TypeScript: декораторы — пример кода

// Same intent, both worlds. Legacy mutates the descriptor in place:
class ApiLegacy {
  @logged                     // experimentalDecorators: true
  fetch(id: string) { /* ... */ }
}

// Stage 3 returns a replacement — no descriptor mutation, no flags:
class ApiModern {
  @logged                     // TS 5.0 default
  fetch(id: string) { /* ... */ }
}

// Class decorator returning a subclass (Stage 3) — the "replace the declaration" power
function withId<T extends new (...args: any[]) => object>(Base: T, _c: ClassDecoratorContext) {
  return class extends Base {
    id = crypto.randomUUID();
  };
}
@withId class Widget {}
// new Widget().id is present at runtime, but note: the added field
// is NOT visible on the static type unless you also declare it.