Babel Plugin: React CSS Modules

Babel plugin for advanced CSS modules support in React:

• It transforms styleName attribute of JSX components into className using compile-time CSS module resolution, allowing for a cleaner use of CSS modules in React.
• For server-side rendering (SSR) scenarios it can replace named stylesheet imports by classname mapping objects, and remove anonymous stylesheet imports.

Content

• Usage examples
• Installation
  • React Native
• Configuration
  • Plugin options
  • Configurate syntax loaders
  • Custom Attribute Mapping
• Under the hood
  • How does it work?
  • Project history
  • Migration from babel-plugin-react-css-modules
  • css-loader compatibility

Usage Examples

Assuming style.css in the following examples is compiled as CSS Module.

Without this plugin

import S from './styles.css';

export default function Component() {
  return (
    <div className={S.container}>
      <h1 className={S.title}>Example</div>
      <p styleName={S.text}>Sample text paragraph.</p>
      <p className={`${S.text} ${S.special}`}>
        Sample text paragraph with special style.
      </p>
    </div>
  );
}

With this plugin

import './styles.css';

export default function Component() {
  return (
    <div styleName="container">
      <h1 styleName="title">Example</div>
      <p styleName="text">Sample text paragraph.</p>
      <p styleName="text special">
        Sample text paragraph with special style.
      </p>
    </div>
  );
}

With this plugin and multiple stylesheets

Assuming:

• Styles container, title, and text are defined in styles-01.css.
• Style special is defined in styles-02.css.
• The plugin's autoResolveMultipleImports option is enabled (default).

import './styles-01.css';
import './styles-02.css';

export default function Component() {
  return (
    <div styleName="container">
      <h1 styleName="title">Example</div>
      <p styleName="text">Sample text paragraph.</p>
      <p styleName="text special">
        Sample text paragraph with special style.
      </p>
    </div>
  );
}

If both files, styles-01.css and styles-02.css contain styles with the same names, thus making auto resolution impossible, this plugin allows explicit stylesheet prefixes:

import S1 from './styles-01.css';
import S2 from './styles-02.css';

export default function Component() {
  return (
    <div styleName="S1.container">
      <h1 styleName="S1.title">Example</div>
      <p styleName="S1.text">Sample text paragraph.</p>
      <p styleName="S1.text S2.special">
        Sample text paragraph with special style.
      </p>
    </div>
  );
}

With this plugin and runtime resolution

import './styles-01.css';
import './styles-02.css';

export default function Component({ special }) {
  let textStyle = 'text';
  if (special) textStyle += ' special';

  return (
    <div styleName="container">
      <h1 styleName="title">Example</div>
      <p styleName={textStyle}>Sample text paragraph.</p>
      <p styleName={textStyle}>
        Sample text paragraph with special style.
      </p>
    </div>
  );
}

In the case when the exact style value is not known at the compile time, like in this example, the plugin will inject necessary code to correctly resolve the styleName at runtime (which is somewhat less performant, but otherwise works fine).

SSR scenario

Consider such component, which uses a named stylesheet import in order to use it in some other ways, beside simple styling, e.g. to also display the classname mapping:

import S from './style.css';

export default function Component() {
  return (
    <div styleName="container">
      {JSON.stringify(S)}
    </div>
  )
}

While by default this plugin transforms it into (leaving it to the Webpack's css-loader to handle ./style.scss import for the actual CSS bundling, and leaving a correct JS in place of it):

import S from './style.css';

export default function Component() {
  return (
    <div className="12345">
      {JSON.stringify(S)}
    </div>
  )
}

For server-side environment, if you don't compile server-side code with Webpack, you'll need to replace ./style.css with valid JS code. That is exactly what this plugin does with replaceImport option enabled, it outputs:

const S = {
  container: '12345',
  // Other stylesheet keys, if any.
};

export default function Component() {
  return (
    <div className="12345">
      {JSON.stringify(S)}
    </div>
  )
}

CommonJS require() support

The plugin works the same with require('./style.css') CSS imports.

Installation

• The core CSS Modules functionality should be enabled and configured elsewhere in your React project:

  • With Create React App see Adding a CSS Modules Stylesheet.
  • With bare Webpack see modules option of css-loader.
  • With other frameworks refer to their documentation.

• Install this plugin as a direct dependency (in edge-cases not allowing for a compile-time styleName resolution, the plugin falls back to the runtime resolution).

  npm install --save @dr.pogodin/babel-plugin-react-css-modules

• Install Webpack at least as a dev dependency:

  npm install --save-dev webpack

• Add the plugin to Babel configuration:

  {
    "plugins": [
      ["@dr.pogodin/react-css-modules", {
        // The default localIdentName in "css-loader" is "[hash:base64]",
        // it is highly-recommended to explicitly specify the same value
        // both here, and in "css-loader" options, including the hash length
        // (the last digit in the template below).
        "generateScopedName": "[hash:base64:6]"
  
        // See below for all valid options.
      }]
    ]
  }

• The generateScopedName option value MUST match localIdentName option of css-loader to ensure both Webpack and this plugin generate matching class names. The same goes for other options impacting class names (e.g. the default length of hashes generated by Webpack, which is used if you don't specify the hash length explicitly in localIdentName hash placeholders), and also the actuals version of this plugin and css-loader (see css-loader compatibility).

