code: function(err) {
                  if (err) {
                      // eslint-disable-next-line no-console
                      console.log("\n");
                      return Promise.reject(err);
                  }
  
                  // Get 20 files
                  let _files = filesNames.splice(0, batch);
  
                  if (_files.length) {
                      return Promise.all(_files.map(performUpload)).then(() => execStack(), execStack);
                  } else {
                      return Promise.resolve();
                  }
              }
docs: Execute stack according to `batch` option