File Uploads with Multer in Express

Multer handles multipart/form-data - the format for file uploads.

Setup

npm install multer

Basic Configuration

const multer = require('multer');

// Simple memory storage
const upload = multer({ dest: 'uploads/' });

// Or disk storage with custom naming
const storage = multer.diskStorage({
  destination: (req, file, cb) => {
    cb(null, 'uploads/');
  },
  filename: (req, file, cb) => {
    const uniqueName = Date.now() + '-' + file.originalname;
    cb(null, uniqueName);
  }
});

const upload = multer({ storage });

Single File Upload

app.post('/upload', upload.single('avatar'), (req, res) => {
  console.log(req.file);
  // {
  //   fieldname: 'avatar',
  //   originalname: 'photo.jpg',
  //   filename: '1234567890-photo.jpg',
  //   path: 'uploads/1234567890-photo.jpg',
  //   size: 12345
  // }
  res.json({ file: req.file });
});

Multiple Files

// Multiple files, same field
app.post('/photos', upload.array('photos', 5), (req, res) => {
  console.log(req.files);  // Array of files
  res.json({ files: req.files });
});

// Multiple fields
const cpUpload = upload.fields([
  { name: 'avatar', maxCount: 1 },
  { name: 'gallery', maxCount: 8 }
]);

app.post('/profile', cpUpload, (req, res) => {
  console.log(req.files.avatar);   // Single file
  console.log(req.files.gallery);  // Array of files
});

File Filtering

const imageFilter = (req, file, cb) => {
  const allowedTypes = ['image/jpeg', 'image/png', 'image/gif'];
  
  if (allowedTypes.includes(file.mimetype)) {
    cb(null, true);
  } else {
    cb(new Error('Only images allowed'), false);
  }
};

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

Error Handling

app.post('/upload', (req, res) => {
  upload.single('file')(req, res, (err) => {
    if (err instanceof multer.MulterError) {
      if (err.code === 'LIMIT_FILE_SIZE') {
        return res.status(400).json({ error: 'File too large' });
      }
      return res.status(400).json({ error: err.message });
    }
    if (err) {
      return res.status(400).json({ error: err.message });
    }
    res.json({ file: req.file });
  });
});

Memory Storage (for cloud upload)

const upload = multer({ storage: multer.memoryStorage() });

app.post('/upload', upload.single('file'), async (req, res) => {
  // req.file.buffer contains file data
  // Upload to S3, Cloudinary, etc.
  
  const buffer = req.file.buffer;
  // await uploadToCloud(buffer);
  
  res.json({ message: 'Uploaded' });
});

Complete Example

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

const app = express();

const storage = multer.diskStorage({
  destination: 'uploads/',
  filename: (req, file, cb) => {
    const ext = path.extname(file.originalname);
    cb(null, `${Date.now()}${ext}`);
  }
});

const upload = multer({
  storage,
  limits: { fileSize: 5 * 1024 * 1024 },
  fileFilter: (req, file, cb) => {
    const allowed = /jpeg|jpg|png|gif/;
    const ext = allowed.test(path.extname(file.originalname).toLowerCase());
    const mime = allowed.test(file.mimetype);
    
    if (ext && mime) cb(null, true);
    else cb(new Error('Images only!'));
  }
});

// Serve uploaded files
app.use('/uploads', express.static('uploads'));

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

app.listen(3000);

Key Takeaway

Multer handles file uploads in Express. Use single() for one file, array() for multiple. Filter by type/size for security. Use memory storage for cloud uploads, disk storage for local files.