Writing an image compression app with NodeJS, imagemin & pngquant (Part III)

Part III into writing an image compression app with NodeJS, imagemin & pngquant.

Writing an image compression app with NodeJS, imagemin & pngquant (Part III)

Hey guys, welcome back to Part III of my mini-app-cli-ish image compression series! If you haven't read part Part II, you can do so here. By the way, ghost added image compression to their core so your images will be compressed automatically by default!

Anyway, I'll continue this tutorial series because I'm starting a new Fotohaecker Project from scratch with Elixir & Phoenix Framework and I need a compression CLI to process users photo uploads.

Updating our dependencies

Because my last post is a bit old, we first want to check for dependency updates. For this, I find two libraries very useful: npm-check-updates and  npm-check. The first one is more popular but the second has a graphical User-Interface - so you decide!

npm-check-updates: ncu

with ncu, you can preview the dependencies that are out of date. After that, you can run `ncu -u` to update all dependencies.

npm-check: npm-check (note, I added the -u flag to skip unused dependencies warnings, as we haven't actually used mozjpeg etc., yet.)

With npm-check, dependencies are grouped by the changes of the outdated dependencies. You can select those dependencies to update with Spacebar and install them via Enter.

Into compression

Compression model

Now, lets get started with the actual compression of our images. It's triggered by the CLI command node run.js foo.jpg --compress [compression-level] and evaluated in our code src/cli-controller.js:18~25.

We can start adding the imagemin calls right in the controller but I'd much rather prefer a model for this. First, let's download a sample image to compress and place it into  /images/test.jpg. Also, add a ./src/compression-model.js :

"use strict";

const imagemin = require("imagemin");
const imageminMozjpeg = require("imagemin-mozjpeg");
const fs = require("fs");

module.exports.compress = async (options) => {
  const { sourcePath } = options;
  // Check if file exists
    .catch((err) => console.error(err));

const compressImage = async ({ sourcePath, destinationPath }) => {
  const compressed_image = await imagemin([sourcePath], {
    destination: destinationPath,
    plugins: [imageminMozjpeg()],

basic ./src/compression-model.js that throws an error if the file doesn't exist.

Note the following things:

  • with fs.promises.access, we check if the sourcePath-File exists. Technically, we're introducing race-conditions with that but that's fine for me.
  • We're passing options to our  compressImage function but only use sourcePath & destinationPath for now.
  • imagemin is asynchronous so our compressImage function needs to be async aswell to call await imagemin. We'll come to that later.
  • Finally, we're printing out the result of imagemin with console.debug().

updating our CLI controller

Then, we want to expand our CLI with a parameter that determines the destination to save the compressed image as. Our CLI will look like this then: node run.js [sourceFile] [desinationFile] [--compress] [compression-level]. With this, we'll also refactor our code a bit so missing parameters are catched earlier:

"use strict";

const compressionModel = require("./compression-model.js");

// function that decides what do do
// remember the syntax: `node run.js [sourceFile] [desinationFile] [--compress/--resize] [compression-level]`
module.exports.pass = async (args) => {
  const errors = await checkArgsForErrors(args);

  if (errors) {
  } else {
    const [sourcePath, destinationPath, command, ...options] = args;
    // only run this if args where provided
    switch (command) {
      case "--compress": // fall-through to case "compress"
      case "compress":
          sourcePath: sourcePath,
          destinationPath: destinationPath,
          options: options,
      case "--resize": // fall-through to case "resize"
      case "resize":
        console.log(`** Insert resizing here, using options ${options} **`);
          `${command} is not a valid action, either use --resize or --compress.`

const checkArgsForErrors = async (args) => {
  let errors = "";
  if (args.length < 1) {
    // warning the user that he did not provide a source file. !TODO we might want to print out our apps syntax here.
    errors += `You did not provide a source file. `;
  if (args.length < 2) {
    // warning the user that he did not provide a destination file. !TODO we might want to print out our apps syntax here.
    errors += `You did not provide a destination file. `;
  if (args.length < 3) {
    // warning the user that he did not provide an action
    errors += ` You didn't provide an action, such as --compress or --resize. `;

  return errors;
./src/cli-controller.js that now can get more parameters

Note the following things:

  • As imagemin is asynchronous (That means that we have to wait for imagemin to finish processing our image before we can actually say that it succeeded and furthermore process it), we also needed to change all functions upstream to be async. You can find a nice article about asynchronous concepts on MDN
  • I added a checkArgsForErrors function that adds up errors that might occur. When we start resizing, we can also move the check if the source file exists here
  • With const [sourcePath, destinationPath, command, ...options] = args;, args are destructured into more meaningful variables that we can use in the switch.

Make sure to also update the controller function call in run.js:9~12:

// function to handle user input, args is the array containing the arguments
const handleCliInput = async (args) => {
  await controller.pass(args);

Run it!

You can now run our CLI with node run.js ./images/test.jpg ./images/compressed/test.jpg --compress

It works!

If the source file can't be found, our CLI will print out an error just like this:

If the file doesn't exist, we print an error!

Wrapping it up

Today, we refactored our cli-controller and added an asynchronous compression-model that takes different options to pass it to imagemin.

If you want to clone the project, you can: https://github.com/fschoenfeldt/ghost-compression

Thanks for checking out my guide series for writing an image compression app with NodeJS. If you have any questions, feel free to contact me via https://fschoenfeldt.de - Part IV is coming, very soon! x

Photo by Tim Mossholder on Unsplash