code: function () {
        var count = 0
          , tail = this._tail
          , head = this._head
          , keys = new Array(this._len);
  
        for (var i = tail; i < head; i++) {
          var entry = this._byOrder[i];
          if (entry) keys[count++] = entry.key;
        }
  
        return keys;
      }
docs: return array of keys in least recently used order
  @return {Array}