount: number, level: string): number {
  if (amount === 1000 && level === 'gold') return 100;
  if (amount === 500 && level === 'silver') return 25;
  return 0;
}
// CORRECT — real logic
function calculateDiscount(amount: number, level: string): number {
  const rates: Record<string, number> = { gold: 0.1, silver: 0.05, bronze: 0.02 };
  const rate = rates[level] ?? 0;
  return amount * rate;
}

Correct order:
1. Discover bug
2. Write a test that reproduces the bug
3. Run the test — confirm it FAILS (red)
4. Fix the code
5. Run the test — confirm it PASSES (green)

FORBIDDEN workflow:
1. Human asks to add debug logs
2. AI adds logs
3. Human runs code, shares log output
4. AI "finds the problem", applies fix AND removes debug logs in the same edit
5. Fix doesn't work — logs are gone, must re-add them
 
CORRECT workflow:
1. Human asks to add debug logs
2. AI adds logs
3. Human runs code, shares log output
4. AI applies fix ONLY — debug logs stay untouched
5. Human verifies the fix
6. Human decides when to remove debug logs (or asks AI to remove them)