• Optional. css-loader is known for eventual minor updates in their default class name generation logic that require counterpart upgrades of this plugin to keep it compatible. They denied to expose the default class name generator for re-used by 3rd party libraries, and suggested to rely on getLocalIdent option if unwanted class name changes due to css-loader updates are a problem for a particular project.

  To alleviate this issue, this plugin provides stable default implementation for getLocalIdent function (taken from a selected earlier version of css-loader). Consider to use it:

  Within Webpack Config

  const { getLocalIdent } = require('@dr.pogodin/babel-plugin-react-css-modules/utils');
  
  const cssLoaderOptions = {
    modules: {
      getLocalIdent,
      localIdentName: '[path]___[name]__[local]___[hash:base64:6]'
    }
  };

  Within Babel Config

  const { generateScopedNameFactory } = require('@dr.pogodin/babel-plugin-react-css-modules/utils');
  
  module.exports = {
    plugins: [
      ["@dr.pogodin/react-css-modules", {
        generateScopedName:
          // The classname template MUST match "localIdentName" option value
          // you passed to "css-loader".
          generateScopedNameFactory("[path]___[name]__[local]___[hash:base64:6]"),
      }]
    ]
  };

  In addition to the standard class name template placeholders mentioned in css-loader documentation the version of getLocalIdent() and generateScopedName() provided by this plugin also support [package] placeholder. If used, it looks up from CSS file for the closest package.json file, takes the package name from it, and inserts it into the class name (this is useful for CSS bundling for libraries).

React Native

If you'd like to get this working in React Native, you're going to have to allow custom import extensions, via a rn-cli.config.js file:

module.exports = {
  getAssetExts() {
    return ["scss"];
  }
}

Remember, also, that the bundler caches things like plugins and presets. If you want to change your .babelrc (to add this plugin) then you'll want to add the --reset-cache flag to the end of the package command.

Configuration

Plugin Options

These are valid plugin options. All are optional, but the overall configuration should be compatible with that of css-loader, thus defaults may not work for you.

• context - string - Must match webpack context configuration. css-loader inherits context values from webpack. Other CSS module implementations might use different context resolution logic. Defaults process.cwd().

• exclude - string - A RegExp that will exclude otherwise included files e.g., to exclude all styles from node_modules: exclude: 'node_modules'.

• filetypes - Configurate syntax loaders like sugarss, LESS and SCSS, and extra plugins for them.

• generateScopedName - function | string - Allows to customize the exact styleName to className conversion algorithm. For details see Generating scoped names. Defaults [path]___[name]__[local]___[hash:base64:5].

• replaceImport - boolean - Replaces / removes stylesheet imports for server-side rendering purposes. See details below. Defaults false.

• webpackHotModuleReloading - boolean | "commonjs" - Enables injection of Hot Module Reloading code.

• handleMissingStyleName - string - Determines what should be done for undefined CSS modules (using a styleName for which there is no CSS module defined). Valid values: "throw", "warn", "ignore". Setting this option to "ignore" is equivalent to setting errorWhenNotFound: false in react-css-modules. Defaults "throw".

• attributeNames - Custom Attribute Mapping

• skip - boolean - Whether to apply plugin if no matching attributeNames found in the file. Defaults false.

• transform - function - If provided, each CSS source loaded by the plugin will be passed through this function, alongside its path, and this plugin's options, and the output of this function will be used in place of the original CSS.

• autoResolveMultipleImports - boolean - Allows multiple anonymous imports if styleName is only in one of them. Defaults true.

Deprecated Plugin Options

• ~~removeImport - boolean~~ - Use replaceImport option instead.

Configurate syntax loaders

To add support for different CSS syntaxes (e.g. SCSS), perform the following two steps:

1. Add the postcss syntax loader as a development dependency:

   npm install postcss-scss --save-dev

2. Add a filetypes syntax mapping to the Babel plugin configuration. For example for SCSS:

   "filetypes": {
     ".scss": {
       "syntax": "postcss-scss"
     }
   }

   And optionally specify extra plugins:

   "filetypes": {
     ".scss": {
       "syntax": "postcss-scss",
       "plugins": [
         "postcss-nested"
       ]
     }
   }

   NOTE: postcss-nested is added as an extra plugin for demonstration purposes only. It's not needed with postcss-scss because SCSS already supports nesting.

   Postcss plugins can have options specified by wrapping the name and an options object in an array inside your config:

     "plugins": [
       ["postcss-import-sync2", {
         "path": ["src/styles", "shared/styles"]
       }],
       "postcss-nested"
     ]

Hot Module Reloading

If you don't know what is Hot Module Reloading (HMR), refer to the Webpack documentation.

