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

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

Getting Started

Warning

To use the latest version of css-loader, webpack@5 is required

To begin, you'll need to install css-loader:

npm install --save-dev css-loader

or

yarn add -D css-loader

or

pnpm add -D css-loader

Then add the plugin to your webpack config. For example:

file.js

import * as css from "file.css";

webpack.config.js

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

And run webpack via your preferred method.

If, for one reason or another, you need to extract CSS as a file (i.e. do not store CSS in a JS module) you might want to check out the recommend example.

Options

• url
• import
• modules
• sourceMap
• importLoaders
• esModule
• exportType

url

Type:

type url =
  | boolean
  | {
      filter: (url: string, resourcePath: string) => boolean;
    };

Default: true

Allow to enable/disables handling the CSS functions url and image-set. If set to false, css-loader will not parse any paths specified in url or image-set. A function can also be passed to control this behavior dynamically based on the path to the asset. Starting with version 4.0.0, absolute paths are parsed based on the server root.

Examples resolutions:

url(image.png) => require('./image.png')
url('image.png') => require('./image.png')
url(./image.png) => require('./image.png')
url('./image.png') => require('./image.png')
url('http://dontwritehorriblecode.com/2112.png') => require('http://dontwritehorriblecode.com/2112.png')
image-set(url('image2x.png') 1x, url('image1x.png') 2x) => require('./image1x.png') and require('./image2x.png')

To import assets from a node_modules path (include resolve.modules) and for alias, prefix it with a ~:

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

boolean

Enable/disable url() resolving.

webpack.config.js

module.exports = {
  module: {
    rules: [
      {
        test: /\.css$/i,
        loader: "css-loader",
        options: {
          url: true,
        },
      },
    ],
  },
};

object

Allow to filter url(). All filtered url() will not be resolved (left in the code as they were written).

webpack.config.js

module.exports = {
  module: {
    rules: [
      {
        test: /\.css$/i,
        loader: "css-loader",
        options: {
          url: {
            filter: (url, resourcePath) => {
              // resourcePath - path to css file

              // Don't handle `img.png` urls
              if (url.includes("img.png")) {
                return false;
              }

              // Don't handle images under root-relative /external_images/
              if (/^\/external_images\//.test(path)) {
                return false;
              }

              return true;
            },
          },
        },
      },
    ],
  },
};

import

Type:

type importFn =
  | boolean
  | {
      filter: (
        url: string,
        media: string,
        resourcePath: string,
        supports?: string,
        layer?: string,
      ) => boolean;
    };

Default: true

Allows to enables/disables @import at-rules handling. Control @import resolving. Absolute urls in @import will be moved in runtime code.

Examples resolutions:

@import 'style.css' => require('./style.css')
@import url(style.css) => require('./style.css')
@import url('style.css') => require('./style.css')
@import './style.css' => require('./style.css')
@import url(./style.css) => require('./style.css')
@import url('./style.css') => require('./style.css')
@import url('http://dontwritehorriblecode.com/style.css') => @import url('http://dontwritehorriblecode.com/style.css') in runtime

To import styles from a node_modules path (include resolve.modules) and for alias, prefix it with a ~:

@import url(~module/style.css) => require('module/style.css')
@import url('~module/style.css') => require('module/style.css')
@import url(~aliasDirectory/style.css) => require('otherDirectory/style.css')

boolean

Enable/disable @import resolving.

webpack.config.js

module.exports = {
  module: {
    rules: [
      {
        test: /\.css$/i,
        loader: "css-loader",
        options: {
          import: true,
        },
      },
    ],
  },
};

object

filter

Type:

type filter = (url: string, media: string, resourcePath: string) => boolean;

Default: undefined

Allow to filter @import. All filtered @import will not be resolved (left in the code as they were written).

webpack.config.js

module.exports = {
  module: {
    rules: [
      {
        test: /\.css$/i,
        loader: "css-loader",
        options: {
          import: {
            filter: (url, media, resourcePath) => {
              // resourcePath - path to css file

              // Don't handle `style.css` import
              if (url.includes("style.css")) {
                return false;
              }

              return true;
            },
          },
        },
      },
    ],
  },
};

modules

Type:

type modules =
  | boolean
  | "local"
  | "global"
  | "pure"
  | "icss"
  | {
      auto: boolean | regExp | ((resourcePath: string) => boolean);
      mode:
        | "local"
        | "global"
        | "pure"
        | "icss"
        | ((resourcePath) => "local" | "global" | "pure" | "icss");
      localIdentName: string;
      localIdentContext: string;
      localIdentHashSalt: string;
      localIdentHashFunction: string;
      localIdentHashDigest: string;
      localIdentRegExp: string | regExp;
      getLocalIdent: (
        context: LoaderContext,
        localIdentName: string,
        localName: string,
      ) => string;
      namedExport: boolean;
      exportGlobals: boolean;
      exportLocalsConvention:
        | "as-is"
        | "camel-case"
        | "camel-case-only"
        | "dashes"
        | "dashes-only"
        | ((name: string) => string);
      exportOnlyLocals: boolean;
      getJSON: ({
        resourcePath,
        imports,
        exports,
        replacements,
      }: {
        resourcePath: string;
        imports: object[];
        exports: object[];
        replacements: object[];
      }) => Promise<void> | void;
    };

