code: function executeBatch(bulkOperation, batch, options, callback) {
    function resultHandler(err, result) {
      // Error is a driver related error not a bulk op error, terminate
      if (((err && err.driver) || (err && err.message)) && !(err instanceof MongoWriteConcernError)) {
        return handleCallback(callback, err);
      }
  
      // If we have and error
      if (err) err.ok = 0;
      if (err instanceof MongoWriteConcernError) {
        return handleMongoWriteConcernError(batch, bulkOperation.s.bulkResult, false, err, callback);
      }
      handleCallback(
        callback,
        null,
        mergeBatchResults(false, batch, bulkOperation.s.bulkResult, err, result)
      );
    }
  
    bulkOperation.finalOptionsHandler({ options, batch, resultHandler }, callback);
  }
docs: Execute the command
  
  @param {UnorderedBulkOperation} bulkOperation
  @param {object} batch
  @param {object} options
  @param {function} callback