gajus/react-css-modules

README

React CSS Modules

React CSS Modules implement automatic mapping of CSS modules. Every CSS class is assigned a local-scoped identifier with a global unique name. CSS Modules enable a modular and reusable CSS!

⚠️⚠️⚠️ DEPRECATION NOTICE ⚠️⚠️⚠️

If you are considering to use react-css-modules, evaluate if babel-plugin-react-css-modules covers your use case. babel-plugin-react-css-modules is a lightweight alternative of react-css-modules.

babel-plugin-react-css-modules is not a drop-in replacement and does not cover all the use cases of react-css-modules. However, it has a lot smaller performance overhead (0-10% vs +50%; see Performance) and a lot smaller size footprint (less than 2kb vs +17kb).

It is easy to get started! See the demo https://github.com/gajus/babel-plugin-react-css-modules/tree/master/demo

CSS Modules
- webpack css-loader
What's the Problem?
The Implementation
Usage
SASS, SCSS, LESS and other CSS Preprocessors
- Enable Sourcemaps
Class Composition
- What Problems does Class Composition Solve?
- Class Composition Using CSS Preprocessors
Global CSS
Multiple CSS Modules

CSS Modules

CSS Modules are awesome. If you are not familiar with CSS Modules, it is a concept of using a module bundler such as webpack to load CSS scoped to a particular document. CSS module loader will generate a unique name for each CSS class at the time of loading the CSS document (Interoperable CSS to be precise). To see CSS Modules in practice, webpack-demo.

In the context of React, CSS Modules look like this:

import React from 'react';
import styles from './table.css';

export default class Table extends React.Component {
    render () {
        return <div className={styles.table}>
            <div className={styles.row}>
                <div className={styles.cell}>A0</div>
                <div className={styles.cell}>B0</div>
            </div>
        </div>;
    }
}

Rendering the component will produce a markup similar to:

<div class="table__table___32osj">
    <div class="table__row___2w27N">
        <div class="table__cell___1oVw5">A0</div>
        <div class="table__cell___1oVw5">B0</div>
    </div>
</div>

and a corresponding CSS file that matches those CSS classes.

Awesome!

webpack `css-loader`

CSS Modules is a specification that can be implemented in multiple ways. react-css-modules leverages the existing CSS Modules implementation webpack css-loader.

What's the Problem?

webpack css-loader itself has several disadvantages:

You have to use camelCase CSS class names.
You have to use styles object whenever constructing a className.
Mixing CSS Modules and global CSS classes is cumbersome.
Reference to an undefined CSS Module resolves to undefined without a warning.

React CSS Modules component automates loading of CSS Modules using styleName property, e.g.

import React from 'react';
import CSSModules from 'react-css-modules';
import styles from './table.css';

class Table extends React.Component {
    render () {
        return <div styleName='table'>
            <div styleName='row'>
                <div styleName='cell'>A0</div>
                <div styleName='cell'>B0</div>
            </div>
        </div>;
    }
}

export default CSSModules(Table, styles);

Using react-css-modules:

You are not forced to use the camelCase naming convention.
You do not need to refer to the styles object every time you use a CSS Module.
There is clear distinction between global CSS and CSS Modules, e.g.

<div className='global-css' styleName='local-module'></div>

You are warned when styleName refers to an undefined CSS Module (handleNotFoundStyleName option).
You can enforce use of a single CSS module per ReactElement (allowMultiple option).

The Implementation

react-css-modules extends render method of the target component. It will use the value of styleName to look for CSS Modules in the associated styles object and will append the matching unique CSS class names to the ReactElement className property value.

Awesome!

Usage

Setup consists of:

Setting up a module bundler to load the Interoperable CSS.
Decorating your component using react-css-modules.

Module Bundler

webpack

Development

In development environment, you want to Enable Sourcemaps and webpack Hot Module Replacement (HMR). style-loader already supports HMR. Therefore, Hot Module Replacement will work out of the box.

