docs: function (fn) {
      if (!fn || !_.isFunction(fn)) {
        var err = new Error('Please pass a valid function!');
        debug(err);
        throw err;
      }
  
      if (fn.length === 1) {
        fn = _.wrap(fn, function(func, cmd, callback) {
          func(cmd);
          callback(null);
        });
      }
  
      this.onCommandHandle = fn;
  
      return this;
    }