Let's learn Multer in Express.js step by step — from basics to advanced usage — with clear examples.

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📦 What is Multer?

Multer is a middleware for Express.js used to handle multipart/form-data, which is the format used for file uploads (images, PDFs, videos, etc.).

It lets you:

• Upload single or multiple files
• Store files locally or in memory
• Filter files (e.g., only images)
• Control file names and storage location

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✅ 1. Install Multer

npm install multer

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✅ 2. Basic Setup — Upload a Single File

➤ Folder structure (simple)

project/
 ├─ uploads/
 ├─ server.js

➤ server.js

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

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

app.post('/upload', upload.single('file'), (req, res) => {
  console.log(req.file);
  res.send('File uploaded!');
});

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

➤ How to test

Use Postman:

• POST → http://localhost:3000/upload
• Body → form-data
• Add key file → type: file → choose a file

🎉 The file will save in uploads/ with a random filename.

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✅ 3. Upload Multiple Files

Upload multiple files in the same field

app.post('/upload-multi', upload.array('photos', 10), (req, res) => {
  console.log(req.files);
  res.send('Multiple files uploaded!');
});

• array(fieldName, maxCount)

Upload files from different fields

app.post('/upload-fields',
  upload.fields([
    { name: 'avatar', maxCount: 1 },
    { name: 'gallery', maxCount: 5 }
  ]),
  (req, res) => {
    console.log(req.files);
    res.send('Files uploaded from multiple fields');
  }
);

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📁 4. Custom Storage with Filenames

By default, Multer gives random filenames. Let’s customize:

const storage = multer.diskStorage({
  destination: function (req, file, cb) {
    cb(null, 'uploads/');
  },
  filename: function (req, file, cb) {
    const ext = file.originalname.split('.').pop();
    cb(null, Date.now() + '-' + file.fieldname + '.' + ext);
  }
});

const upload = multer({ storage });

Now the uploaded files have more readable names like:

1732459023000-file.png

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🔒 5. Filter File Types (e.g., Only Images)

function fileFilter(req, file, cb) {
  if (file.mimetype.startsWith('image/')) {
    cb(null, true);   // accept file
  } else {
    cb(new Error('Only images are allowed!'), false);
  }
}

const upload = multer({
  storage,
  fileFilter
});

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🧠 6. Limit File Size

const upload = multer({
  storage,
  limits: { fileSize: 2 * 1024 * 1024 } // 2 MB
});

If a file is too large, Multer automatically throws an error.

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🚫 7. Handling Multer Errors with Express Middleware

Instead of manually wrapping(commented script) upload.single(...), you can use Express error-handling middleware to catch Multer errors globally:

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

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

// app.post('/upload', (req, res) => {
//   upload.single('file')(req, res, function (err) {
//     if (err instanceof multer.MulterError) {
//       return res.status(400).send('Multer error: ' + err.message);
//     } else if (err) {
//       return res.status(400).send('Error: ' + err.message);
//     }
//     res.send('File uploaded successfully');
//   });
// });

app.post('/upload', upload.single('file'), (req, res) => {
  res.send('File uploaded successfully');
});

// Error-handling middleware
app.use((err, req, res, next) => {
  if (err instanceof multer.MulterError) {
    // Multer-specific errors
    res.status(400).send('Multer error: ' + err.message);
  } else if (err) {
    // Other errors
    res.status(400).send('Error: ' + err.message);
  } else {
    next();
  }
});

✅ Benefits of this approach:

• Separates route logic from error handling
• Works for multiple routes
• Cleaner and scalable for large apps

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☁️ 8. Bonus: Upload to Cloud Storage (S3, Cloudinary)

Once you're comfortable, you can:

• Store files in AWS S3
• Store images in Cloudinary
• Store only metadata in your database

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🎯 Summary — What You’ve Learned

• Install and use Multer
• Upload single & multiple files
• Customize filenames
• Validate file type
• Limit file size
• Handle errors properly using Express middleware

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