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!
⚠️ ⚠️ ⚠️ Note:
If you are considering to use
react-css-modules
, evaluate ifbabel-plugin-react-css-modules
covers your use case.babel-plugin-react-css-modules
is a lightweight alternative ofreact-css-modules
.
babel-plugin-react-css-modules
is not a drop-in replacement and does not cover all the use cases ofreact-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
- What's the Problem?
- The Implementation
- Usage
- SASS, SCSS, LESS and other CSS Preprocessors
- Class Composition
- 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!
css-loader
webpack 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 aclassName
. - 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.
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"
andcss-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
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>;
}
}
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 usingconsole.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.