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Composing Configuration

Even though not a lot has been done with webpack yet, the amount of configuration is starting to feel substantial. Now you have to be careful about the way you compose it as you have separate production and development targets in the project. The situation can only get worse as you want to add more functionality for testing and other purposes.

Using a single monolithic configuration file impacts comprehension and removes any potential for reusability. As the needs of your project grow, you have to figure out the means to manage webpack configuration more effectively.

Possible ways to manage configuration

You can manage webpack configuration in the following ways:

  • Maintain configuration within multiple files for each environment and point webpack to each through the --config parameter, sharing configuration through module imports.
  • Push configuration to a library, which you then consume. Examples: webpack-config-pluginsβ†—, Neutrinoβ†—, webpack-blocksβ†—.
  • Push configuration to a tool. Examples: create-react-appβ†—, kytβ†—, nwbβ†—.
  • Maintain all configuration within a single file and branch there and rely on the --mode parameter. The approach is explained in detail later in this chapter.

My preferred approach is to compose webpack configuration out of smaller functions that I put together. The development of this book motivated the direction as it gives you something you can approach piece-wise while giving you a small API over webpack configuration and related techniques.

Composing configuration by merging

In composition based approach, you split webpack configuration and then merge it together. The problem is that a normal way of merging objects using a feature such as Object.assign doesn’t do the right thing with arrays as if two objects have arrays attached to them, it’s going to lose data. It’s for this reason that I developed webpack-mergeβ†—.

At its core, webpack-merge does two things: it concatenates arrays and merges objects instead of overriding them allowing composition. The example below shows the behavior in detail:

> { merge } = require("webpack-merge")
> merge(
... { a: [1], b: 5, c: 20 },
... { a: [2], b: 10, d: 421 }
... )
{ a: [ 1, 2 ], b: 10, c: 20, d: 421 }

webpack-merge provides even more control through strategies that enable you to control its behavior per field. They allow you to force it to append, prepend, or replace content.

Even though webpack-merge was designed for this book, it has proven to be an invaluable tool beyond it. You can consider it as a learning tool and pick it up in your work if you find it handy.

webpack-chain↗ provides a fluent API for configuring webpack allowing you to avoid configuration shape-related problems while enabling composition.

Setting up webpack-merge

To get started, add webpack-merge to the project:

npm add webpack-merge -D

To give a degree of abstraction, you can define webpack.config.js for higher level configuration and for configuration parts to consume. Here is the development server as a function:

const { WebpackPluginServe } = require("webpack-plugin-serve");
const {
} = require("mini-html-webpack-plugin");

exports.devServer = () => ({
  watch: true,
  plugins: [
    new WebpackPluginServe({
      port: parseInt(process.env.PORT, 10) || 8080,
      static: "./dist", // Expose if output.path changes
      liveReload: true,
      waitForBuild: true,
}); = ({ title }) => ({
  plugins: [new MiniHtmlWebpackPlugin({ context: { title } })],

For the sake of simplicity, we’ll develop all of the configuration using JavaScript. It would be possible to use TypeScript here as well. If you want to go that route, see the Loading JavaScript chapter for the required TypeScript setup.

To connect this configuration part, set up webpack.config.js as in the code example below:


const { mode } = require("webpack-nano/argv");
const { merge } = require("webpack-merge");
const parts = require("./");

const commonConfig = merge([
  { entry: ["./src"] },{ title: "Demo" }),

const productionConfig = merge([]);

const developmentConfig = merge([
  { entry: ["webpack-plugin-serve/client"] },

const getConfig = (mode) => {
  switch (mode) {
    case "production":
      return merge(commonConfig, productionConfig, { mode });
    case "development":
      return merge(commonConfig, developmentConfig, { mode });
      throw new Error(`Trying to use an unknown mode, ${mode}`);

module.exports = getConfig(mode);

After these changes, the build should behave the same way as before. This time, however, you have room to expand, and you don’t have to worry about how to combine different parts of the configuration.

