code: function paramCallback(err) {
      var fn = paramCallbacks[paramIndex++];
  
      // store updated value
      paramCalled.value = req.params[key.name];
  
      if (err) {
        // store error
        paramCalled.error = err;
        param(err);
        return;
      }
  
      if (!fn) return param();
  
      try {
        fn(req, res, paramCallback, paramVal, key.name);
      } catch (e) {
        paramCallback(e);
      }
    }
docs: single param callbacks