Since webpack 4, the production output gets minified using UglifyJS by default. That said, it's good to understand the technique and further possibilities.

Minifying JavaScript#

The point of minification is to convert the code into a smaller form. Safe transformations do this without losing any meaning by rewriting code. Good examples of this include renaming variables or even removing entire blocks of code based on the fact that they are unreachable (if (false)).

Unsafe transformations can break code as they can lose something implicit the underlying code relies upon. For example, Angular 1 expects specific function parameter naming when using modules. Rewriting the parameters breaks code unless you take precautions against it in this case.

Modifying JavaScript Minification Process#

In webpack 4, minification process is controlled through two configuration fields: optimization.minimize flag to toggle it and optimization.minimizer array to configure the process.

To tune the defaults, we'll attach uglifyjs-webpack-plugin to the project so that it's possible to tune it.

To get started, include the plugin to the project:

npm install uglifyjs-webpack-plugin --save-dev

To attach it to the configuration, define a part for it first:

const UglifyWebpackPlugin = require("uglifyjs-webpack-plugin");

exports.minifyJavaScript = () => ({
  optimization: {
    minimizer: [new UglifyWebpackPlugin({ sourceMap: true })],

Hook it up to the configuration:


const productionConfig = merge([
... ]);

If you execute npm run build now, you should see result close to the same as before. The outcome may be a slightly better as you are likely using a newer version of UglifyJS this way.

Source maps are disabled by default. You can enable them through the sourceMap flag. You should check uglifyjs-webpack-plugin for more options.
To strip console.log calls from the resulting source, set uglifyOptions.compress.drop_console to true as discussed on Stack Overflow.

Other Ways to Minify JavaScript#

Although the defaults and uglifyjs-webpack-plugin works for this use case, there are more options you can consider:

Speeding Up JavaScript Execution#

Specific solutions allow you to preprocess code so that it will run faster. They complement the minification technique and can be split into scope hoisting, pre-evaluation, and improving parsing. It's possible these techniques grow overall bundle size sometimes while allowing faster execution.

Scope Hoisting#

Since webpack 4, it applies scope hoisting in production mode by default. It hoists all modules to a single scope instead of writing a separate closure by each. Doing this slows down the build but gives you bundles that are faster to execute. Read more about scope hoisting at the webpack blog.

Pass --display-optimization-bailout flag to webpack to gain debugging information related to hoisting results.


prepack-webpack-plugin uses Prepack, a partial JavaScript evaluator. It rewrites computations that can be done compile-time and therefore speeds up code execution. See also val-loader and babel-plugin-preval for alternative solutions.

Improving Parsing#

optimize-js-plugin complements the other solutions by wrapping eager functions, and it enhances the way your JavaScript code gets parsed initially. The plugin relies on optimize-js by Nolan Lawson.

Minifying HTML#

If you consume HTML templates through your code using html-loader, you can preprocess it through posthtml with posthtml-loader. You can use posthtml-minifier to minify your HTML through it.

Minifying CSS#

css-loader allows minifying CSS through cssnano. Minification needs to be enabled explicitly using the minimize option. You can also pass cssnano specific options to the query to customize the behavior further.

clean-css-loader allows you to use a popular CSS minifier clean-css.

optimize-css-assets-webpack-plugin is a plugin based option that applies a chosen minifier on CSS assets. Using ExtractTextPlugin can lead to duplicated CSS given it only merges text chunks. OptimizeCSSAssetsPlugin avoids this problem by operating on the generated result and thus can lead to a better result.

Setting Up CSS Minification#

Out of the available solutions, OptimizeCSSAssetsPlugin composes the best. To attach it to the setup, install it and cssnano first:

npm install optimize-css-assets-webpack-plugin cssnano --save-dev

Like for JavaScript, you can wrap the idea in a configuration part:

const OptimizeCSSAssetsPlugin = require(
const cssnano = require("cssnano");

exports.minifyCSS = ({ options }) => ({
  plugins: [
    new OptimizeCSSAssetsPlugin({
      cssProcessor: cssnano,
      cssProcessorOptions: options,
      canPrint: false,
If you use --json output with webpack as discussed in the Build Analysis chapter, you should set canPrint: false for the plugin.

Then, connect with the main configuration:


const productionConfig = merge([
parts.minifyCSS({ options: { discardComments: { removeAll: true, }, // Run cssnano in safe mode to avoid // potentially unsafe transformations. safe: true, }, }),
... ]);

If you build the project now (npm run build), you should notice that CSS has become smaller as it's missing comments:

Hash: f51ecf99e0da4db99834
Version: webpack 4.1.1
Time: 3125ms
Built at: 3/16/2018 5:32:55 PM
           Asset       Size  Chunks             Chunk Names
      chunk.0.js  162 bytes       0  [emitted]
      chunk.1.js   96.8 KiB       1  [emitted]  vendors~main
         main.js   2.19 KiB       2  [emitted]  main
main.css 1.2 KiB 2 [emitted] main vendors~main.css 1.32 KiB 1 [emitted] vendors~main 204 bytes 0 [emitted] 235 KiB 1 [emitted] vendors~main ...
compression-webpack-plugin allows you to push the problem of generating compressed files to webpack to potentially save processing time on the server.

Minifying Images#

Image size can be reduced by using img-loader, imagemin-webpack, and imagemin-webpack-plugin. The packages use image optimizers underneath.

It can be a good idea to use cache-loader and thread-loader with these as discussed in the Performance chapter given they can be substantial operations.


Minification is the most comfortable step you can take to make your build smaller. To recap:

  • Minification process analyzes your source code and turns it into a smaller form with the same meaning if you use safe transformations. Specific unsafe transformations allow you to reach even smaller results while potentially breaking code that relies, for example, on exact parameter naming.
  • Webpack performs minification in production mode using UglifyJS by default. Other solutions, such as babel-minify-webpack-plugin, provide similar functionality with costs of their own.
  • Besides JavaScript, it's possible to minify other assets, such as CSS, HTML, and images, too. Minifying these requires specific technologies that have to be applied through loaders and plugins of their own.

You'll learn to apply tree shaking against code in the next chapter.

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