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Типы данных и типизация в TypeScript — Type predicates (`value is T`)

Фрагмент из «Типы данных и типизация в TypeScript»: Type predicates (`value is T`).

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TypeScript main.ts 
type Payment = { id: string; amount: number };

function isPayment(value: unknown): value is Payment {
  if (typeof value !== "object" || value === null) return false;
  const o = value as Record<string, unknown>;
  return typeof o.id === "string" && typeof o.amount === "number";
}

function charge(raw: unknown): void {
  if (!isPayment(raw)) {
    throw new Error("Invalid payment payload");
  }
  console.log(raw.amount); // Payment, не unknown
}
type Payment = { id: string; amount: number };

function isPayment(value: unknown): value is Payment {
  if (typeof value !== "object" || value === null) return false;
  const o = value as Record<string, unknown>;
  return typeof o.id === "string" && typeof o.amount === "number";
}

function charge(raw: unknown): void {
  if (!isPayment(raw)) {
    throw new Error("Invalid payment payload");
  }
  console.log(raw.amount); // Payment, не unknown
}