Default: undefined

Allows to enable/disable CSS Modules or ICSS and setup configuration:

• undefined - enable CSS modules for all files matching /\.module\.\w+$/i.test(filename) and /\.icss\.\w+$/i.test(filename) regexp.
• true - enable CSS modules for all files.
• false - disables CSS Modules for all files.
• string - disables CSS Modules for all files and set the mode option, more information you can read here
• object - enable CSS modules for all files, if modules.auto option is not specified, otherwise the modules.auto option will determine whether if it is CSS modules or not, more information you can read here

The modules option enables/disables the CSS Modules specification and setup basic behaviour.

Using false value increase performance because we avoid parsing CSS Modules features, it will be useful for developers who use vanilla css or use other technologies.

webpack.config.js

module.exports = {
  module: {
    rules: [
      {
        test: /\.css$/i,
        loader: "css-loader",
        options: {
          modules: true,
        },
      },
    ],
  },
};

Features

Scope

Using local value requires you to specify :global classes. Using global value requires you to specify :local classes. Using pure value requires selectors must contain at least one local class or id.

You can find more information here.

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. The :global(.className) notation 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;
}

Note

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.

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.

Note

We strongly recommend that you specify the extension when importing a file, since it is possible to import a file with any extension and it is not known in advance which file to use.

: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 highlight from "./edit.css",
    button from "module/button.css",
    classFromThisModule;
  background: red;
}

or

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

Values

You can use @value to specific values to be reused throughout a document.

We recommend use prefix v- for values, s- for selectors and m- for media at-rules.

@value v-primary: #BF4040;
@value s-black: black-selector;
@value m-large: (min-width: 960px);

.header {
  color: v-primary;
  padding: 0 10px;
}

.s-black {
  color: black;
}

@media m-large {
  .header {
    padding: 0 20px;
  }
}

boolean

Enable CSS Modules features.

webpack.config.js

module.exports = {
  module: {
    rules: [
      {
        test: /\.css$/i,
        loader: "css-loader",
        options: {
          modules: true,
        },
      },
    ],
  },
};

string

Enable CSS Modules features and setup mode.

webpack.config.js

module.exports = {
  module: {
    rules: [
      {
        test: /\.css$/i,
        loader: "css-loader",
        options: {
          // Using `local` value has same effect like using `modules: true`
          modules: "global",
        },
      },
    ],
  },
};

object

Enable CSS Modules features and setup options for them.

webpack.config.js

module.exports = {
  module: {
    rules: [
      {
        test: /\.css$/i,
        loader: "css-loader",
        options: {
          modules: {
            mode: "local",
            auto: true,
            exportGlobals: true,
            localIdentName: "[path][name]__[local]--[hash:base64:5]",
            localIdentContext: path.resolve(__dirname, "src"),
            localIdentHashSalt: "my-custom-hash",
            namedExport: true,
            exportLocalsConvention: "as-is",
            exportOnlyLocals: false,
            getJSON: ({ resourcePath, imports, exports, replacements }) => {},
          },
        },
      },
    ],
  },
};

auto

Type:

type auto =
  | boolean
  | regExp
  | ((
      resourcePath: string,
      resourceQuery: string,
      resourceFragment: string,
    ) => boolean);

Default: undefined

Allows auto enable CSS modules/ICSS based on the filename, query or fragment when modules option is object.

Possible values:

• undefined - enable CSS modules for all files.
• true - enable CSS modules for all files matching /\.module\.\w+$/i.test(filename) and /\.icss\.\w+$/i.test(filename) regexp.
• false - disables CSS Modules.
• RegExp - enable CSS modules for all files matching /RegExp/i.test(filename) regexp.
• function - enable CSS Modules for files based on the filename satisfying your filter function check.

boolean

Possible values:

• true - enables CSS modules or interoperable CSS format, sets the modules.mode option to local value for all files which satisfy /\.module(s)?\.\w+$/i.test(filename) condition or sets the modules.mode option to icss value for all files which satisfy /\.icss\.\w+$/i.test(filename) condition
• false - disables CSS modules or interoperable CSS format based on filename

webpack.config.js

module.exports = {
  module: {
    rules: [
      {
        test: /\.css$/i,
        loader: "css-loader",
        options: {
          modules: {
            auto: true,
          },
        },
      },
    ],
  },
};

RegExp

Enable CSS modules for files based on the filename satisfying your regex check.

webpack.config.js

module.exports = {
  module: {
    rules: [
      {
        test: /\.css$/i,
        loader: "css-loader",
        options: {
          modules: {
            auto: /\.custom-module\.\w+$/i,
          },
        },
      },
    ],
  },
};

function

Enable CSS modules for files based on the filename, query or fragment satisfying your filter function check.

webpack.config.js

module.exports = {
  module: {
    rules: [
      {
        test: /\.css$/i,
        loader: "css-loader",
        options: {
          modules: {
            auto: (resourcePath, resourceQuery, resourceFragment) => {
              return resourcePath.endsWith(".custom-module.css");
            },
          },
        },
      },
    ],
  },
};

mode

Type:

type mode =
  | "local"
  | "global"
  | "pure"
  | "icss"
  | ((
      resourcePath: string,
      resourceQuery: string,
      resourceFragment: string,
    ) => "local" | "global" | "pure" | "icss");

Default: 'local'

Setup mode option. You can omit the value when you want local mode.