You can add more targets by expanding the package.json definition and branching at webpack.config.js based on the need. grows to contain specific techniques you can then use to compose the configuration.

Webpack does not set global NODE_ENV↗ based on mode by default. If you have any external tooling, such as Babel, relying on it, make sure to set it explicitly. To do this, set process.env.NODE_ENV = mode; within getConfig.

Benefits of composing configuration

There are several benefits to composing configuration:

  • Splitting configuration into smaller functions lets you keep on expanding the setup.
  • You can type the functions assuming you are using a language such as TypeScript.
  • If you consume the configuration across multiple projects, you can publish the configuration as a package and then have only one place to optimize and upgrade as the underlying configuration changes. SurviveJS - Maintenanceβ†— covers practices related to the approach.
  • Treating configuration as a package allows you to version it as any other and deliver change logs to document the changes to the consumers.
  • Taken far enough, you can end up with your own create-react-app that can be used to bootstrap projects quickly with your preferred setup.

Configuration layouts

In the book project, you will push all of the configuration into two files: webpack.config.js and The former contains higher level configuration while the lower level isolates you from webpack specifics. The chosen approach allows more file layouts than the one we have.

Split per configuration target

If you split the configuration per target, you could end up with a file structure as below:

└── config
    β”œβ”€β”€ webpack.common.js
    β”œβ”€β”€ webpack.development.js
    └── webpack.production.js

In this case, you would point to the targets through webpack --config parameter and merge common configuration through module.exports = merge(common, config);.

Split parts per purpose

To add hierarchy to the way configuration parts are managed, you could decompose per category:

└── config
    β”œβ”€β”€ parts
    β”‚   β”œβ”€β”€ devserver.js
    β”‚   β”œβ”€β”€ index.js
    β”‚   └── javascript.js
    └── ...

This arrangement can make it faster to find configuration related to a category. Additionally, it can also reduce your build time if you’re consuming parts from a published package as then only the required plugins will have to be loaded. A good alternative for better readability would be to arrange the functions within a single file and use comments to split it up.

Guidelines for building your own configuration packages

If you go with the configuration package approach I mentioned, consider the guidelines below:

  • It can make sense to develop the package using TypeScript to document the interface well. It’s particularly useful if you are authoring your configuration in TypeScript as discussed in the Loading JavaScript chapter.
  • Expose functions that cover only one piece of functionality at a time as it lets you to replace a Hot Module Replacement implementation easily for example.
  • Provide enough customization options through function parameters. It can be a good idea to expose an object as that lets you mimic named parameters in JavaScript. You can then destructure the parameters from that while combining this with good defaults and TypeScript types.
  • Include all related dependencies within the configuration package. In specific cases you could use peerDependencies if you want that the consumer is able to control specific versions. Doing this means you’ll likely download more dependencies that you would need but it’s a good compromise.
  • For parameters that have a loader string within them, use require.resolve to resolve against a loader within the configuration package. Otherwise the build can fail as it’s looking into the wrong place for the loaders.
  • When wrapping loaders, use the associated TypeScript type in parameters.
  • Consider testing the package by using snapshots (expect().toMatchSnapshot() in Jest) to assert output changes. See the Extending with Plugins chapters for an example of a test harness.


Even though the configuration is technically the same as before, now you have room to grow it through composition.

To recap:

  • Given webpack configuration is JavaScript code underneath, there are many ways to manage it.
  • You should choose a method to compose configuration that makes the most sense to you. webpack-mergeβ†— was developed to provide a light approach for composition, but you can find many other options in the wild.
  • Composition can enable configuration sharing. Instead of having to maintain a custom configuration per repository, you can share it across repositories this way. Using npm packages allows this. Developing configuration is close to developing any other code. This time, however, you codify your practices as packages.

The next parts of this book cover different techniques, and sees a lot of action as a result. The changes to webpack.config.js, fortunately, remain minimal.