code: function clear (store) {
    const forDeletion = []
    const now = Date.now()
  
    for (const [key, entry] of store) {
      // mark for deletion entries that have timed out
      if (now - entry.created > entry.ttl) {
        forDeletion.push(key)
      }
    }
  
    const forDeletionLength = forDeletion.length
    // if nothing to clean then exit
    if (forDeletionLength === 0) {
      return 0
    }
  
    debug(`local revalidation flags that timed out: ${forDeletion}`)
    forDeletion.forEach(key => store.delete(key))
    return forDeletionLength
  }
docs: It marks and deletes all the keys that have their ttl expired
  @param {Map} store Map Object that stores all the cached keys
  @returns {Number} ammount of entries deleted