How to modify the multer filename with front variables?

Question

Good morning, I am creating an application React and Node, and at the moment I need to upload a PDF file, the code works well, but I wanted to know how when it comes to mounting the filename of the file by the multer, I use variables of the front itself? thank you.

The code is as follows:

Front:

const sendFile = () => {
      const dado = new FormData() 
      dado.append('file', file)
      axios.post("http://localhost:8080/api/comprebem/file", dado, { // receive two parameter endpoint url ,form data 
    })
    .then(res => { // then print response status
      console.log(res.statusText)
    })
    }

Back:

const storage = multer.diskStorage({
  destination:"../login/public/comprebem/ibama/",
  filename: function (req, file, cb) {
    cb(null, `unidade${Date.now()}_data${file.originalname}` )
}
})

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

app.post('/api/comprebem/file',function(req, res) {
     
  upload(req, res, function (err) {
         if (err instanceof multer.MulterError) {
             return res.status(500).json(err)
         } else if (err) {
             return res.status(500).json(err)
         }
    return res.status(200).send(req.file)

  })

});

If anyone can answer me too why you’re making that mistake:

It gives when I send the file, but it is working all correctly, why there is error?