Setup:

Install style-loader.
Install css-loader.
Setup /\.css$/ loader:

{
    test: /\.css$/,
    loaders: [
        'style-loader?sourceMap',
        'css-loader?modules&importLoaders=1&localIdentName=[path]___[name]__[local]___[hash:base64:5]'
    ]
}

Production

In production environment, you want to extract chunks of CSS into a single stylesheet file.

Advantages:

Fewer style tags (older IE has a limit)

CSS SourceMap (with devtool: "source-map" and css-loader?sourceMap)

CSS requested in parallel

CSS cached separate

Faster runtime (less code and DOM operations)

Caveats:

Additional HTTP request

Longer compilation time

More complex configuration

No runtime public path modification

No Hot Module Replacement

– extract-text-webpack-plugin

Setup:

Install style-loader.
Install css-loader.
Use extract-text-webpack-plugin to extract chunks of CSS into a single stylesheet.

Setup /\.css$/ loader:

ExtractTextPlugin v1x:

{
  test: /\.css$/,
  loader: ExtractTextPlugin.extract('style', 'css?modules&importLoaders=1&localIdentName=[name]__[local]___[hash:base64:5]')
}

ExtractTextPlugin v2x:

{
  test: /\.css$/,
  use: ExtractTextPlugin.extract({
      fallback: 'style-loader',
      use: 'css-loader?modules,localIdentName="[name]-[local]-[hash:base64:6]"'
  }),
}

Setup extract-text-webpack-plugin plugin:

ExtractTextPlugin v1x:

new ExtractTextPlugin('app.css', {
    allChunks: true
})

ExtractTextPlugin v2x:

new ExtractTextPlugin({
  filename: 'app.css',
  allChunks: true
})

Refer to webpack-demo or react-css-modules-examples for an example of a complete setup.

Browserify

Refer to css-modulesify.

Extending Component Styles

Use styles property to overwrite the default component styles.

Explanation using Table component:

import React from 'react';
import CSSModules from 'react-css-modules';
import styles from './table.css';

class Table extends React.Component {
    render () {
        return <div styleName='table'>
            <div styleName='row'>
                <div styleName='cell'>A0</div>
                <div styleName='cell'>B0</div>
            </div>
        </div>;
    }
}

export default CSSModules(Table, styles);

In this example, CSSModules is used to decorate Table component using ./table.css CSS Modules. When Table component is rendered, it will use the properties of the styles object to construct className values.

Using styles property you can overwrite the default component styles object, e.g.

import customStyles from './table-custom-styles.css';

<Table styles={customStyles} />;

Interoperable CSS can extend other ICSS. Use this feature to extend default styles, e.g.

/* table-custom-styles.css */
.table {
    composes: table from './table.css';
}

.row {
    composes: row from './table.css';
}

/* .cell {
    composes: cell from './table.css';
} */

.table {
    width: 400px;
}

.cell {
    float: left; width: 154px; background: #eee; padding: 10px; margin: 10px 0 10px 10px;
}

In this example, table-custom-styles.css selectively extends table.css (the default styles of Table component).

Refer to the UsingStylesProperty example for an example of a working implementation.

`styles` Property

Decorated components inherit styles property that describes the mapping between CSS modules and CSS classes.

class extends React.Component {
    render () {
        <div>
            <p styleName='foo'></p>
            <p className={this.props.styles.foo}></p>
        </div>;
    }
}

In the above example, styleName='foo' and className={this.props.styles.foo} are equivalent.

styles property is designed to enable component decoration of Loops and Child Components.

Loops and Child Components

styleName cannot be used to define styles of a ReactElement that will be generated by another component, e.g.

import React from 'react';
import CSSModules from 'react-css-modules';
import List from './List';
import styles from './table.css';

class CustomList extends React.Component {
    render () {
        let itemTemplate;

        itemTemplate = (name) => {
            return <li styleName='item-template'>{name}</li>;
        };

        return <List itemTemplate={itemTemplate} />;
    }
}

export default CSSModules(CustomList, styles);

The above example will not work. CSSModules is used to decorate CustomList component. However, it is the List component that will render itemTemplate.

