How to return an error back to ExpressJS from middleware?

Question

I am using [Multer][1] as middleware to process multipart form data. Multer offers some configuration options for setting destination of file uploads and names called diskStorage . It is within this area that one can do some error checking and control whether Multer authorises a file upload or not.

My Express route is basically this:

expressRouter.post(['/create'],
    MulterUpload.single("FileToUpload"), // if this throws an error then have Express return that error to the user
    async function(req, res) {
      // handle the form text fields in req.body here
});

MulterUpload.single() takes the file input field named "FileToUpload" and sends it off to do this:

const MulterUpload = multer({
    storage: MulterStorage
)}

const MulterStorage = multer.diskStorage({
    destination: async function (req, file, cb) {
        try {
            if ("postID" in req.body && req.body.postID != null && req.body.postID.toString().length) {

                const Result = await api.verifyPost(req.body.postID)
                if (Result[0].postverified == false) {
                    const Err = new Error("That is not your post!");
                    Err.code = "ILLEGAL_OPERATION";
                    Err.status = 403;
                    throw(Err); // not authorised to upload
                } else {
                    cb(null, '/tmp/my-uploads') // authorised to upload
                }
            }
        } catch (err) {
            // How do I return the err back to Express so it can send it to the user? The err is an unresolved Promise as I am using async/await
        }
    }
    ,
    filename: function (req, file, cb) {
        cb(null, file.fieldname + '-' + Date.now())
    }
})

I just can't seem to work out how to get the error from MulterStorage back to Express so that it is sent back the browser/user as an error. [1]: https://www.npmjs.com/package/multer

Answer 1

You can call the completion callback with an Error object as the first argument. So, instead of

cb(null, someResult)

you call the callback with an error object

cb(new Error("I got a disk error"));

Then, if you have multer set up as plain middleware, this will result in next(err) being called and in Express, your generic error handler will receive the error.

Here are a couple examples:

https://www.npmjs.com/package/multer#error-handling

https://github.com/expressjs/multer/issues/336#issuecomment-242906859