mongodb aggregate

Expense
        .aggregate(
            [
                {
                    $match: {
                        creator: new mongoose.Types.ObjectId(req.userId)
                    }
                },
                {
                  $group :
                    {
                      _id : "$category",
                      totalSaleAmount: { $sum: "$amount" }
                    }
                 }
            ]
        )
        .then(result => {
            res.status(200).json({ message: 'Expense By Category for current User', expensesByCategory: result });
        })
        .catch(err => {
            if (!err.statusCode) {
                err.statusCode = 500;
            }
            next(err);
        })