For that purpose, the decorated component inherits styles property that you can use just as a regular CSS Modules object. The earlier example can be therefore rewritten to:

import React from 'react';
import CSSModules from 'react-css-modules';
import List from './List';
import styles from './table.css';

class CustomList extends React.Component {
    render () {
        let itemTemplate;

        itemTemplate = (name) => {
            return <li className={this.props.styles['item-template']}>{name}</li>;
        };

        return <List itemTemplate={itemTemplate} />;
    }
}

export default CSSModules(CustomList, styles);

You can use styleName property within the child component if you decorate the child component using CSSModules before passing it to the rendering component, e.g.

import React from 'react';
import CSSModules from 'react-css-modules';
import List from './List';
import styles from './table.css';

class CustomList extends React.Component {
    render () {
        let itemTemplate;

        itemTemplate = (name) => {
            return <li styleName='item-template'>{name}</li>;
        };

        itemTemplate = CSSModules(itemTemplate, this.props.styles);

        return <List itemTemplate={itemTemplate} />;
    }
}

export default CSSModules(CustomList, styles);

Decorator

/**
 * @typedef CSSModules~Options
 * @see {@link https://github.com/gajus/react-css-modules#options}
 * @property {Boolean} allowMultiple
 * @property {String} handleNotFoundStyleName
 */

/**
 * @param {Function} Component
 * @param {Object} defaultStyles CSS Modules class map.
 * @param {CSSModules~Options} options
 * @return {Function}
 */

You need to decorate your component using react-css-modules, e.g.

import React from 'react';
import CSSModules from 'react-css-modules';
import styles from './table.css';

class Table extends React.Component {
    render () {
        return <div styleName='table'>
            <div styleName='row'>
                <div styleName='cell'>A0</div>
                <div styleName='cell'>B0</div>
            </div>
        </div>;
    }
}

export default CSSModules(Table, styles);

Thats it!

As the name implies, react-css-modules is compatible with the ES7 decorators syntax:

import React from 'react';
import CSSModules from 'react-css-modules';
import styles from './table.css';

@CSSModules(styles)
export default class extends React.Component {
    render () {
        return <div styleName='table'>
            <div styleName='row'>
                <div styleName='cell'>A0</div>
                <div styleName='cell'>B0</div>
            </div>
        </div>;
    }
}

Awesome!

Refer to the react-css-modules-examples repository for an example of webpack setup.

Options

Options are supplied as the third parameter to the CSSModules function.

CSSModules(Component, styles, options);

or as a second parameter to the decorator:

@CSSModules(styles, options);

`allowMultiple`

Default: false.

Allows multiple CSS Module names.

When false, the following will cause an error:

<div styleName='foo bar' />

`handleNotFoundStyleName`

Default: throw.

Defines the desired action when styleName cannot be mapped to an existing CSS Module.

Available options:

throw throws an error
log logs a warning using console.warn
ignore silently ignores the missing style name

SASS, SCSS, LESS and other CSS Preprocessors

Interoperable CSS is compatible with the CSS preprocessors. To use a preprocessor, all you need to do is add the preprocessor to the chain of loaders, e.g. in the case of webpack it is as simple as installing sass-loader and adding !sass to the end of the style-loader loader query (loaders are processed from right to left):

{
    test: /\.scss$/,
    loaders: [
        'style',
        'css?modules&importLoaders=1&localIdentName=[path]___[name]__[local]___[hash:base64:5]',
        'resolve-url',
        'sass'
    ]
}

Enable Sourcemaps

To enable CSS Source maps, add sourceMap parameter to the css-loader and to the sass-loader:

{
    test: /\.scss$/,
    loaders: [
        'style?sourceMap',
        'css?modules&importLoaders=1&localIdentName=[path]___[name]__[local]___[hash:base64:5]',
        'resolve-url',
        'sass?sourceMap'
    ]
}

Class Composition

CSS Modules promote composition pattern, i.e. every CSS Module that is used in a component should define all properties required to describe an element, e.g.

