Static & Private Class Fields

class Cache {
  #data = new Map();
  static #DEFAULT_TTL = 3600000;
  
  set(key, value, ttl = Cache.#DEFAULT_TTL) {
    this.#data.set(key, {
      value,
      expires: Date.now() + ttl
    });
  }
  
  get(key) {
    const entry = this.#data.get(key);
    if (!entry) return null;
    if (Date.now() > entry.expires) {
      this.#data.delete(key);
      return null;
    }
    return entry.value;
  }
}