code: function exists (store, key) {
    const entry = store.get(key)
    // checks entry exists and it has not timed-out
    return !!entry && Date.now() - entry.created < entry.ttl
  }
docs: True if a key exists, and it has not expired
  @param {Map} store Map Object that stores all the cached keys
  @param {String} key key value
  @returns {Boolean} true if key exists, false otherwise