Controls the level of compilation applied to the input styles.

The local, global, and pure handles class and id scoping and @value values. The icss will only compile the low level Interoperable CSS format for declaring :import and :export dependencies between CSS and other languages.

ICSS underpins CSS Module support, and provides a low level syntax for other tools to implement CSS-module variations of their own.

string

Possible values - local, global, pure, and icss.

webpack.config.js

module.exports = {
  module: {
    rules: [
      {
        test: /\.css$/i,
        loader: "css-loader",
        options: {
          modules: {
            mode: "global",
          },
        },
      },
    ],
  },
};

function

Allows set different values for the mode option based on the filename, query or fragment.

Possible return values - local, global, pure and icss.

webpack.config.js

module.exports = {
  module: {
    rules: [
      {
        test: /\.css$/i,
        loader: "css-loader",
        options: {
          modules: {
            // Callback must return "local", "global", or "pure" values
            mode: (resourcePath, resourceQuery, resourceFragment) => {
              if (/pure.css$/i.test(resourcePath)) {
                return "pure";
              }

              if (/global.css$/i.test(resourcePath)) {
                return "global";
              }

              return "local";
            },
          },
        },
      },
    ],
  },
};

localIdentName

Type:

type localIdentName = string;

Default: '[hash:base64]'

Allows to configure the generated local ident name.

For more information on options see:

• webpack template strings,
• output.hashDigest,
• output.hashDigestLength,
• output.hashFunction,
• output.hashSalt.

Supported template strings:

• [name] the basename of the resource
• [folder] the folder the resource relative to the compiler.context option or modules.localIdentContext option.
• [path] the path of the resource relative to the compiler.context option or modules.localIdentContext option.
• [file] - filename and path.
• [ext] - extension with leading ..
• [hash] - the hash of the string, generated based on localIdentHashSalt, localIdentHashFunction, localIdentHashDigest, localIdentHashDigestLength, localIdentContext, resourcePath and exportName
• [<hashFunction>:hash:<hashDigest>:<hashDigestLength>] - hash with hash settings.
• [local] - original class.

Recommendations:

• use '[path][name]__[local]' for development
• use '[hash:base64]' for production

The [local] placeholder contains original class.

Note: all reserved (<>:"/\|?*) and control filesystem characters (excluding characters in the [local] placeholder) will be converted to -.

webpack.config.js

module.exports = {
  module: {
    rules: [
      {
        test: /\.css$/i,
        loader: "css-loader",
        options: {
          modules: {
            localIdentName: "[path][name]__[local]--[hash:base64:5]",
          },
        },
      },
    ],
  },
};

localIdentContext

Type:

type localIdentContex = string;

Default: compiler.context

Allows to redefine basic loader context for local ident name.

webpack.config.js

module.exports = {
  module: {
    rules: [
      {
        test: /\.css$/i,
        loader: "css-loader",
        options: {
          modules: {
            localIdentContext: path.resolve(__dirname, "src"),
          },
        },
      },
    ],
  },
};

localIdentHashSalt

Type:

type localIdentHashSalt = string;

Default: undefined

Allows to add custom hash to generate more unique classes. For more information see output.hashSalt.

webpack.config.js

module.exports = {
  module: {
    rules: [
      {
        test: /\.css$/i,
        loader: "css-loader",
        options: {
          modules: {
            localIdentHashSalt: "hash",
          },
        },
      },
    ],
  },
};

localIdentHashFunction

Type:

type localIdentHashFunction = string;

Default: md4

Allows to specify hash function to generate classes . For more information see output.hashFunction.

webpack.config.js

module.exports = {
  module: {
    rules: [
      {
        test: /\.css$/i,
        loader: "css-loader",
        options: {
          modules: {
            localIdentHashFunction: "md4",
          },
        },
      },
    ],
  },
};

localIdentHashDigest

Type:

type localIdentHashDigest = string;

Default: hex

Allows to specify hash digest to generate classes. For more information see output.hashDigest.

webpack.config.js

module.exports = {
  module: {
    rules: [
      {
        test: /\.css$/i,
        loader: "css-loader",
        options: {
          modules: {
            localIdentHashDigest: "base64",
          },
        },
      },
    ],
  },
};

localIdentHashDigestLength

Type:

type localIdentHashDigestLength = number;

Default: 20

Allows to specify hash digest length to generate classes. For more information see output.hashDigestLength.

webpack.config.js

module.exports = {
  module: {
    rules: [
      {
        test: /\.css$/i,
        loader: "css-loader",
        options: {
          modules: {
            localIdentHashDigestLength: 5,
          },
        },
      },
    ],
  },
};

hashStrategy

Type: 'resource-path-and-local-name' | 'minimal-subset' Default: 'resource-path-and-local-name'

Should local name be used when computing the hash.

