Method Caching TTL Decorator

interface CacheEntry<T> {
  value: T;
  expiresAt: number;
}

function Cached<T>(ttlMs: number) {
  return function (
    target: object,
    propertyKey: string,
    descriptor: TypedPropertyDescriptor<(...args: never[]) => Promise<T>>,
  ) {
    const original = descriptor.value!;
    const store = new Map<string, CacheEntry<T>>();

    descriptor.value = async function (this: unknown, ...args: never[]) {
      const key = propertyKey + ':' + JSON.stringify(args);
      const hit = store.get(key);
      const now = Date.now();

      if (hit && hit.expiresAt > now) {
        (this as { __cacheHits?: number }).__cacheHits ??= 0;
        return hit.value;
      }

      const value = await original.apply(this, args);
      store.set(key, { value, expiresAt: now + ttlMs });
      return value;
    };

    return descriptor;
  };
}

class ExchangeRates {
  private hits = 0;

  @Cached(30_000)
  async ratesFor(base: string): Promise<Record<string, number>> {
    const res = await fetch('https://api.example.com/rates?base=' + base);
    return res.json();
  }

  @Cached(300_000)
  async currencies(): Promise<string[]> {
    return ['USD', 'EUR', 'JPY'];
  }
}

const rates = new ExchangeRates();
const first = await rates.ratesFor('USD');
const second = await rates.ratesFor('USD');
console.log('cached identity:', first === second);