Observer & Singleton Patterns

class TestableSingleton {
  static #instances = new Map();
  #state;

  constructor(key = 'default') {
    const existingInstance = TestableSingleton.#instances.get(key);
    if (existingInstance) {
      return existingInstance;
    }

    this.#state = {};
    TestableSingleton.#instances.set(key, this);
  }

  static getInstance(key = 'default') {
    if (!TestableSingleton.#instances.has(key)) {
      new TestableSingleton(key);
    }
    return TestableSingleton.#instances.get(key);
  }

  static resetInstances() {
    TestableSingleton.#instances.clear();
  }
}