Minify and compress network payloads with gzip

This codelab explores how both minifying and compressing the JavaScript bundle for the following application improves page performance by reducing the app's request size.

App screenshot


Before diving in to add optimizations, it's always a good idea to first analyze the current state of the application.

  • To preview the site, press View App. Then press Fullscreen fullscreen.

This app, which was also covered in the "Remove unused code" codelab, lets you vote for your favorite kitten. 🐈

Now take a look at how large this application is:

  1. Press `Control+Shift+J` (or `Command+Option+J` on Mac) to open DevTools.
  2. Click the Network tab.
  3. Select the Disable cache checkbox.
  4. Reload the app.

Original bundle size in Network panel

Although a lot of progress was made in the "Remove unused code" codelab to trim this bundle size down, 225 KB is still quite large.


Consider the following block of code.

function soNice() {
  let counter = 0;

  while (counter < 100) {

If this function is saved in a file of its own, the file size is around 112 B (bytes).

If all whitespace is removed, the resulting code looks like this:

function soNice(){let counter=0;while(counter<100){console.log("nice");counter++;}}

The file size would now be around 83 B. If it gets further mangled by reducing the length of variable name and modifying some expressions, the final code may end up looking like this:

function soNice(){for(let i=0;i<100;)console.log("nice"),i++}

The file size now reaches 62 B.

With each step, the code is becoming harder to read. However, the browser's JavaScript engine interprets each of these in the exact same way. The benefit of obfuscating code in this manner can help achieve smaller file sizes. 112 B really was not much to begin with, but there was still a 50% reduction in size!

In this application, webpack version 4 is used as a module bundler. The specific version can be seen in package.json.

"devDependencies": {
  "webpack": "^4.16.4",

Version 4 already minifies the bundle by default during production mode. It uses TerserWebpackPlugin a plugin for Terser. Terser is a popular tool used to compress JavaScript code.

To get an idea of what the minified code looks like, go ahead and click main.bundle.js while still in the DevTools Network panel. Now click the Response tab.

Minified response

The code in its final form, minified and mangled, is shown in the response body. To find out how large the bundle may have been if it was not minified, open webpack.config.js and update the mode configuration.

module.exports = {
  mode: 'production',
  mode: 'none',

Reload the application and take a look at the bundle size again through the DevTools Network panel

Bundle size of 767 KB

That's a pretty big difference! 😅

Make sure to revert the changes here before continuing.

module.exports = {
  mode: 'production',
  mode: 'none',

Including a process to minify code in your application depends on the tools that you use:

  • If webpack v4 or greater is used, no additional work needs to be done since code is minified by default in production mode. 👍
  • If an older version of webpack is used, install and include TerserWebpackPlugin into the webpack build process. The documentation explains this in detail.
  • Other minification plugins also exist and can be used instead, such as BabelMinifyWebpackPlugin and ClosureCompilerPlugin.
  • If a module bundler is not being used at all, use Terser as a CLI tool or include it directly as a dependency.


Although the term "compression" is sometimes loosely used to explain how code is reduced during the minification process, it isn't actually compressed in the literal sense.

Compression usually refers to code that has been modified using a data compression algorithm. Unlike minification which ends up providing perfectly valid code, compressed code needs to be decompressed before being used.

With every HTTP request and response, browsers and web servers can add headers to include additional information about the asset being fetched or received. This can be seen in the Headers tab within the DevTools Network panel where three types are shown:

  • General represents general headers relevant to the entire request-response interaction.
  • Response Headers shows a list of headers specific to the actual response from the server.
  • Request Headers shows a list of headers attached to the request by the client.

Take a look at the accept-encoding header in the Request Headers.

Accept encoding header

accept-encoding is used by the browser to specify which content encoding formats, or compression algorithms, it supports. There are many text-compression algorithms out there, but there are only three that are supported here for the compression (and decompression) of HTTP network requests:

  • Gzip (gzip): The most widely used compression format for server and client interactions. It builds on top of the Deflate algorithm and is supported in all current browsers.
  • Deflate (deflate): Not commonly used.
  • Brotli (br): A newer compression algorithm that aims to further improve compression ratios, which can result in even faster page loads. It is supported in the latest versions of most browsers.

