UpdateFrequency.js

  // if key was accessed, increment its frequency
  #updateFrequency = key => {
    let frequencyObject = this.#frequencyMap.get(key);

    // if existing key, update the frequency 
    if (frequencyObject)
      frequencyObject.update();
    else
      this.#frequencyMap.set(key, frequencyObject = new Frequency());

    debug(`Updated { ${key} => ${frequencyObject} }`);

    // Maybe we went over the maxSize, truncate the Map
    this.#resizeMap();
  }