Unions and narrowing

function showId(id: string | number): string {
  if (typeof id === "number") {
    return "#" + id.toFixed(0); // here id is a number
  }
  return id.toUpperCase();      // here id is a string
}

type PaymentStatus = "pending" | "paid" | "failed";

let status: PaymentStatus = "pending";
status = "paid";     // OK
status = "refunded"; // Error: Type '"refunded"' is not assignable to type 'PaymentStatus'.

type User = { name: string; city?: string };
declare function findUser(id: string): User | undefined;

const user = findUser("u1");
user.name;          // Error: 'user' is possibly 'undefined'.
if (user) {
  user.name;        // OK: narrowed to User
}
const city = user?.city ?? "Not given"; // optional chaining + default

type Payment =
  | { kind: "upi"; upiId: string }
  | { kind: "card"; last4: string }
  | { kind: "cash" };

function describe(p: Payment): string {
  switch (p.kind) {
    case "upi":
      return "UPI: " + p.upiId;          // p has upiId here
    case "card":
      return "Card ending " + p.last4;   // p has last4 here
    case "cash":
      return "Cash on delivery";
    default: {
      const unreachable: never = p;      // error if a case is missing
      return unreachable;
    }
  }
}

type Product = { id: string; name: string; price: number };

function isProduct(value: unknown): value is Product {
  return (
    typeof value === "object" && value !== null &&
    "id" in value && typeof value.id === "string" &&
    "name" in value && typeof value.name === "string" &&
    "price" in value && typeof value.price === "number"
  );
}

if (isProduct(data)) {
  console.log(data.price.toFixed(2)); // data is a Product here
}