• 'resource-path-and-local-name' Both resource path and local name are used when hashing. Each identifier in a module gets its own hash digest, always.
• 'minimal-subset' Auto detect if identifier names can be omitted from hashing. Use this value to optimize the output for better GZIP or Brotli compression.

webpack.config.js

module.exports = {
  module: {
    rules: [
      {
        test: /\.css$/i,
        loader: "css-loader",
        options: {
          modules: {
            hashStrategy: "minimal-subset",
          },
        },
      },
    ],
  },
};

localIdentRegExp

Type:

type localIdentRegExp = string | RegExp;

Default: undefined

webpack.config.js

module.exports = {
  module: {
    rules: [
      {
        test: /\.css$/i,
        loader: "css-loader",
        options: {
          modules: {
            localIdentRegExp: /page-(.*)\.css/i,
          },
        },
      },
    ],
  },
};

getLocalIdent

Type:

type getLocalIdent = (
  context: LoaderContext,
  localIdentName: string,
  localName: string,
) => string;

Default: undefined

Allows to specify a function to generate the classname. By default we use built-in function to generate a classname. If the custom function returns null or undefined, we fallback to the built-in function to generate the classname.

webpack.config.js

module.exports = {
  module: {
    rules: [
      {
        test: /\.css$/i,
        loader: "css-loader",
        options: {
          modules: {
            getLocalIdent: (context, localIdentName, localName, options) => {
              return "whatever_random_class_name";
            },
          },
        },
      },
    ],
  },
};

namedExport

Type:

type namedExport = boolean;

Default: Depends on the value of the esModule option. If the value of the esModule options is true, this value will also be true, otherwise it will be false.

Enables/disables ES modules named export for locals.

Warning

Because it is not allowed to use the default class in CSS when the namedExport is true (since ECMA modules have a reserved keyword default for default export), it will be automatically renamed to the _default class.

styles.css

.foo-baz {
  color: red;
}
.bar {
  color: blue;
}
.default {
  color: green;
}

index.js

import * as styles from "./styles.css";

// If using `exportLocalsConvention: "as-is"` (default value):
console.log(styles["foo-baz"], styles.bar);

// If using `exportLocalsConvention: "camel-case-only"`:
console.log(styles.fooBaz, styles.bar);

// For the `default` classname
console.log(styles["_default"]);

You can enable a ES module named export using:

webpack.config.js

module.exports = {
  module: {
    rules: [
      {
        test: /\.css$/i,
        loader: "css-loader",
        options: {
          esModule: true,
          modules: {
            namedExport: true,
          },
        },
      },
    ],
  },
};

To set a custom name for namedExport, can use exportLocalsConvention option as a function. Example below in the examples section.

exportGlobals

Type:

type exportsGLobals = boolean;

Default: false

Allow css-loader to export names from global class or id, so you can use that as local name.

webpack.config.js

module.exports = {
  module: {
    rules: [
      {
        test: /\.css$/i,
        loader: "css-loader",
        options: {
          modules: {
            exportGlobals: true,
          },
        },
      },
    ],
  },
};

exportLocalsConvention

Type:

type exportLocalsConvention =
  | "as-is"
  | "camel-case"
  | "camel-case-only"
  | "dashes"
  | "dashes-only"
  | ((name: string) => string);

Default: Depends on the value of the modules.namedExport option, if true - as-is, otherwise camel-case-only.

Warning

Names of locals are converted to camelcase when the named export is false, i.e. the exportLocalsConvention option has camelCaseOnly value by default. You can set this back to any other valid option but selectors which are not valid JavaScript identifiers may run into problems which do not implement the entire modules specification.

Style of exported class names.

string

By default, the exported JSON keys mirror the class names (i.e as-is value).

Name	Type	Description
'as-is'	string	Class names will be exported as is.
'camel-case'	string	Class names will be camelized, the original class name will not to be removed from the locals
'camel-case-only'	string	Class names will be camelized, the original class name will be removed from the locals
'dashes'	string	Only dashes in class names will be camelized
'dashes-only'	string	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

module.exports = {
  module: {
    rules: [
      {
        test: /\.css$/i,
        loader: "css-loader",
        options: {
          modules: {
            exportLocalsConvention: "camel-case-only",
          },
        },
      },
    ],
  },
};

function

webpack.config.js

module.exports = {
  module: {
    rules: [
      {
        test: /\.css$/i,
        loader: "css-loader",
        options: {
          modules: {
            exportLocalsConvention: function (name) {
              return name.replace(/-/g, "_");
            },
          },
        },
      },
    ],
  },
};

webpack.config.js

module.exports = {
  module: {
    rules: [
      {
        test: /\.css$/i,
        loader: "css-loader",
        options: {
          modules: {
            exportLocalsConvention: function (name) {
              return [
                name.replace(/-/g, "_"),
                // dashesCamelCase
                name.replace(/-+(\w)/g, (match, firstLetter) =>
                  firstLetter.toUpperCase(),
                ),
              ];
            },
          },
        },
      },
    ],
  },
};

exportOnlyLocals

Type:

type exportOnlyLocals = boolean;

Default: false

Export only locals.

