How to handle file uploads in Express with Multer

File Uploads with Multer

Multer is the standard Express middleware for handling multipart/form-data — the encoding used for file uploads.

npm install multer

Single file upload with validation

const multer = require('multer');
const path = require('path');

const storage = multer.diskStorage({
  destination: (req, file, cb) => cb(null, 'uploads/'),
  filename: (req, file, cb) => {
    const uniqueName = `${Date.now()}-${Math.round(Math.random()*1e9)}${path.extname(file.originalname)}`;
    cb(null, uniqueName);
  }
});

const fileFilter = (req, file, cb) => {
  const allowed = ['image/jpeg', 'image/png', 'image/webp'];
  if (allowed.includes(file.mimetype)) cb(null, true);
  else cb(new Error('Only JPEG, PNG and WebP allowed'), false);
};

const upload = multer({
  storage,
  limits: { fileSize: 5 * 1024 * 1024 }, // 5MB
  fileFilter
});

app.post('/upload', upload.single('avatar'), (req, res) => {
  if (!req.file) return res.status(400).json({ error: 'No file uploaded' });
  res.json({
    filename: req.file.filename,
    path: `/uploads/${req.file.filename}`,
    size: req.file.size
  });
});

// Multiple files
app.post('/gallery', upload.array('images', 10), (req, res) => {
  res.json({ files: req.files.map(f => f.filename) });
});

Serve the uploads directory statically: app.use('/uploads', express.static('uploads')). Always validate MIME type — never trust the file extension alone.