docs: function delExpiredKeys(num, threshold) {
  
    function removeExpiredKeys(count) {
      if(!this._ekeys.length) return 0;
      var num = num || 100
        , threshold = threshold || 25
        , count = count || 0
        , i
        , key
        , ind;
  
      for(i = 0;i < num;i++) {
        ind = Math.floor(Math.random() * this._ekeys.length);
        key = this._ekeys[ind];
        if(this._expires[key] <= Date.now()) {
          this.emit('expired', key);
          this.delKey(key);
          count++;
        }
      }
  
      if(count >= threshold) {
        setImmediate(removeExpiredKeys);
      }
  
      return count;
    }
  
    removeExpiredKeys = removeExpiredKeys.bind(this);
    return removeExpiredKeys();
  }