Challenge 3: Version a Type — Possible Solution
====================================================================

interface ProductV1 {
  version: 1;
  id: string;
  title: string;      // v1 called it "title"
  priceInCents: number;
}

interface ProductV2 {
  version: 2;
  id: string;
  name: string;        // v2 renamed "title" to "name"
  price: number;        // v2 uses dollars (number), not cents
  currency: string;     // v2 added a currency field
}

type VersionedProduct = ProductV1 | ProductV2;

// A normalized shape the rest of the app can use regardless of which
// version the data came from.
interface NormalizedProduct {
  id: string;
  name: string;
  price: number;   // always in dollars
  currency: string;
}

function normalizeProduct(product: VersionedProduct): NormalizedProduct {
  if (product.version === 1) {
    // Narrowed to ProductV1 here — title/priceInCents exist, name/price/currency don't.
    return {
      id: product.id,
      name: product.title,
      price: product.priceInCents / 100,
      currency: "USD", // v1 never stored a currency — assume USD
    };
  }

  // Narrowed to ProductV2 here.
  return {
    id: product.id,
    name: product.name,
    price: product.price,
    currency: product.currency,
  };
}

// --- Usage ---

const oldProduct: ProductV1 = { version: 1, id: "p1", title: "Widget", priceInCents: 999 };
const newProduct: ProductV2 = { version: 2, id: "p2", name: "Gadget", price: 19.99, currency: "EUR" };

console.log(normalizeProduct(oldProduct)); // { id: 'p1', name: 'Widget', price: 9.99, currency: 'USD' }
console.log(normalizeProduct(newProduct)); // { id: 'p2', name: 'Gadget', price: 19.99, currency: 'EUR' }

WHY THIS WORKS
--------------
- `version` is a literal-typed discriminant (1 vs 2), so checking
  `product.version === 1` narrows the whole union the same way `status`
  did in Challenge 1's ApiResponse.
- Each version keeps its own honest shape — ProductV1 never pretends to
  have a `currency` field it never actually stored — instead of one type
  bloated with optional fields for every version that ever existed.
- normalizeProduct() is the single place that understands both versions,
  so the rest of the codebase only ever deals with NormalizedProduct.