Useful when you use css modules for pre-rendering (for example SSR). For pre-rendering with mini-css-extract-plugin you should use this option instead of style-loader!css-loader in the pre-rendering bundle. It doesn't embed CSS but only exports the identifier mappings.

webpack.config.js

module.exports = {
  module: {
    rules: [
      {
        test: /\.css$/i,
        loader: "css-loader",
        options: {
          modules: {
            exportOnlyLocals: true,
          },
        },
      },
    ],
  },
};

getJSON

Type:

type getJSON = ({
  resourcePath,
  imports,
  exports,
  replacements,
}: {
  resourcePath: string;
  imports: object[];
  exports: object[];
  replacements: object[];
}) => Promise<void> | void;

Default: undefined

Enables a callback to output the CSS modules mapping JSON. The callback is invoked with an object containing the following:

• resourcePath: the absolute path of the original resource, e.g., /foo/bar/baz.module.css

• imports: an array of import objects with data about import types and file paths, e.g.,

[
  {
    "type": "icss_import",
    "importName": "___CSS_LOADER_ICSS_IMPORT_0___",
    "url": "\"-!../../../../../node_modules/css-loader/dist/cjs.js??ruleSet[1].rules[4].use[1]!../../../../../node_modules/postcss-loader/dist/cjs.js!../../../../../node_modules/sass-loader/dist/cjs.js!../../../../baz.module.css\"",
    "icss": true,
    "index": 0
  }
]

(Note that this will include all imports, not just those relevant to CSS modules.)

• exports: an array of export objects with exported names and values, e.g.,

[
  {
    "name": "main",
    "value": "D2Oy"
  }
]

• replacements: an array of import replacement objects used for linking imports and exports, e.g.,

{
  "replacementName": "___CSS_LOADER_ICSS_IMPORT_0_REPLACEMENT_0___",
  "importName": "___CSS_LOADER_ICSS_IMPORT_0___",
  "localName": "main"
}

Using getJSON, it's possible to output a files with all CSS module mappings. In the following example, we use getJSON to cache canonical mappings and add stand-ins for any composed values (through composes), and we use a custom plugin to consolidate the values and output them to a file:

webpack.config.js

const path = require("path");
const fs = require("fs");

const CSS_LOADER_REPLACEMENT_REGEX =
  /(___CSS_LOADER_ICSS_IMPORT_\d+_REPLACEMENT_\d+___)/g;
const REPLACEMENT_REGEX = /___REPLACEMENT\[(.*?)]\[(.*?)]___/g;
const IDENTIFIER_REGEX = /\[(.*?)]\[(.*?)]/;
const replacementsMap = {};
const canonicalValuesMap = {};
const allExportsJson = {};

function generateIdentifier(resourcePath, localName) {
  return `[${resourcePath}][${localName}]`;
}

function addReplacements(resourcePath, imports, exportsJson, replacements) {
  const importReplacementsMap = {};

  // create a dict to quickly identify imports and get their absolute stand-in strings in the currently loaded file
  // e.g., { '___CSS_LOADER_ICSS_IMPORT_0_REPLACEMENT_0___': '___REPLACEMENT[/foo/bar/baz.css][main]___' }
  importReplacementsMap[resourcePath] = replacements.reduce(
    (acc, { replacementName, importName, localName }) => {
      const replacementImportUrl = imports.find(
        (importData) => importData.importName === importName,
      ).url;
      const relativePathRe = /.*!(.*)"/;
      const [, relativePath] = replacementImportUrl.match(relativePathRe);
      const importPath = path.resolve(path.dirname(resourcePath), relativePath);
      const identifier = generateIdentifier(importPath, localName);
      return { ...acc, [replacementName]: `___REPLACEMENT${identifier}___` };
    },
    {},
  );

  // iterate through the raw exports and add stand-in variables
  // ('___REPLACEMENT[<absolute_path>][<class_name>]___')
  // to be replaced in the plugin below
  for (const [localName, classNames] of Object.entries(exportsJson)) {
    const identifier = generateIdentifier(resourcePath, localName);

    if (CSS_LOADER_REPLACEMENT_REGEX.test(classNames)) {
      // if there are any replacements needed in the concatenated class names,
      // add them all to the replacements map to be replaced altogether later
      replacementsMap[identifier] = classNames.replaceAll(
        CSS_LOADER_REPLACEMENT_REGEX,
        (_, replacementName) =>
          importReplacementsMap[resourcePath][replacementName],
      );
    } else {
      // otherwise, no class names need replacements so we can add them to
      // canonical values map and all exports JSON verbatim
      canonicalValuesMap[identifier] = classNames;

      allExportsJson[resourcePath] = allExportsJson[resourcePath] || {};
      allExportsJson[resourcePath][localName] = classNames;
    }
  }
}

function replaceReplacements(classNames) {
  return classNames.replaceAll(
    REPLACEMENT_REGEX,
    (_, resourcePath, localName) => {
      const identifier = generateIdentifier(resourcePath, localName);

      if (identifier in canonicalValuesMap) {
        return canonicalValuesMap[identifier];
      }

