Handle File Upload with Express and Multer

Express undoubtedly has become a popular framework for building web applications based on Node.js. It is shipped with support for handling file uploading using middleware that takes the user requests, parses the contents for the files, and provides the next handler with the files information. The following snippet shows a basic example of handling files uploading in Express using Multer.

const multer = require('multer');

const storage = multer.diskStorage({
  destination: (req, file, cb) => {
    // store files to "uploads/" directory
    cb(null, 'uploads/');
  },
});

const upload = multer({ storage });

// initiate an upload handler that can accept multiple fields and multiple files
const uploadHandler = upload.fields([
  { name: 'galleryImages', maxCount: 10 }, 
  { name: 'userFiles', maxCount: 2 },
]);

app.post('/upload', uploadHandler, (req, res) => {
  res.json(req.files);
});

The sample above can be used if we just want to store our files in the disk storage of our server. If we want to store it somewhere else, for example in the cloud storage, we can perform an additional action after we get the files information from req.files object.

If we want a more uncomplicated solution, without a need for additional action for managing files manually, we can look for a custom Multer storage engine in the NPM repository that aligns with our intention. In our common use cases for building web-based applications, we usually store the files in the cloud, especially in the object storage that is compatible with S3 protocols, to decouple our application with the storage. Then, we oftentimes need to resize some image files to be more optimized to be displayed on the web like for the thumbnails.

In that scenario, we can utilize my library named multer-object-storage which provides a storage engine that can handle both disk storage and S3-compatible object storage uploading. Besides, it can handle image resizing before the uploading process ensued. The following snippet shows how to configure this storage engine to store files both on disk and object storage.

require('dotenv').config();
const express = require('express');
const multer  = require('multer')
const { ObjectStorage } = require('multer-object-storage');
const app = express();
const port = 3000;

const storage = new ObjectStorage({
  
  // optional: set file name generator 
  filename: function (req, file, cb) {
    cb(
      null,
      Math.ceil(Math.random() * 10000) + file.originalname,
    );
  },

  // optional: set destination directory for disk saving 
  destination: function (req, file, cb) {
    cb(null, 'uploads/');
  },

  // optional: set list of image resize options, based on Sharp resize options
  resize: function (req, file, cb) {
    cb(null, {
      // set allowed image mime-types that can be resized
      mimeTypes: ['image/jpg', 'image/jpeg', 'image/png'], 
      options: [
        {
          width: 200,
          height: 150,
          // by default, file name will be appended by string with format "w{width}-h{height}"
          fileNameTail: 'thumb', 
        },
        {
          width: 300, // height will be auto based on image ratio
        },
      ],
    });
  },

  // optional: set S3-compatible object storage credentials
  bucket: function (req, file, cb) {
    cb(null, {
      name: process.env.BUCKET_NAME,
      endpoint: process.env.BUCKET_ENDPOINT, // include protocol, eg. https://sgp1.digitaloceanspaces.com
      accessKeyId: process.env.BUCKET_ACCESS_ID,
      secretAccessKey: process.env.BUCKET_SECRET_KEY,
    });
  },
});

const upload = multer({ storage });

// allows 5 fields with name "image" and 2 fields with name "file"
const uploadHandler = upload.fields([{ name: 'image', maxCount: 5 }, { name: 'file', maxCount: 2 }]);

// send a multipart form-data that contains files to this endpoint
app.post('/upload', uploadHandler, (req, res) => {
  res.json(req.files);
});

app.listen(port, () => {
  console.log(`Listening on port ${port}`)
});

We can test the program above using Postman by sending a POST request with a multipart form-data payload to http://localhost:3000. We will get a result with a format that is similar to the following example.

// ...
"image": [
   {
      "fieldname": "image",
      "originalname": "my-image.jpg",
      "encoding": "7bit",
      "mimetype": "image/jpeg",
      "destination": "uploads/",
      "filename": "123-my-image.jpg",
      "path": "/path/to/uploads/123-my-image.jpg",
        "size": 111083,
        "url": "https://x.myspaces.com/123-my-image.jpg",
        "width": 669,
        "height": 562,
        "resize": [
          {
            "path": "/path/to/uploads/123-my-image-thumb.jpg",
            "url": "https://x.myspaces.com/123-my-image-thumb.jpg",
            "width": 200,
            "height": 150
          },
          {
            "path": "/path/to/uploads/123-my-image-thumb.jpg",
            "url": "https://x.myspaces.com/123-my-image-w300hauto.jpg",
            "width": 300,
            "height": null
// ...

We can read more on its documentation on the repository page.