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File Uploads in Express.js

Handling file uploads (like images, PDFs, or videos) is a common requirement for many web applications. In the Express.js ecosystem, Multer is the standard middleware used for handling multipart/form-data, which is primarily used for uploading files.

In this guide, we’ll cover how to install, configure, and use Multer to handle single and multiple file uploads.

What is Multer?

Multer is a Node.js middleware for handling multipart/form-data. It adds a body object and a file (or files) object to the request object. The body object contains the values of the text fields of the form, and the file object contains the files uploaded via the form.

Getting Started

First, install Multer via npm:

Terminal window
npm install multer

Basic Setup

Here’s a simple example of how to set up Multer to store uploaded files in a local folder.

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


// Configure storage
const storage = multer.diskStorage({
  destination: function (req, file, cb) {
    cb(null, 'uploads/'); // Ensure this folder exists
  },
  filename: function (req, file, cb) {
    const uniqueSuffix = Date.now() + '-' + Math.round(Math.random() * 1E9);
    cb(null, file.fieldname + '-' + uniqueSuffix + path.extname(file.originalname));
  }
});


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


// Single file upload route
app.post('/upload-profile', upload.single('profile_pic'), (req, res) => {
  // req.file is the `profile_pic` file
  // req.body will hold the text fields, if there were any
  console.log(req.file);
  res.send('File uploaded successfully!');
});


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

Uploading Multiple Files

To upload multiple files, you can use upload.array() for multiple files with the same field name, or upload.fields() for multiple files with different field names.

Using upload.array()

app.post('/upload-photos', upload.array('photos', 12), (req, res) => {
  // req.files is array of `photos` files
  console.log(req.files);
  res.send('Photos uploaded!');
});

File Validation and Filtering

You can limit file sizes and filter file types (e.g., only allowing images) using the fileFilter option.

const upload = multer({
  storage: storage,
  limits: { fileSize: 1024 * 1024 * 5 }, // Limit size to 5MB
  fileFilter: (req, file, cb) => {
    const filetypes = /jpeg|jpg|png|gif/;
    const extname = filetypes.test(path.extname(file.originalname).toLowerCase());
    const mimetype = filetypes.test(file.mimetype);


    if (mimetype && extname) {
      return cb(null, true);
    } else {
      cb('Error: Images Only!');
    }
  }
});

Key Takeaways

  • Multer is the go-to middleware for multipart/form-data.
  • Use diskStorage for fine-grained control over file destinations and naming.
  • upload.single() is for one file, upload.array() for many files under one name.
  • Always implement file size limits and type filtering for security and performance.

Now you can easily integrate file upload functionality into your Express.js applications!