How to handle file uploads in Express.js with Multer?

File Uploads in Express.js

Express.js doesn't handle file uploads natively. Multer is the most popular middleware for handling multipart/form-data (file uploads).

Setup

bash

npm install multer

Basic File Upload

const multer = require('multer');

// Store in memory (for processing/uploading to cloud)
const upload = multer({ storage: multer.memoryStorage() });

// Single file upload
app.post('/upload/avatar', upload.single('avatar'), (req, res) => {
  console.log(req.file);
  // {
  //   fieldname: 'avatar',
  //   originalname: 'photo.jpg',
  //   mimetype: 'image/jpeg',
  //   size: 123456,
  //   buffer: <Buffer ...>
  // }
  res.json({ message: 'Uploaded!', file: req.file.originalname });
});

Disk Storage with Custom Naming

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

const upload = multer({ storage });

app.post('/upload', upload.single('file'), (req, res) => {
  res.json({ path: req.file.path });
});

Multiple Files

// Multiple files with same field name
app.post('/upload/photos', upload.array('photos', 10), (req, res) => {
  console.log(req.files); // Array of file objects
  res.json({ count: req.files.length });
});

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

app.post('/upload/profile', cpUpload, (req, res) => {
  console.log(req.files['avatar']);   // Array with 1 file
  console.log(req.files['gallery']); // Array with up to 8 files
});

File Validation

const upload = multer({
  storage: multer.memoryStorage(),
  limits: {
    fileSize: 5 * 1024 * 1024, // 5MB max
    files: 5                    // Max 5 files
  },
  fileFilter: (req, file, cb) => {
    const allowedTypes = ['image/jpeg', 'image/png', 'image/webp'];

    if (allowedTypes.includes(file.mimetype)) {
      cb(null, true);
    } else {
      cb(new Error('Only JPEG, PNG, and WebP images are allowed'), false);
    }
  }
});

// Handle Multer errors
app.post('/upload', upload.single('image'), (req, res) => {
  res.json({ success: true });
});

app.use((err, req, res, next) => {
  if (err instanceof multer.MulterError) {
    if (err.code === 'LIMIT_FILE_SIZE') {
      return res.status(400).json({ error: 'File too large (max 5MB)' });
    }
    return res.status(400).json({ error: err.message });
  }
  next(err);
});

Upload to Cloud (S3 Example)

const { S3Client, PutObjectCommand } = require('@aws-sdk/client-s3');
const upload = multer({ storage: multer.memoryStorage() });

const s3 = new S3Client({ region: 'us-east-1' });

app.post('/upload', upload.single('file'), async (req, res) => {
  const key = `uploads/${Date.now()}-${req.file.originalname}`;

  await s3.send(new PutObjectCommand({
    Bucket: process.env.S3_BUCKET,
    Key: key,
    Body: req.file.buffer,
    ContentType: req.file.mimetype
  }));

  res.json({ url: `https://${process.env.S3_BUCKET}.s3.amazonaws.com/${key}` });
});

Summary

Feature	Implementation
Single file	`upload.single('field')`
Multiple files	`upload.array('field', max)`
Multiple fields	`upload.fields([...])`
File size limit	`limits: { fileSize: ... }`
Type validation	`fileFilter` callback
Memory storage	`multer.memoryStorage()`
Disk storage	`multer.diskStorage({...})`

Production tip: Always validate file type and size. Store files in cloud storage (S3, GCS, Azure Blob) rather than on disk. Use signed URLs for secure access. Process images asynchronously (resize, compress) using libraries like sharp.

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