If you use HMR in your development setup (you probably should), depending on your particular configuration you might need to enable webpackHotModuleReloading option of this plugin, or you may need to leave it disabled (default), as other loaders / plugins in your Webpack pipeline for CSS may already inject required HMR code.

In case you decide to enable it in this plugin, webpackHotModuleReloading option may be set equal:

• true - this plugin will inject HMR accept code for each imported CSS module, using import.meta.webpackHot (ESM) syntax (see for details).
• commonjs string - this plugin will inject HMR accept code using the legacy module.hot syntax.

The default value is false - this plugin does not inject HMR accept code.

transform

function transform(cssSource, cssSourceFilePath, pluginOptions): string

The transform function, if provided as the transform option of this plugin, will be called for each loaded CSS source with three arguments:

• cssSource - string - The loaded CSS code.
• cssSourceFilePath - string - The path of loaded CSS file.
• pluginOptions - object - The options set for this plugin.

It should return a string, the actual CSS code to use.

Custom Attribute

You can set your own attribute mapping rules using the attributeNames option.

It's an object, where keys are source attribute names and values are destination attribute names.

For example, the <NavLink> component from React Router has an activeClassName attribute to accept an additional class name. You can set "attributeNames": { "activeStyleName": "activeClassName" } to transform it.

The default styleName -> className transformation will not be affected by an attributeNames value without a styleName key. Of course you can use { "styleName": "somethingOther" } to change it, or use { "styleName": null } to disable it.

Server-Side Rendering

If replaceImport flag is set, this plugin will remove or replace original stylesheet imports, which is needed for server-side rendering:

// Anonymous imports are removed from the code:
import 'path/to/style.css';

// Default and named imports are replaced in the following manner:

// Before:
import styles, {
  className,
  otherClassName as alias,
} from 'path/to/style.css';

// After:
const styles = {
  className: 'generatedClassName',
  otherClassName: 'otherGeneratedClassName',
},
className = 'generatedClassName',
alias = 'otherGeneratedClassName';

// Also this kind of import:
import * as style from 'path/to/style.css';

// is replaced by:
const style = {
  className: 'generatedClassName',
  otherClassName: 'otherGeneratedClassName',
};

Under the hood

How does it work?

This plugin does the following:

1. Builds index of all stylesheet imports per file (imports of files with .css or .scss extension).
2. Uses postcss to parse the matching CSS files into a lookup of CSS module references.
3. Iterates through all JSX element declarations.
4. Parses the styleName attribute value into anonymous and named CSS module references.
5. Finds the CSS class name matching the CSS module reference:
   • If styleName value is a string literal, generates a string literal value.
   • If styleName value is a jSXExpressionContainer, uses a helper function (getClassName) to construct the className value at the runtime.
6. Removes the styleName attribute from the element.
7. Appends the resulting className to the existing className value (creates className attribute if one does not exist).

Project history

This plugin is an up-to-date, well-maintained fork of the original babel-plugin-react-css-modules:

• It generates class names matching current css-loader versions (see css-loader compatibility for details).
• All dependencies are upgraded to the latest versions.
• Follow-up maintenance and improvements are performed as necessary.

The original babel-plugin-react-css-modules plugin is largely abandoned by its author since March 2019. When an year later updates of css-loader and Webpack broke dependant projects, with no reaction from babel-plugin-react-css-modules author on emerging issue reports in GitHub, I (birdofpreyru) created this fork to ensure stability of my own projects relying on it.

I am banned from commenting in the original project repo since I tried a little self-promo, trying to encourage people to switch over to my fork. If you read this, consider to spread the word to encourage more users to move to this fork.

Migration from babel-plugin-react-css-modules

• Prefix plugin name in your Babel config by @dr.pogodin/ scope, i.e.: @dr.pogodin/babel-plugin-react-css-modules or @dr.pogodin/react-css-moudles instead of babel-plugin-react-css-modules or react-css-modules.

• Be sure to have webpack installed (it is a must-to-have peer dependency of this plugin starting from v6.2.0).

css-loader compatibility

| css-loader versions | this plugin versions | | ------------------------- | ------------------------- | | 7.0.0 – 7.1.2 (latest) | 6.13.0 – 6.13.2 (latest) | | 6.7.1 – 6.11.0 | 6.7.0 – 6.12.0 | | 6.5.0 – 6.7.0 | 6.5.1 – 6.6.1 | | 6.4.0 | 6.4.0 – 6.4.1 | | 6.0.0 – 6.3.0 | 6.2.1 – 6.3.1 | | 5.2.5 – 5.2.7 | 6.1.1 | | 5.2.4 | 6.1.0 | | 5.1.3 – 5.2.3 | 6.0.11 / 6.1.0(1) | | 5.0.0 – 5.1.2 | 6.0.7 – 6.0.11 | | 4.2.0 – 4.3.0 | 6.0.3 – 6.0.6 | | ≤ 3.6.0 | original plugin |

1) There might be some corner-case differences in class name transformation between these versions of css-loader and this plugin, but most probably they won't break compatibility for most users.