I've been sipping coffee, rolling cigarettes, and pulling my hair since the day before yesterday.

I have trouble setting what I thought a useful dynamic middle-ware for later implementations. (Post request with so much constraints, now and surely in future).

The issue is with Multer (or connect-sequence see later). If you know, Multer is a Node.js middleware for handling (including parsing) multipart/form-data.

What I expect could differ though and this is why I'm setting this super intelligent middle-ware.

I found how to chain middle-wares on runtime, which is not a very usual task, but possible and very useful for my case:

connect-sequence

The solution dates a bit as you can see. Anyway, here is a slice of my middle-ware code (that constructs and runs other middle-wares in return).

async function magicMiddleware(req, res, next) {
    const contentType = req.get('content-type')
    const form = formidable({})

    if (contentType.indexOf('multipart/form-data') > -1) {
        await form.parse(req, function (err, fields, files) {
            if (err) {
                console.log(String(err));
                return;
            }
            magicMiddleware_(fields)
        })
    } else {
        magicMiddleware_(req)
    }

    const section = req.params[0]
    const method = req.method

    const magicMiddleware_ = (body) => {
        const {
            secured,
            upload,
            geolocation,
            illustrations,
            schema
        } = constraints[process.env.NODE_ENV][method][section]
        singletonSchema = schema()

        // Third party middlewares
        var seq = new ConnectSequence(req, res, next)
        debugger;
        const validating = celebrate({
            [Segments.BODY]: singletonSchema,
        })
        if (upload) { 
            console.log('attaching multer')
            seq.append(ops.multer) 
        }
        if (!singletonSchema.called) {
            seq.append(validating)
        }
        seq.append(errors())
        if (secured) { 
            console.log('attaching multer')
            seq.append(ops.auth0) 
        }
        seq.run()
        console.log(req.body)
    }
}

It is to know that third party middle-wares could mismatch. One adds something to req and one remove the same thing for instance.

I run debugger and couldn't find out but I think multer specifically is not running well, It is running but not well; req.body is {}, not undefined and not anything else.

I put multer at first in the sequence to be able to parse multipart/form-data data I'm testing right now, then there is celebrate (Joi based validation middle-ware). Then there is auth0 too (for passwordless authentication).

Of course there are multiple application middle-ware attached when the app starts. But none is susceptible of causing harm as if I push multer, celebrate and auth0 as middle-wares directly (as usual), all works fine.

So data is parsed, req.body is there with the uploaded file and input text fields.

I'm pulling my hair, really. As I don't want to cancel all this and start from zero pushing middle-wares directly on multiple post requests (with different constraints: like one runs without authentication, the other expects text/plain all combinations might behave differently on different environments, like no upload when testing with Postman).

If you have a hint, I will follow up with test. If you want to jump on code, here is the file and here is the repo.

Note: there is a bug in connect-sequence. It is expecting err (passed through next(err)) to be different that undefined. But Multer appears to push null when there is no error. So there is this, I modified code accordingly.

From this:
if (err !== undefined) {

To this:
if(err)

Final note: I want to believe in in connect-sequence as I couldn't find my way adding middle-wares at runtime. I went through code (not debugging) and all seems well to my eyes.

Thanks a lot !!!