code: function prepareNativeRequest (self) {
  
      if (self._emitter) {
          self._doUploadProgress = function (eventType, event) {
              event.lengthComputable = (event.loaded >= 0) && (event.total > 0); 
              self._emitter.emit(eventType, event);
          };
      }
  
      return self;
  }
docs: *****************************
  Preparing the request, either for native or browser's own.