The sample application in this tutorial is identical to the app completed in the "Remove unused code" codelab except for the fact that Express is now used as a server framework. In the next few sections, both static and dynamic compression is explored.

Dynamic compression

Dynamic compression involves compressing assets on-the-fly as they get requested by the browser.


  • Creating and updating saved compressed versions of assets does not need to be done.
  • Compressing on-the-fly works especially well for web pages that are dynamically generated.


  • Compressing files at higher levels to achieve better compression ratios takes longer. This can cause a performance hit as the user waits for assets to compress before they are sent by the server.

Dynamic compression with Node/Express

The server.js file is responsible for setting up the Node server that hosts the application.

const express = require('express');

const app = express();


const listener = app.listen(process.env.PORT, function() {
  console.log('Your app is listening on port ' + listener.address().port);

All this currently does is import express and use the express.static middleware to load all the static HTML, JS and CSS files in the public/ directory (and those files are created by webpack with every build).

To make sure all of the assets are compressed every time they're requested, the compression middleware library can be used. Begin by adding it as a devDependency in package.json:

"devDependencies": {
  "compression": "^1.7.3"

And import it into the server file, server.js:

const express = require('express');
const compression = require('compression');

And add it as a middleware before express.static is mounted:


const app = express();




Now reload the app and take a look at the bundle size in the Network panel.

Bundle size with dynamic compression

From 225 KB to 61.6 KB! In the Response Headers now, a content-encoding header shows that the server is sending down this file encoded with gzip.

Content encoding header

Static compression

The idea behind static compression is to have assets compressed and saved ahead of time.


  • Latency due to high compression levels is not a concern anymore. Nothing needs to happen on-the-fly to compress files as they can now be fetched directly.


  • Assets need to compressed with every build. Build times can increase significantly if high compression levels are used.

Static compression with Node/Express and webpack

Since static compression involves compressing files ahead of time, webpack settings can be modified to compress assets as part of the build step. CompressionPlugin can be used for this.

Begin by adding it as a devDependency in package.json:

"devDependencies": {
  "compression-webpack-plugin": "^1.1.11"

Like any other webpack plugin, import it in the configurations file, webpack.config.js:

const path = require("path");


const CompressionPlugin = require("compression-webpack-plugin");

And include it within the plugins array:

module.exports = {
  plugins: [
    new CompressionPlugin()

By default, the plugin compresses the build files using gzip. Take a look at the documentation to learn how to add options to use a different algorithm or include/exclude certain files.

When the app reloads and rebuilds, a compressed version of the main bundle is now created. Open the Glitch Console to take a look at what's inside the final public/ directory that's served by the Node server.

  • Click the Tools button.
  • Click the Console button.
  • In the console, run the following commands to change into the public directory and see all of its files:
cd public

Final outputted files in public directory

The gzipped version of the bundle, main.bundle.js.gz, is now saved here as well. CompressionPlugin also compresses index.html by default.

The next thing that needs to be done is tell the server to send these gzipped files whenever their original JS versions are being requested. This can be done by defining a new route in server.js before the files are served with express.static.

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

app.get('*.js', (req, res, next) => {
  req.url = req.url + '.gz';
  res.set('Content-Encoding', 'gzip');



app.get is used to tell the server how to respond to a GET request for a specific endpoint. A callback function is then used to define how to handle this request. The route works like this:

  • Specifying '*.js' as the first argument means that this works for every endpoint that is fired to fetch a JS file.
  • Within the callback, .gz is attached to the URL of the request and the Content-Encoding response header is set to gzip.
  • Finally, next() ensures that the sequence continues to any callback that may be next.

Once the app reloads, take a look at the Network panel once more.

Bundle size reduction with static compression

Just like before, a significant reduction in bundle size!


This codelab covered the process of minifying and compressing source code. Both these techniques are becoming a default in many of the tools available today, so it's important to find out whether your toolchain already supports them or if you should begin applying both processes yourself.