Frequency.js

// Class to store frequency and recency of the key
class Frequency {

  // Private fields
  #frequency;   // Number of times the key has been accessed
  #recency;     // Last timestamp of access

  constructor(frequency = 1, recency = new Date()) {
    this.#frequency = frequency;
    this.#recency = recency.getTime() + recency.getMilliseconds();
  }

  // Update frequency and recency 
  update = () => {
    this.#frequency++;

    let now = new Date();
    this.#recency = now.getTime() + now.getMilliseconds();
  }

  get frequency() { return this.#frequency; }
  get recency() { return this.#recency; }

  toString = () => {
    return `{ frequency: ${this.#frequency}, recency: ${this.#recency} }`;
  }
}