      // Recurse through other stand-in that may be imports
      const canonicalValue = replaceReplacements(replacementsMap[identifier]);

      canonicalValuesMap[identifier] = canonicalValue;

      return canonicalValue;
    },
  );
}

function getJSON({ resourcePath, imports, exports, replacements }) {
  const exportsJson = exports.reduce((acc, { name, value }) => {
    return { ...acc, [name]: value };
  }, {});

  if (replacements.length > 0) {
    // replacements present --> add stand-in values for absolute paths and local names,
    // which will be resolved to their canonical values in the plugin below
    addReplacements(resourcePath, imports, exportsJson, replacements);
  } else {
    // no replacements present --> add to canonicalValuesMap verbatim
    // since all values here are canonical/don't need resolution
    for (const [key, value] of Object.entries(exportsJson)) {
      const id = `[${resourcePath}][${key}]`;

      canonicalValuesMap[id] = value;
    }

    allExportsJson[resourcePath] = exportsJson;
  }
}

class CssModulesJsonPlugin {
  constructor(options) {
    this.options = options;
  }

  // eslint-disable-next-line class-methods-use-this
  apply(compiler) {
    compiler.hooks.emit.tap("CssModulesJsonPlugin", () => {
      for (const [identifier, classNames] of Object.entries(replacementsMap)) {
        const adjustedClassNames = replaceReplacements(classNames);

        replacementsMap[identifier] = adjustedClassNames;

        const [, resourcePath, localName] = identifier.match(IDENTIFIER_REGEX);

        allExportsJson[resourcePath] = allExportsJson[resourcePath] || {};
        allExportsJson[resourcePath][localName] = adjustedClassNames;
      }

      fs.writeFileSync(
        this.options.filepath,
        JSON.stringify(
          // Make path to be relative to `context` (your project root)
          Object.fromEntries(
            Object.entries(allExportsJson).map((key) => {
              key[0] = path
                .relative(compiler.context, key[0])
                .replace(/\\/g, "/");

              return key;
            }),
          ),
          null,
          2,
        ),
        "utf8",
      );
    });
  }
}

module.exports = {
  module: {
    rules: [
      {
        test: /\.css$/i,
        loader: "css-loader",
        options: { modules: { getJSON } },
      },
    ],
  },
  plugins: [
    new CssModulesJsonPlugin({
      filepath: path.resolve(__dirname, "./output.css.json"),
    }),
  ],
};

In the above, all import aliases are replaced with ___REPLACEMENT[<resourcePath>][<localName>]___ in getJSON, and they're resolved in the custom plugin. All CSS mappings are contained in allExportsJson:

{
  "foo/bar/baz.module.css": {
    "main": "D2Oy",
    "header": "thNN"
  },
  "foot/bear/bath.module.css": {
    "logo": "sqiR",
    "info": "XMyI"
  }
}

This is saved to a local file named output.css.json.

importLoaders

Type:

type importLoaders = number;

Default: 0

Allows to enables/disables or setups number of loaders applied before CSS loader for @import at-rules, CSS modules and ICSS imports, i.e. @import/composes/@value value from './values.css'/etc.

The option importLoaders allows you to configure how many loaders before css-loader should be applied to @imported resources and CSS modules/ICSS imports.

webpack.config.js

module.exports = {
  module: {
    rules: [
      {
        test: /\.css$/i,
        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.

sourceMap

Type:

type sourceMap = boolean;

Default: depends on the compiler.devtool value

By default generation of source maps depends on the devtool option. All values enable source map generation except eval and false value.

webpack.config.js

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

esModule

Type:

type esModule = boolean;

Default: true

By default, css-loader generates JS modules that use the ES modules syntax. There are some cases in which using ES modules is beneficial, like in the case of module concatenation and tree shaking.

You can enable a CommonJS modules syntax using:

webpack.config.js

module.exports = {
  module: {
    rules: [
      {
        test: /\.css$/i,
        loader: "css-loader",
        options: {
          esModule: false,
        },
      },
    ],
  },
};

exportType

Type:

type exportType = "array" | "string" | "css-style-sheet";

Default: 'array'

Allows exporting styles as array with modules, string or constructable stylesheet (i.e. CSSStyleSheet). Default value is 'array', i.e. loader exports array of modules with specific API which is used in style-loader or other.

webpack.config.js

module.exports = {
  module: {
    rules: [
      {
        assert: { type: "css" },
        loader: "css-loader",
        options: {
          exportType: "css-style-sheet",
        },
      },
    ],
  },
};

src/index.js

import sheet from "./styles.css" assert { type: "css" };

document.adoptedStyleSheets = [sheet];
shadowRoot.adoptedStyleSheets = [sheet];

'array'

The default export is array of modules with specific API which is used in style-loader or other.

webpack.config.js

module.exports = {
  module: {
    rules: [
      {
        test: /\.(sa|sc|c)ss$/i,
        use: ["style-loader", "css-loader", "postcss-loader", "sass-loader"],
      },
    ],
  },
};

src/index.js

// `style-loader` applies styles to DOM
import "./styles.css";

'string'

Warning

You should not use style-loader or mini-css-extract-plugin with this value.