.box {
    width: 100px;
    height: 100px;
}

.empty {
    composes: box;

    background: #4CAF50;
}

.full {
    composes: box;

    background: #F44336;
}

Composition promotes better separation of markup and style using semantics that would be hard to achieve without CSS Modules.

Because CSS Module names are local, it is perfectly fine to use generic style names such as "empty" or "full", without "box-" prefix.

To learn more about composing CSS rules, I suggest reading Glen Maddern article about CSS Modules and the official spec of the CSS Modules.

What Problems does Class Composition Solve?

Consider the same example in CSS and HTML:

.box {
    width: 100px;
    height: 100px;
}

.box-empty {
    background: #4CAF50;
}

.box-full {
    background: #F44336;
}

<div class='box box-empty'></div>

This pattern emerged with the advent of OOCSS. The biggest disadvantage of this implementation is that you will need to change HTML almost every time you want to change the style.

Class Composition Using CSS Preprocessors

This section of the document is included as a learning exercise to broaden the understanding about the origin of Class Composition. CSS Modules support a native method of composing CSS Modules using composes keyword. CSS Preprocessor is not required.

You can write compositions in SCSS using @extend keyword and using Mixin Directives, e.g.

Using @extend:

%box {
    width: 100px;
    height: 100px;
}

.box-empty {
    @extend %box;

    background: #4CAF50;
}

.box-full {
    @extend %box;

    background: #F44336;
}

This translates to:

.box-empty,
.box-full {
    width: 100px;
    height: 100px;
}

.box-empty {
    background: #4CAF50;
}

.box-full {
    background: #F44336;
}

Using mixins:

@mixin box {
    width: 100px;
    height: 100px;
}

.box-empty {
    @include box;

    background: #4CAF50;
}

.box-full {
    @include box;

    background: #F44336;
}

This translates to:

.box-empty {
    width: 100px;
    height: 100px;
    background: #4CAF50;
}

.box-full {
    width: 100px;
    height: 100px;
    background: #F44336;
}

Global CSS

CSS Modules does not restrict you from using global CSS.

:global .foo {

}

However, use global CSS with caution. With CSS Modules, there are only a handful of valid use cases for global CSS (e.g. normalization).

Multiple CSS Modules

Avoid using multiple CSS Modules to describe a single element. Read about Class Composition.

That said, if you require to use multiple CSS Modules to describe an element, enable the allowMultiple option. When multiple CSS Modules are used to describe an element, react-css-modules will append a unique class name for every CSS Module it matches in the styleName declaration, e.g.

.button {

}

.active {

}

<div styleName='button active'></div>

This will map both Interoperable CSS CSS classes to the target element.

README

React CSS Modules

React CSS Modules implement automatic mapping of CSS modules. Every CSS class is assigned a local-scoped identifier with a global unique name. CSS Modules enable a modular and reusable CSS!

⚠️⚠️⚠️ DEPRECATION NOTICE ⚠️⚠️⚠️

If you are considering to use react-css-modules, evaluate if babel-plugin-react-css-modules covers your use case. babel-plugin-react-css-modules is a lightweight alternative of react-css-modules.

babel-plugin-react-css-modules is not a drop-in replacement and does not cover all the use cases of react-css-modules. However, it has a lot smaller performance overhead (0-10% vs +50%; see Performance) and a lot smaller size footprint (less than 2kb vs +17kb).

It is easy to get started! See the demo https://github.com/gajus/babel-plugin-react-css-modules/tree/master/demo

CSS Modules
- webpack css-loader
What's the Problem?
The Implementation
Usage
SASS, SCSS, LESS and other CSS Preprocessors
- Enable Sourcemaps
Class Composition
- What Problems does Class Composition Solve?
- Class Composition Using CSS Preprocessors
Global CSS
Multiple CSS Modules

CSS Modules

In the context of React, CSS Modules look like this:

import React from 'react';
import styles from './table.css';

export default class Table extends React.Component {
    render () {
        return <div className={styles.table}>
            <div className={styles.row}>
                <div className={styles.cell}>A0</div>
                <div className={styles.cell}>B0</div>
            </div>
        </div>;
    }
}

Rendering the component will produce a markup similar to:

<div class="table__table___32osj">
    <div class="table__row___2w27N">
        <div class="table__cell___1oVw5">A0</div>
        <div class="table__cell___1oVw5">B0</div>
    </div>
</div>

and a corresponding CSS file that matches those CSS classes.

