multer: dynamic destination path

edited

See https://github.com/expressjs/multer#diskstorage

Note that req.body might not have been fully populated yet. It depends on the order that the client transmits fields and files to the server.

Due to that, first write file in temp directory, read directory name from req and move file:

fs = require('fs-extra'); //npm install fs.extra
...

var storage = multer.diskStorage({ 
    destination: function (req, file, cb) {
        cb(null, '../tempDir/')
    },
    filename: function (req, file, cb) {
        cb(null, file.originalname)
    }
});

var upload = multer({ 
    storage: storage
}).single('file');

upload(req, res, function (err) {
    if (err) {
        res.json({});
        return;
    }

    var dir = JSON.parse(req.body.data).directory;
    var filename = req.file.filename;

    fs.move('../tempDir/' + fileName, '../tempDir/' + dir + '/' + fileName, function (err) {
        if (err) {
            return console.error(err);
        }

        res.json({});
    });

});

Make sure you append first the textfields on the client-side and only then do you append the files. In my case i had something like this:

` 

for(let i=0; i<files.length;i++)
{
  formData.append("files[]",files[i]);
}
formData.append("username",username);

  `

The fix was to first append the textfield like so:

`

formData.append("username",username);
for(let i=0; i<files.length;i++)
{
  formData.append("files[]",files[i]);
}

`

Here's what I do for uploading files to dynamic directories.

In frontend I use URL parameters to pass user IDs.

await axios({
  method: 'post',
  data: formData,
  url: '/api/upload?userId=123',
  headers: { 'content-type': 'multipart/form-data' }
})

In backend get that parameter and use for destination. Also create the directory if it doesn't exist.

const upload = multer({
  storage: multer.diskStorage({
    destination: (req, file, cb) => {
      const directory = `./public/uploads/${req.query.userId}`

      if (!fs.existsSync(directory)) {
        fs.mkdirSync(directory, { recursive: true })
      }

      cb(null, directory)
    },
    filename: (req, file, cb) => {
      cb(null, `${Date.now()}-${file.originalname}`)
    }
  })
})