code: function() {
  
              // return list
              var ret = [];
  
              // get current timestamp
              var timestamp = this.getTimestamp();
  
              // iterate over storage keys to find all counters
              var that = this;
              this._iterKeys(function(storageKey) {
  
                  // if its not a timestamp key, skip it
                  if (storageKey.indexOf(that._expiration_key_prefix) === 0) {
  
                      // get item key
                      var itemKey = storageKey.substr(that._expiration_key_prefix.length);
  
                      // if expired remove it + the item
                      if (that.isExpired(itemKey)) {
  
                          that.removeItem(itemKey);
                          ret.push(itemKey);
                      }
                  }
              });
  
              // return list with removed keys
              return ret;
          }
docs: Clear expired keys.
  If you never call this function, expired keys will remain until you try to get them / reset a new value.
  
  @param return: List of removed keys due to expiration.