Awesome!

webpack `css-loader`

CSS Modules is a specification that can be implemented in multiple ways. react-css-modules leverages the existing CSS Modules implementation webpack css-loader.

What's the Problem?

webpack css-loader itself has several disadvantages:

You have to use camelCase CSS class names.
You have to use styles object whenever constructing a className.
Mixing CSS Modules and global CSS classes is cumbersome.
Reference to an undefined CSS Module resolves to undefined without a warning.

React CSS Modules component automates loading of CSS Modules using styleName property, e.g.

import React from 'react';
import CSSModules from 'react-css-modules';
import styles from './table.css';

class Table extends React.Component {
    render () {
        return <div styleName='table'>
            <div styleName='row'>
                <div styleName='cell'>A0</div>
                <div styleName='cell'>B0</div>
            </div>
        </div>;
    }
}

export default CSSModules(Table, styles);

Using react-css-modules:

You are not forced to use the camelCase naming convention.
You do not need to refer to the styles object every time you use a CSS Module.
There is clear distinction between global CSS and CSS Modules, e.g.

<div className='global-css' styleName='local-module'></div>

You are warned when styleName refers to an undefined CSS Module (handleNotFoundStyleName option).
You can enforce use of a single CSS module per ReactElement (allowMultiple option).

The Implementation

Awesome!

Usage

Setup consists of:

Setting up a module bundler to load the Interoperable CSS.
Decorating your component using react-css-modules.

Module Bundler

webpack

Development

In development environment, you want to Enable Sourcemaps and webpack Hot Module Replacement (HMR). style-loader already supports HMR. Therefore, Hot Module Replacement will work out of the box.

Setup:

Install style-loader.
Install css-loader.
Setup /\.css$/ loader:

{
    test: /\.css$/,
    loaders: [
        'style-loader?sourceMap',
        'css-loader?modules&importLoaders=1&localIdentName=[path]___[name]__[local]___[hash:base64:5]'
    ]
}

Production

In production environment, you want to extract chunks of CSS into a single stylesheet file.

Advantages:

Fewer style tags (older IE has a limit)

CSS SourceMap (with devtool: "source-map" and css-loader?sourceMap)

CSS requested in parallel

CSS cached separate

Faster runtime (less code and DOM operations)

Caveats:

Additional HTTP request

Longer compilation time

More complex configuration

No runtime public path modification

No Hot Module Replacement

– extract-text-webpack-plugin

Setup:

Install style-loader.
Install css-loader.
Use extract-text-webpack-plugin to extract chunks of CSS into a single stylesheet.

Setup /\.css$/ loader:

ExtractTextPlugin v1x:

{
  test: /\.css$/,
  loader: ExtractTextPlugin.extract('style', 'css?modules&importLoaders=1&localIdentName=[name]__[local]___[hash:base64:5]')
}

ExtractTextPlugin v2x:

{
  test: /\.css$/,
  use: ExtractTextPlugin.extract({
      fallback: 'style-loader',
      use: 'css-loader?modules,localIdentName="[name]-[local]-[hash:base64:6]"'
  }),
}

Setup extract-text-webpack-plugin plugin:

ExtractTextPlugin v1x:

new ExtractTextPlugin('app.css', {
    allChunks: true
})

ExtractTextPlugin v2x:

new ExtractTextPlugin({
  filename: 'app.css',
  allChunks: true
})

Refer to webpack-demo or react-css-modules-examples for an example of a complete setup.

Browserify

Refer to css-modulesify.

Extending Component Styles

Use styles property to overwrite the default component styles.