Warning

The esModule option should be enabled if you want to use it with CSS modules, by default for locals will be used named export.

The default export is string.

webpack.config.js

module.exports = {
  module: {
    rules: [
      {
        test: /\.(sa|sc|c)ss$/i,
        use: ["css-loader", "postcss-loader", "sass-loader"],
      },
    ],
  },
};

src/index.js

import sheet from "./styles.css";

console.log(sheet);

'css-style-sheet'

Warning

@import rules not yet allowed, more information

Warning

You don't need style-loader anymore, please remove it.

Warning

The esModule option should be enabled if you want to use it with CSS modules, by default for locals will be used named export.

Warning

Source maps are not currently supported in Chrome due bug

The default export is a constructable stylesheet (i.e. CSSStyleSheet).

Useful for custom elements and shadow DOM.

More information:

• Using CSS Module Scripts to import stylesheets
• Constructable Stylesheets: seamless reusable styles

webpack.config.js

module.exports = {
  module: {
    rules: [
      {
        assert: { type: "css" },
        loader: "css-loader",
        options: {
          exportType: "css-style-sheet",
        },
      },

      // For Sass/SCSS:
      //
      // {
      //   assert: { type: "css" },
      //   rules: [
      //     {
      //       loader: "css-loader",
      //       options: {
      //         exportType: "css-style-sheet",
      //         // Other options
      //       },
      //     },
      //     {
      //       loader: "sass-loader",
      //       options: {
      //         // Other options
      //       },
      //     },
      //   ],
      // },
    ],
  },
};

src/index.js

// Example for Sass/SCSS:
// import sheet from "./styles.scss" assert { type: "css" };

// Example for CSS modules:
// import sheet, { myClass } from "./styles.scss" assert { type: "css" };

// Example for CSS:
import sheet from "./styles.css" assert { type: "css" };

document.adoptedStyleSheets = [sheet];
shadowRoot.adoptedStyleSheets = [sheet];

For migration purposes, you can use the following configuration:

module.exports = {
  module: {
    rules: [
      {
        test: /\.css$/i,
        oneOf: [
          {
            assert: { type: "css" },
            loader: "css-loader",
            options: {
              exportType: "css-style-sheet",
              // Other options
            },
          },
          {
            use: [
              "style-loader",
              {
                loader: "css-loader",
                options: {
                  // Other options
                },
              },
            ],
          },
        ],
      },
    ],
  },
};

Examples

Recommend

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, because it creates separate css files. For development mode (including webpack-dev-server) you can use style-loader, because it injects CSS into the DOM using multiple <style></style> and works faster.

Note

Do not use style-loader and mini-css-extract-plugin together.

webpack.config.js

const MiniCssExtractPlugin = require("mini-css-extract-plugin");
const devMode = process.env.NODE_ENV !== "production";

module.exports = {
  module: {
    rules: [
      {
        // If you enable `experiments.css` or `experiments.futureDefaults`, please uncomment line below
        // type: "javascript/auto",
        test: /\.(sa|sc|c)ss$/i,
        use: [
          devMode ? "style-loader" : MiniCssExtractPlugin.loader,
          "css-loader",
          "postcss-loader",
          "sass-loader",
        ],
      },
    ],
  },
  plugins: [].concat(devMode ? [] : [new MiniCssExtractPlugin()]),
};

Disable url resolving using the /* webpackIgnore: true */ comment

With the help of the /* webpackIgnore: true */comment, it is possible to disable sources handling for rules and for individual declarations.

/* webpackIgnore: true */
@import url(./basic.css);
@import /* webpackIgnore: true */ url(./imported.css);

.class {
  /* Disabled url handling for the all urls in the 'background' declaration */
  color: red;
  /* webpackIgnore: true */
  background: url("./url/img.png"), url("./url/img.png");
}

.class {
  /* Disabled url handling for the first url in the 'background' declaration */
  color: red;
  background:
    /* webpackIgnore: true */ url("./url/img.png"), url("./url/img.png");
}

.class {
  /* Disabled url handling for the second url in the 'background' declaration */
  color: red;
  background:
    url("./url/img.png"),
    /* webpackIgnore: true */ url("./url/img.png");
}

/* prettier-ignore */
.class {
  /* Disabled url handling for the second url in the 'background' declaration */
  color: red;
  background: url("./url/img.png"),
    /* webpackIgnore: true */
    url("./url/img.png");
}

/* prettier-ignore */
.class {
  /* Disabled url handling for third and sixth urls in the 'background-image' declaration */
  background-image: image-set(
    url(./url/img.png) 2x,
    url(./url/img.png) 3x,
    /* webpackIgnore:  true */ url(./url/img.png) 4x,
    url(./url/img.png) 5x,
    url(./url/img.png) 6x,
    /* webpackIgnore:  true */
    url(./url/img.png) 7x
  );
}

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.

