Node.js and Multer - Handle the destination of the uploaded file in callback function (req,res)

Update

Quite a few things have changed since I posted the original answer.

With multer 1.2.1.

1. You need to use DiskStorage to specify where & how of the stored file.
2. By default, multer will use the operating system's default directory. In our case, since we are particular about the location. We need to ensure that the folder exists before we could save the file over there.

Note: You are responsible for creating the directory when providing destination as a function.

More here

'use strict';

let multer = require('multer');
let fs = require('fs-extra');

let upload = multer({
  storage: multer.diskStorage({
    destination: (req, file, callback) => {
      let userId = req.user._id;
      let path = `./public/uploads//${userId}`;
      fs.mkdirsSync(path);
      callback(null, path);
    },
    filename: (req, file, callback) => {
      //originalname is the uploaded file's name with extn
      callback(null, file.originalname);
    }
  })
});

app.post('/', upload.single('file'), (req, res) => {
  res.status(200).send();
});

fs-extra for creating directory, just in case it doesn't exists

Original answer

You can use changeDest

Function to rename the directory in which to place uploaded files.

It is available from v0.1.8

app.post('/', multer({
dest: './public/uploads/',
changeDest: function(dest, req, res) {
    var newDestination = dest + req.user._id;
    var stat = null;
    try {
        stat = fs.statSync(newDestination);
    } catch (err) {
        fs.mkdirSync(newDestination);
    }
    if (stat && !stat.isDirectory()) {
        throw new Error('Directory cannot be created because an inode of a different type exists at "' + dest + '"');
    }
    return newDestination
}
}), function(req, res) {
     //set your response
});