// Bad - assertion lets invalid data into the system
const user = JSON.parse(responseText) as User;

// Good - boundary handles parsing, validation, and source context
const user = parseUserResponse(responseText);

function parseUserResponse(responseText: string): User {
  const payload = parseJson(responseText, "user response");
  return parseUser(payload);
}

type PaymentReceipt = {
  id: string;
  totalCents: number;
};

interface Payments {
  charge(request: ChargeRequest): Promise<PaymentReceipt>;
}