Fixing Node.js & Express File Upload Error: req.files undefined

When you’re working with file uploads in Node.js and Express, encountering ‘req.files undefined‘ can be a common issue. This error typically arises because the middleware required to parse multipart/form-data, which is the enctype that forms must use to upload files, is not configured or is misconfigured in your Express application.

Reasons Behind the Error

• Lack of middleware for handling multipart/form-data.
• Incorrect usage or configuration of the file upload middleware.
• Front-end form might not be set to enctype='multipart/form-data'.

Steps to Fix the Error

1. Ensure that the express-fileupload or multer middleware is installed.
2. Correctly configure the middleware in your Express app.
3. Verify that the front-end form is correctly set up to send files.

Complete Code Examples

Using express-fileupload

const express = require('express');
const fileUpload = require('express-fileupload');

const app = express();

// Default options
app.use(fileUpload());

// POST endpoint for file upload
app.post('/upload', function(req, res) {
  if (!req.files || Object.keys(req.files).length === 0) {
    return res.status(400).send('No files were uploaded.');
  }

  // Accessing the uploaded file via the name attribute of the input field
  let uploadedFile = req.files.myFile;

  // Use the mv() method to place the file on the server
  uploadedFile.mv('/desired/path/' + uploadedFile.name, function(err) {
    if (err)
      return res.status(500).send(err);

    res.send('File uploaded!');
  });
});

app.listen(3000, () => console.log('Server started on port 3000'));

Using multer

const express = require('express');
const multer = require('multer');
const upload = multer({ dest: 'uploads/' });

const app = express();

// POST endpoint for file upload
app.post('/upload', upload.single('myFile'), function(req, res, next) {
  const file = req.file;
  if (!file) {
    return res.status(400).send('Please upload a file.');
  }
  res.send('File uploaded successfully.');
});

app.listen(3000, () => console.log('Server started on port 3000'));

Note that when using multer, the upload.single('myFile') middleware specifically handles one file, where ‘myFile’ is the name of the form field in your front-end. Additional configurations can be made to handle multiple files or to customize the storage.

Verifying Front-end Form Setup

<!-- Front-end HTML form -->
<form action="/upload" method="post" enctype="multipart/form-data">
  <input type="file" name="myFile" />
  <input type="submit" value="Upload File" />
</form>

Make sure that your HTML form includes the attribute enctype="multipart/form-data" to handle file uploads properly.

By following these steps and using the provided code examples, you should be able to resolve the ‘req.files undefined‘ error in your Node.js and Express applications when attempting to upload files.