Money in Floats in TypeScript

A computer stores numbers with decimals, floats, in binary. In binary, 0.1 has no exact value, just as 1/3 has none in decimal (0.333...). So in TypeScript 0.1 + 0.2 is 0.30000000000000004, a tiny bit more than 0.3. For money, that's a bug. Store amounts as whole numbers of the smallest unit, cents, instead: 19.99 euros is 1999 cents, and 1999 + 1 is exactly 2000.

An AI assistant wrote the cart total for the shop's stationery corner, with the prices in euros, as floats. A customer who bought one sticker and one pencil was told to pay 0.30000000000000004:

const PRICES: { [product: string]: number } = { sticker: 0.10, pencil: 0.20, eraser: 0.45, postcard: 1.15 };

app.post("/cart/total", (req: Request, res: Response) => {
  const items: CartItem[] = req.body.items;
  let total = 0;
  for (const item of items) {
    total = total + PRICES[item.product] * item.quantity;
  }
  res.json({ total: total });
});

For 3 postcards it said 3.4499999999999997, a little less than the real price. Some totals came out right, like 3 postcards and 2 erasers (4.35), and that's how the bug got past a quick try. TypeScript has one type, number, for whole numbers and floats alike, so the types don't warn you either. When AI code works with money, look for floats.

The client sends the cart as JSON, like {"items": [{"product": "sticker", "quantity": 3}]}. Your version must answer whole cents:

POST /cart/total  {"items": [{"product": "sticker", "quantity": 1}, {"product": "pencil", "quantity": 1}]}  →  200  {"total_cents": 30}
POST /cart/total  {"items": [{"product": "sticker", "quantity": 3}]}  →  200  {"total_cents": 30}
POST /cart/total  {"items": [{"product": "postcard", "quantity": 3}]}  →  200  {"total_cents": 345}
POST /cart/total  {"items": [{"product": "postcard", "quantity": 3}, {"product": "eraser", "quantity": 2}]}  →  200  {"total_cents": 435}
POST /cart/total  {"items": [{"product": "sticker", "quantity": 7}]}  →  200  {"total_cents": 70}
POST /cart/total  {"items": []}  →  200  {"total_cents": 0}

Your task: write the price list PRICES in cents (sticker 10, pencil 20, eraser 45, postcard 115), typed { [product: string]: number }, and rewrite the function of the /cart/total route: add up each item's price times its quantity, and answer {"total_cents": ...}.

Assignment
Follow the Coding Tutorial to rewrite the price list and the /cart/total route so every amount is a whole number of cents.

Hint
Keep every number whole, from the price list to the total: prices in cents, and a total that starts at 0. Dividing by 100 is for whoever shows the price to a person, not for this route.