css-loader

What is css-loader?

The css-loader is a loader for webpack that interprets `@import` and `url()` like `import/require()` and will resolve them. It allows you to load CSS files with JavaScript and use CSS modules for local scope CSS.

What are css-loader's main functionalities?

Interpreting @import and url()

Allows importing CSS within JavaScript modules using @import and url() which will be resolved by webpack.

import './style.css';

CSS Modules

Enables the use of CSS Modules, where CSS classes are imported as an object of strings, allowing for local scope and composition of CSS classes.

import styles from './style.module.css'; console.log(styles.className);

Source Maps

Supports generation of source maps to enable debugging of CSS by linking the transformed code back to the original source.

module.exports = { module: { rules: [{ test: /\.css$/, use: ['style-loader', { loader: 'css-loader', options: { sourceMap: true } }] }] } };

Minification

In combination with other tools like CssMinimizerPlugin, it can be used to minify CSS for production builds.

module.exports = { optimization: { minimizer: ['...', new CssMinimizerPlugin()] } };

Import Loaders

Allows to configure how many loaders before css-loader should be applied to @imported resources.

module.exports = { module: { rules: [{ test: /\.css$/, use: ['style-loader', { loader: 'css-loader', options: { importLoaders: 1 } }, 'postcss-loader'] }] } };

Other packages similar to css-loader

style-loader

Injects CSS into the DOM using multiple <style> tags. It is often used in conjunction with css-loader.

sass-loader

Loads and compiles Sass/SCSS files. It extends css-loader's functionality to work with Sass/SCSS syntax.

less-loader

Processes Less files and compiles them to CSS. Similar to sass-loader, it's specific to Less preprocessor language.

postcss-loader

Processes CSS with PostCSS, a tool for transforming CSS with JavaScript plugins. It can be used after css-loader to apply various transformations and optimizations.

mini-css-extract-plugin

Extracts CSS into separate files. It creates a CSS file per JS file which contains CSS. It is used for CSS splitting, and is more performant than style-loader for production builds.

README

CSS Loader

Install

npm install --save-dev css-loader

Usage

The css-loader interprets @import and url() like import/require() and will resolve them.

Good loaders for requiring your assets are the file-loader and the url-loader which you should specify in your config (see below).

file.js

import css from 'file.css';

webpack.config.js

module.exports = {
  module: {
    rules: [
      {
        test: /\.css$/,
        use: [ 'style-loader', 'css-loader' ]
      }
    ]
  }
}

toString

You can also use the css-loader results directly as a string, such as in Angular's component style.

webpack.config.js

{
   test: /\.css$/,
   use: [
     'to-string-loader',
     'css-loader'
   ]
}

or

const css = require('./test.css').toString();

console.log(css); // {String}

If there are SourceMaps, they will also be included in the result string.

If, for one reason or another, you need to extract CSS as a plain string resource (i.e. not wrapped in a JS module) you might want to check out the extract-loader. It's useful when you, for instance, need to post process the CSS as a string.

webpack.config.js

{
   test: /\.css$/,
   use: [
     'handlebars-loader', // handlebars loader expects raw resource string
     'extract-loader',
     'css-loader'
   ]
}

Options

Name	Type	Default	Description
url	{Boolean}	true	Enable/Disable url() handling
import	{Boolean}	true	Enable/Disable @import handling
modules	{Boolean}	false	Enable/Disable CSS Modules
localIdentName	{String}	[hash:base64]	Configure the generated ident
sourceMap	{Boolean}	false	Enable/Disable Sourcemaps
camelCase	{Boolean|String}	false	Export Classnames in CamelCase
importLoaders	{Number}	0	Number of loaders applied before CSS loader

url

To disable url() resolving by css-loader set the option to false.

To be compatible with existing css files (if not in CSS Module mode).

url(image.png) => require('./image.png')
url(~module/image.png) => require('module/image.png')

import

To disable @import resolving by css-loader set the option to false

@import url('https://fonts.googleapis.com/css?family=Roboto');

⚠️ Use with caution, since this disables resolving for all @imports, including css modules composes: xxx from 'path/to/file.css' feature.

modules

The query parameter modules enables the CSS Modules spec.

This enables local scoped CSS by default. (You can switch it off with :global(...) or :global for selectors and/or rules.).

Scope

By default CSS exports all classnames into a global selector scope. Styles can be locally scoped to avoid globally scoping styles.

The syntax :local(.className) can be used to declare className in the local scope. The local identifiers are exported by the module.

With :local (without brackets) local mode can be switched on for this selector. :global(.className) can be used to declare an explicit global selector. With :global (without brackets) global mode can be switched on for this selector.

The loader replaces local selectors with unique identifiers. The chosen unique identifiers are exported by the module.

:local(.className) { background: red; }
:local .className { color: green; }
:local(.className .subClass) { color: green; }
:local .className .subClass :global(.global-class-name) { color: blue; }

._23_aKvs-b8bW2Vg3fwHozO { background: red; }
._23_aKvs-b8bW2Vg3fwHozO { color: green; }
._23_aKvs-b8bW2Vg3fwHozO ._13LGdX8RMStbBE9w-t0gZ1 { color: green; }
._23_aKvs-b8bW2Vg3fwHozO ._13LGdX8RMStbBE9w-t0gZ1 .global-class-name { color: blue; }

ℹ️ Identifiers are exported

exports.locals = {
  className: '_23_aKvs-b8bW2Vg3fwHozO',
  subClass: '_13LGdX8RMStbBE9w-t0gZ1'
}

CamelCase is recommended for local selectors. They are easier to use within the imported JS module.

url() URLs in block scoped (:local .abc) rules behave like requests in modules.

file.png => ./file.png
~module/file.png => module/file.png

You can use :local(#someId), but this is not recommended. Use classes instead of ids.

