Resizing images in Node.js applications

Featured in Node Weekly: Issue 73

For the past few months I have been involved, on and off, in building NodeZA - a Node.js information portal and social platform for developers in South Africa. The NodeZA platform is a Node.js application built from the ground up and has CMS capabilities.

Processing images uploaded from user-generated content is one of the problems I encountered while building NodeZA.

This is how I wanted to handle image uploads:

  1. Save the original file
  2. Compress the original file and save the compressed version
  3. Resize the compressed version for different layouts

This turned out to be more challenging than I had imagined. Image processing is a CPU intensive operation that can block the EventLoop if not handled correctly.

I searched the internet for days but could not find a solution that I was happy with. I have been using Gulp to automate image minification and resizing on my local machine with great success. However, Gulp is a command-line build tool, what I wanted was to process images as they were uploaded.

The good news is that Gulp streams all things, a Gulp plugin can be used in any Node.js program without modification.

The Code

NodeZA uses a module called multer for handling image uploads, for the purposes of this tutorial I will create a very simple express application to demonstrate my use case.

We are going to use 2 Gulp plugins for images processing, gulp-image-resize and gulp-imagemin.

Let's create our express app (this requires express to be installed globally npm i expresss -g)

express image-processing

# install express dependencies
cd image-processing && npm install

# install our dependencies
npm install gulp gulp-image-resize gulp-imagemin multer --save

Open app.js in your favourite text editor, remove all the code that we do not need, and include the multer middleware.

var express = require('express');
var path = require('path');
var logger = require('morgan');
var bodyParser = require('body-parser');
var multer = require('multer');
var routes = require('./routes');

var app = express();

// view engine setup
app.set('views', path.join(__dirname, 'views'));
app.set('view engine', 'jade');

// handle image uploads
  dest: './public/images/uploads'

app.use(bodyParser.urlencoded({ extended: false }));
app.use(express.static(path.join(__dirname, 'public')));

app.use('/', routes);

app.listen(app.get('port') || 3000, app.get('ipAddress'),function() {
  console.log("✔ Express server listening on port %d in %s mode", app.get('port') || 3000, app.get('env'));

Up next, open up the routes directory and edit the index.js file.

var express = require('express');
var router = express.Router();
var path = require('path');

// require the image editing file
var editor = path.resolve(__dirname, '../editor.js');

function compressAndResize (imageUrl) {

  // We need to spawn a child process so that we do not block 
  // the EventLoop with cpu intensive image manipulation 
  var childProcess = require('child_process').fork(editor);

  childProcess.on('message', function(message) {

  childProcess.on('error', function(error) {

  childProcess.on('exit', function() {
    console.log('process exited');


/* GET home page. */
router.get('/', function(req, res, next) {
  res.render('index', { title: 'Express' });
});'/upload', function(req, res, next) {
  if (req.files.image_url) {
    compressAndResize('public/images/uploads/' +;
  res.end('Image upload complete');

module.exports = router;

Image processing

Once the form has been submitted and the image uploaded, we need to spawn a child process to handle CPU intensive image manipulation, this prevents us from blocking our precious EventLoop. Create editor.js in the root directory.

var gulp = require('gulp');
var imagemin = require('gulp-imagemin');
var imageResize = require('gulp-image-resize');

function processImg (filesrc) {
 return gulp.src(filesrc)

  // compress and save
  .pipe(imagemin({optimizationLevel: 5}))

  // save 300 x 200
    width: 300,
    height: 200,
    crop: true

  // save 120 x 120
    width: 120,
    height: 120,
    crop: true
  // save 48 x 48
    width: 48,
    height: 48,
    crop: true

process.on('message', function (images) {
  console.log('Image processing started...');

  var stream = processImg(images);

  stream.on('end', function () {
    process.send('Image processing complete');

  stream.on('error', function (err) {

module.exports = {};

Lastly, add form markup to the index.jade file in the views directory.

extends layout

block content
    form(method='post', action='/upload', enctype='multipart/form-data')
      input(type='file', id='image_url', name='image_url')
      input(type='submit', value='Upload image')

That's all. Please make sure you monitor the number of processes running at a time otherwise the app will keep on spawning new processes that could overwhelm your machine.

All the code used in this article can be found on Github - Fork It.

Keep hacking!

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