For webpack v5:

webpack.config.js

module.exports = {
  module: {
    rules: [
      {
        test: /\.css$/i,
        use: ["style-loader", "css-loader"],
      },
      {
        test: /\.(png|jpe?g|gif|svg|eot|ttf|woff|woff2)$/i,
        // More information here https://webpack.js.org/guides/asset-modules/
        type: "asset",
      },
    ],
  },
};

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.

• As an alternative, if seeking better development performance and css outputs that mimic production. extract-css-chunks-webpack-plugin offers a hot module reload friendly, extended version of mini-css-extract-plugin. HMR real CSS files in dev, works like mini-css in non-dev

Pure CSS, CSS modules and PostCSS

When you have pure CSS (without CSS modules), CSS modules and PostCSS in your project you can use this setup:

webpack.config.js

module.exports = {
  module: {
    rules: [
      {
        // For pure CSS - /\.css$/i,
        // For Sass/SCSS - /\.((c|sa|sc)ss)$/i,
        // For Less - /\.((c|le)ss)$/i,
        test: /\.((c|sa|sc)ss)$/i,
        use: [
          "style-loader",
          {
            loader: "css-loader",
            options: {
              // Run `postcss-loader` on each CSS `@import` and CSS modules/ICSS imports, do not forget that `sass-loader` compile non CSS `@import`'s into a single file
              // If you need run `sass-loader` and `postcss-loader` on each CSS `@import` please set it to `2`
              importLoaders: 1,
            },
          },
          {
            loader: "postcss-loader",
            options: { plugins: () => [postcssPresetEnv({ stage: 0 })] },
          },
          // Can be `less-loader`
          {
            loader: "sass-loader",
          },
        ],
      },
      // For webpack v5
      {
        test: /\.(png|jpe?g|gif|svg|eot|ttf|woff|woff2)$/i,
        // More information here https://webpack.js.org/guides/asset-modules/
        type: "asset",
      },
    ],
  },
};

Resolve unresolved URLs using an alias

index.css

.class {
  background: url(/assets/unresolved/img.png);
}

webpack.config.js

module.exports = {
  module: {
    rules: [
      {
        test: /\.css$/i,
        use: ["style-loader", "css-loader"],
      },
    ],
  },
  resolve: {
    alias: {
      "/assets/unresolved/img.png": path.resolve(
        __dirname,
        "assets/real-path-to-img/img.png",
      ),
    },
  },
};

Named export with custom export names

webpack.config.js

module.exports = {
  module: {
    rules: [
      {
        test: /\.css$/i,
        loader: "css-loader",
        options: {
          modules: {
            namedExport: true,
            exportLocalsConvention: function (name) {
              return name.replace(/-/g, "_");
            },
          },
        },
      },
    ],
  },
};

Separating Interoperable CSS-only and CSS Module features

The following setup is an example of allowing Interoperable CSS features only (such as :import and :export) without using further CSS Module functionality by setting mode option for all files that do not match *.module.scss naming convention. This is for reference as having ICSS features applied to all files was default css-loader behavior before v4. Meanwhile all files matching *.module.scss are treated as CSS Modules in this example.

An example case is assumed where a project requires canvas drawing variables to be synchronized with CSS - canvas drawing uses the same color (set by color name in JavaScript) as HTML background (set by class name in CSS).

webpack.config.js

module.exports = {
  module: {
    rules: [
      // ...
      // --------
      // SCSS ALL EXCEPT MODULES
      {
        test: /\.scss$/i,
        exclude: /\.module\.scss$/i,
        use: [
          {
            loader: "style-loader",
          },
          {
            loader: "css-loader",
            options: {
              importLoaders: 1,
              modules: {
                mode: "icss",
              },
            },
          },
          {
            loader: "sass-loader",
          },
        ],
      },
      // --------
      // SCSS MODULES
      {
        test: /\.module\.scss$/i,
        use: [
          {
            loader: "style-loader",
          },
          {
            loader: "css-loader",
            options: {
              importLoaders: 1,
              modules: {
                mode: "local",
              },
            },
          },
          {
            loader: "sass-loader",
          },
        ],
      },
      // --------
      // ...
    ],
  },
};

variables.scss

File treated as ICSS-only.

$colorBackground: red;
:export {
  colorBackgroundCanvas: $colorBackground;
}

Component.module.scss

File treated as CSS Module.

@import "variables.scss";
.componentClass {
  background-color: $colorBackground;
}

Component.jsx

Using both CSS Module functionality as well as SCSS variables directly in JavaScript.

import * as svars from "variables.scss";
import * as styles from "Component.module.scss";

// Render DOM with CSS modules class name
// <div className={styles.componentClass}>
//   <canvas ref={mountsCanvas}/>
// </div>

// Somewhere in JavaScript canvas drawing code use the variable directly
// const ctx = mountsCanvas.current.getContext('2d',{alpha: false});
ctx.fillStyle = `${svars.colorBackgroundCanvas}`;

Contributing

Please take a moment to read our contributing guidelines if you haven't yet done so.

CONTRIBUTING

License

MIT

Changelog

7.1.2 (2024-05-22)

Bug Fixes

• keep order of @imports with the webpackIgnore comment (#1600) (76757ef)