Composing

When declaring a local classname you can compose a local class from another local classname.

:local(.className) {
  background: red;
  color: yellow;
}

:local(.subClass) {
  composes: className;
  background: blue;
}

This doesn't result in any change to the CSS itself but exports multiple classnames.

exports.locals = {
  className: '_23_aKvs-b8bW2Vg3fwHozO',
  subClass: '_13LGdX8RMStbBE9w-t0gZ1 _23_aKvs-b8bW2Vg3fwHozO'
}

._23_aKvs-b8bW2Vg3fwHozO {
  background: red;
  color: yellow;
}

._13LGdX8RMStbBE9w-t0gZ1 {
  background: blue;
}

Importing

To import a local classname from another module.

:local(.continueButton) {
  composes: button from 'library/button.css';
  background: red;
}

:local(.nameEdit) {
  composes: edit highlight from './edit.css';
  background: red;
}

To import from multiple modules use multiple composes: rules.

:local(.className) {
  composes: edit hightlight from './edit.css';
  composes: button from 'module/button.css';
  composes: classFromThisModule;
  background: red;
}

localIdentName

You can configure the generated ident with the localIdentName query parameter. See loader-utils's documentation for more information on options.

webpack.config.js

{
  test: /\.css$/,
  use: [
    {
      loader: 'css-loader',
      options: {
        modules: true,
        localIdentName: '[path][name]__[local]--[hash:base64:5]'
      }
    }
  ]
}

You can also specify the absolute path to your custom getLocalIdent function to generate classname based on a different schema. This requires webpack >= 2.2.1 (it supports functions in the options object).

webpack.config.js

{
  loader: 'css-loader',
  options: {
    modules: true,
    localIdentName: '[path][name]__[local]--[hash:base64:5]',
    getLocalIdent: (context, localIdentName, localName, options) => {
      return 'whatever_random_class_name'
    }
  }
}

ℹ️ For prerendering with extract-text-webpack-plugin you should use css-loader/locals instead of style-loader!css-loader in the prerendering bundle. It doesn't embed CSS but only exports the identifier mappings.

sourceMap

To include source maps set the sourceMap option.

I.e. the extract-text-webpack-plugin can handle them.

They are not enabled by default because they expose a runtime overhead and increase in bundle size (JS source maps do not). In addition to that relative paths are buggy and you need to use an absolute public path which includes the server URL.

webpack.config.js

{
  loader: 'css-loader',
  options: {
    sourceMap: true
  }
}

camelCase

By default, the exported JSON keys mirror the class names. If you want to camelize class names (useful in JS), pass the query parameter camelCase to css-loader.

Name	Type	Description
true	{Boolean}	Class names will be camelized
'dashes'	{String}	Only dashes in class names will be camelized
'only'	{String}	Introduced in 0.27.1. Class names will be camelized, the original class name will be removed from the locals
'dashesOnly'	{String}	Introduced in 0.27.1. Dashes in class names will be camelized, the original class name will be removed from the locals

file.css

.class-name {}

file.js

import { className } from 'file.css';

webpack.config.js

{
  loader: 'css-loader',
  options: {
    camelCase: true
  }
}

importLoaders

The query parameter importLoaders allows you to configure how many loaders before css-loader should be applied to @imported resources.

webpack.config.js

{
  test: /\.css$/,
  use: [
    'style-loader',
    {
      loader: 'css-loader',
      options: {
        importLoaders: 2 // 0 => no loaders (default); 1 => postcss-loader; 2 => postcss-loader, sass-loader
      }
    },
    'postcss-loader',
    'sass-loader'
  ]
}

This may change in the future when the module system (i. e. webpack) supports loader matching by origin.

Examples

Assets

The following webpack.config.js can load CSS files, embed small PNG/JPG/GIF/SVG images as well as fonts as Data URLs and copy larger files to the output directory.

webpack.config.js

module.exports = {
  module: {
    rules: [
      {
        test: /\.css$/,
        use: [ 'style-loader', 'css-loader' ]
      },
      {
        test: /\.(png|jpg|gif|svg|eot|ttf|woff|woff2)$/,
        loader: 'url-loader',
        options: {
          limit: 10000
        }
      }
    ]
  }
}

Extract

For production builds it's recommended to extract the CSS from your bundle being able to use parallel loading of CSS/JS resources later on. This can be achieved by using the mini-css-extract-plugin to extract the CSS when running in production mode.

Maintainers

Juho Vepsäläinen	Joshua Wiens	Kees Kluskens	Sean Larkin
Michael Ciniawsky	Evilebot Tnawi	Joscha Feth

Changelog

1.0.1 (2018-10-29)

Bug Fixes

• loader: trim unquoted import urls (#783) (21fcddf)

<a name="1.0.0"></a>