Issue with throwing error

const errorhandler = (err, req, res, next) => {
    console.log(res.statusCode);
    const statusCode = res.statusCode ? res.statusCode : 500; 

    switch (statusCode) {
        case 400:
            return res.status(400).json({
                title: "Validation failed",
                message: err.message,
                stackTrace: err.stack,
            });
        case 401:
            return res.status(401).json({
                title: "Not Authorized",
                message: err.message,
                stackTrace: err.stack,
            });
        case 403:
            return res.status(403).json({
                title: "Forbidden",
                message: err.message,
                stackTrace: err.stack,
            });
        case 404:
            return res.status(404).json({
                title: "Not Found",
                message: err.message,
                stackTrace: err.stack,
            });
        default:
            console.log("No errors");
            break;
    }
    res.json();
};

module.exports = errorhandler;

const updateBook = asyncHandler(async (req, res) => {
    const book = await Books.findById(req.params.bookId);
    if (!book) {
        res.status(404); // Set the response status to 404
        throw new Error("Book not found");
    } else {
        res.status(200).json(book);
    }
});