Explanation using Table component:

import React from 'react';
import CSSModules from 'react-css-modules';
import styles from './table.css';

class Table extends React.Component {
    render () {
        return <div styleName='table'>
            <div styleName='row'>
                <div styleName='cell'>A0</div>
                <div styleName='cell'>B0</div>
            </div>
        </div>;
    }
}

export default CSSModules(Table, styles);

Using styles property you can overwrite the default component styles object, e.g.

import customStyles from './table-custom-styles.css';

<Table styles={customStyles} />;

Interoperable CSS can extend other ICSS. Use this feature to extend default styles, e.g.

/* table-custom-styles.css */
.table {
    composes: table from './table.css';
}

.row {
    composes: row from './table.css';
}

/* .cell {
    composes: cell from './table.css';
} */

.table {
    width: 400px;
}

.cell {
    float: left; width: 154px; background: #eee; padding: 10px; margin: 10px 0 10px 10px;
}

In this example, table-custom-styles.css selectively extends table.css (the default styles of Table component).

Refer to the UsingStylesProperty example for an example of a working implementation.

`styles` Property

Decorated components inherit styles property that describes the mapping between CSS modules and CSS classes.

class extends React.Component {
    render () {
        <div>
            <p styleName='foo'></p>
            <p className={this.props.styles.foo}></p>
        </div>;
    }
}

In the above example, styleName='foo' and className={this.props.styles.foo} are equivalent.

styles property is designed to enable component decoration of Loops and Child Components.

Loops and Child Components

styleName cannot be used to define styles of a ReactElement that will be generated by another component, e.g.

import React from 'react';
import CSSModules from 'react-css-modules';
import List from './List';
import styles from './table.css';

class CustomList extends React.Component {
    render () {
        let itemTemplate;

        itemTemplate = (name) => {
            return <li styleName='item-template'>{name}</li>;
        };

        return <List itemTemplate={itemTemplate} />;
    }
}

export default CSSModules(CustomList, styles);

The above example will not work. CSSModules is used to decorate CustomList component. However, it is the List component that will render itemTemplate.

For that purpose, the decorated component inherits styles property that you can use just as a regular CSS Modules object. The earlier example can be therefore rewritten to:

import React from 'react';
import CSSModules from 'react-css-modules';
import List from './List';
import styles from './table.css';

class CustomList extends React.Component {
    render () {
        let itemTemplate;

        itemTemplate = (name) => {
            return <li className={this.props.styles['item-template']}>{name}</li>;
        };

        return <List itemTemplate={itemTemplate} />;
    }
}

export default CSSModules(CustomList, styles);

You can use styleName property within the child component if you decorate the child component using CSSModules before passing it to the rendering component, e.g.

import React from 'react';
import CSSModules from 'react-css-modules';
import List from './List';
import styles from './table.css';

class CustomList extends React.Component {
    render () {
        let itemTemplate;

        itemTemplate = (name) => {
            return <li styleName='item-template'>{name}</li>;
        };

        itemTemplate = CSSModules(itemTemplate, this.props.styles);

        return <List itemTemplate={itemTemplate} />;
    }
}

export default CSSModules(CustomList, styles);

Decorator

/**
 * @typedef CSSModules~Options
 * @see {@link https://github.com/gajus/react-css-modules#options}
 * @property {Boolean} allowMultiple
 * @property {String} handleNotFoundStyleName
 */

/**
 * @param {Function} Component
 * @param {Object} defaultStyles CSS Modules class map.
 * @param {CSSModules~Options} options
 * @return {Function}
 */

You need to decorate your component using react-css-modules, e.g.

import React from 'react';
import CSSModules from 'react-css-modules';
import styles from './table.css';

class Table extends React.Component {
    render () {
        return <div styleName='table'>
            <div styleName='row'>
                <div styleName='cell'>A0</div>
                <div styleName='cell'>B0</div>
            </div>
        </div>;
    }
}

export default CSSModules(Table, styles);

Thats it!

