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!

CSS Modules
- webpack css-loader
What's the Problem?
The Implementation
Usage
React Hot Module Replacement
Class Composition
- What Problems does Class Composition Solve?
- Class Composition Using CSS Preprocessors
SASS, SCSS, LESS and other 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 a 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___2w27N">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 as 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 (errorWhenNotFound 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

Install style-loader and css-loader.
You need to use extract-text-webpack-plugin to aggregate the CSS into a single file.
Setup /\.css$/ loader:

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

Setup extract-text-webpack-plugin plugin:

new ExtractTextPlugin('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.

styles property within the component itself is designed to be used only when styleNames cannot be used.

styleNames cannot be used within a component to define styles of a ReactElement that will be generated by another component (https://github.com/gajus/react-css-modules/issues/11), e.g.

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

        itemTemplate = (name) => {
            return <li styleName='foo'>{name}</li>;
        };

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

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:

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

        itemTemplate = (name) => {
            return <li styleName={this.props.styles.foo}>{name}</li>;
        };

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

styles property works with ES6 classes and stateless function components.

Decorator

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

/**
 * @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' />

`errorWhenNotFound`

Default: true.

Throws an error when styleName cannot be mapped to an existing CSS Module.

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

React Hot Module Replacement

Hot module reloading (HMR) does not reload the CSS document (see https://github.com/gajus/react-css-modules/issues/51). It only reloads the class HTML attribute value.

To enable CSS reloading, wrap webpack-dev-server configuration using BrowserSync. BrowserSync enables CSS reloading when it detects a file change.

React CSS Modules examples repository includes a configuration example using BrowserSync configuration using webpack-dev-server.

Note that webpackk-dev-server program does not write bundle files to the disk. Use write-file-webpack-plugin plugin to force writing to the disk. This will enable BrowserSync to detect changes.

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 composting 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.

andris-silis/react-css-modules

React CSS Modules

CSS Modules

webpack css-loader

What's the Problem?

The Implementation

Usage

Module Bundler

webpack

Browserify

Extending Component Styles

Styles Property

Decorator

Options

allowMultiple

errorWhenNotFound

SASS, SCSS, LESS and other CSS Preprocessors

React Hot Module Replacement

Class Composition

What Problems does Class Composition Solve?

Class Composition Using CSS Preprocessors

Global CSS

Multiple CSS Modules

webpack `css-loader`

`allowMultiple`

`errorWhenNotFound`