Eigene Symbol.species Unterklassenbildung für unveränderliche Next.js Request-Kontext-Wrapper

export class HeaderMap extends Map {
  #timestamp = Date.now();
  static get [Symbol.species]() {
    return Map;
  }
  get metadata() {
    return Object.freeze({ created: this.#timestamp, count: this.size });
  }
  filterByPrefix(prefix) {
    const matched = new HeaderMap();
    for (const [key, value] of this) {
      if (key.startsWith(prefix)) matched.set(key, value);
    } 
    return matched;
  }
}