As the name implies, react-css-modules is compatible with the ES7 decorators syntax:

import React from 'react';
import CSSModules from 'react-css-modules';
import styles from './table.css';

@CSSModules(styles)
export default class extends React.Component {
    render () {
        return <div styleName='table'>
            <div styleName='row'>
                <div styleName='cell'>A0</div>
                <div styleName='cell'>B0</div>
            </div>
        </div>;
    }
}

Awesome!

Refer to the react-css-modules-examples repository for an example of webpack setup.

Options

Options are supplied as the third parameter to the CSSModules function.

CSSModules(Component, styles, options);

or as a second parameter to the decorator:

@CSSModules(styles, options);

`allowMultiple`

Default: false.

Allows multiple CSS Module names.

When false, the following will cause an error:

<div styleName='foo bar' />

`handleNotFoundStyleName`

Default: throw.

Defines the desired action when styleName cannot be mapped to an existing CSS Module.

Available options:

throw throws an error
log logs a warning using console.warn
ignore silently ignores the missing style name

SASS, SCSS, LESS and other CSS Preprocessors

{
    test: /\.scss$/,
    loaders: [
        'style',
        'css?modules&importLoaders=1&localIdentName=[path]___[name]__[local]___[hash:base64:5]',
        'resolve-url',
        'sass'
    ]
}

Enable Sourcemaps

To enable CSS Source maps, add sourceMap parameter to the css-loader and to the sass-loader:

{
    test: /\.scss$/,
    loaders: [
        'style?sourceMap',
        'css?modules&importLoaders=1&localIdentName=[path]___[name]__[local]___[hash:base64:5]',
        'resolve-url',
        'sass?sourceMap'
    ]
}

Class Composition

CSS Modules promote composition pattern, i.e. every CSS Module that is used in a component should define all properties required to describe an element, e.g.

.box {
    width: 100px;
    height: 100px;
}

.empty {
    composes: box;

    background: #4CAF50;
}

.full {
    composes: box;

    background: #F44336;
}

Composition promotes better separation of markup and style using semantics that would be hard to achieve without CSS Modules.

Because CSS Module names are local, it is perfectly fine to use generic style names such as "empty" or "full", without "box-" prefix.

To learn more about composing CSS rules, I suggest reading Glen Maddern article about CSS Modules and the official spec of the CSS Modules.

What Problems does Class Composition Solve?

Consider the same example in CSS and HTML:

.box {
    width: 100px;
    height: 100px;
}

.box-empty {
    background: #4CAF50;
}

.box-full {
    background: #F44336;
}

<div class='box box-empty'></div>

This pattern emerged with the advent of OOCSS. The biggest disadvantage of this implementation is that you will need to change HTML almost every time you want to change the style.

Class Composition Using CSS Preprocessors

You can write compositions in SCSS using @extend keyword and using Mixin Directives, e.g.

Using @extend:

%box {
    width: 100px;
    height: 100px;
}

.box-empty {
    @extend %box;

    background: #4CAF50;
}

.box-full {
    @extend %box;

    background: #F44336;
}

This translates to:

.box-empty,
.box-full {
    width: 100px;
    height: 100px;
}

.box-empty {
    background: #4CAF50;
}

.box-full {
    background: #F44336;
}

Using mixins:

@mixin box {
    width: 100px;
    height: 100px;
}

.box-empty {
    @include box;

    background: #4CAF50;
}

.box-full {
    @include box;

    background: #F44336;
}

This translates to:

.box-empty {
    width: 100px;
    height: 100px;
    background: #4CAF50;
}

.box-full {
    width: 100px;
    height: 100px;
    background: #F44336;
}

Global CSS

CSS Modules does not restrict you from using global CSS.

:global .foo {

}

However, use global CSS with caution. With CSS Modules, there are only a handful of valid use cases for global CSS (e.g. normalization).

Multiple CSS Modules

Avoid using multiple CSS Modules to describe a single element. Read about Class Composition.

.button {

}

.active {

}

<div styleName='button active'></div>

This will map both Interoperable CSS CSS classes to the target element.