How to send something after done formidable processing?

I learn to use formidable to process form with code like below.

router.post('/add/', function(req, res) {

    var form = new formidable.IncomingForm();
    form.parse(req, function (err, fields, files) {
        ...
        res. ???

After processing is done, I want to send some message back. I have tried res.write, res.send and res.redirect, none works with error like cannot set header, write after end etc.

So what is the right to do it? Thanks.

via user180574