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css-loader
Advanced tools
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.
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'] }] } };
Injects CSS into the DOM using multiple <style> tags. It is often used in conjunction with css-loader.
Loads and compiles Sass/SCSS files. It extends css-loader's functionality to work with Sass/SCSS syntax.
Processes Less files and compiles them to CSS. Similar to sass-loader, it's specific to Less preprocessor language.
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.
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.
npm install css-loader --save-dev
var css = require("css!./file.css");
// => returns css code from file.css, resolves imports and url(...)
@import
and url(...)
are interpreted like require()
and will be resolved by the css-loader.
Good loaders for requiring your assets are the file-loader
and the url-loader which you should specify in your config (see below).
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")
This webpack config can load css files, embed small png images as Data Urls and jpg images as files.
module.exports = {
module: {
loaders: [
{ test: /\.css$/, loader: "style-loader!css-loader" },
{ test: /\.png$/, loader: "url-loader?limit=100000" },
{ test: /\.jpg$/, loader: "file-loader" }
]
}
};
For urls that start with a /
, the default behavior is to not translate them:
url(/image.png)
=> url(/image.png)
If a root
query parameter is set, however, it will be prepended to the url
and then translated:
With a config like:
loaders: [
{ test: /\.css$/, loader: "style-loader!css-loader?root=." },
...
]
The result is:
url(/image.png)
=> require("./image.png")
Using 'Root-relative' urls is not recommended. You should only use it for legacy CSS files.
By default CSS exports all class names into a global selector scope. This is a feature which offer a local selector scope.
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 choosen unique identifiers are exported by the module.
Example:
:local(.className) { background: red; }
:local .className { color: green; }
:local(.className .subClass) { color: green; }
:local .className .subClass :global(.global-class-name) { color: blue; }
is transformed to
._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; }
and the identifiers are exported:
exports.locals = {
className: "_23_aKvs-b8bW2Vg3fwHozO",
subClass: "_13LGdX8RMStbBE9w-t0gZ1"
}
Camelcasing is recommended for local selectors. They are easier to use in the importing javascript module.
url(...)
URLs in block scoped (:local .abc
) rules behave like requests in modules:
./file.png
instead of file.png
module/file.png
instead of ~module/file.png
You can use :local(#someId)
, but this is not recommended. Use classes instead of ids.
You can configure the generated ident with the localIdentName
query parameter (default [hash:base64]
). Example: css-loader?localIdentName=[path][name]---[local]---[hash:base64:5]
for easier debugging.
Note: 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.
See CSS Modules.
The query parameter modules
enables the CSS Modules spec. (css-loader?modules
)
This enables Local scoped CSS by default. (You can switch it off with :global(...)
or :global
for selectors and/or rules.)
When declaring a local class name you can compose a local class from another local class name.
: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 class names:
exports.locals = {
className: "_23_aKvs-b8bW2Vg3fwHozO",
subClass: "_13LGdX8RMStbBE9w-t0gZ1 _23_aKvs-b8bW2Vg3fwHozO"
}
and CSS is transformed to:
._23_aKvs-b8bW2Vg3fwHozO {
background: red;
color: yellow;
}
._13LGdX8RMStbBE9w-t0gZ1 {
background: blue;
}
To import a local class name 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;
}
To include SourceMaps set the sourceMap
query param.
require("css-loader?sourceMap!./file.css")
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 SourceMap do not). In addition to that relative paths are buggy and you need to use an absolute public path which include the server url.
The query parameter importLoaders
allow to configure which loaders should be applied to @import
ed resources.
importLoaders
(int): That many loaders after the css-loader are used to import resources.
Examples:
require("style-loader!css-loader?importLoaders=1!autoprefixer-loader!...")
// => imported resources are handled this way:
require("css-loader?importLoaders=1!autoprefixer-loader!...")
require("style-loader!css-loader!stylus-loader!...")
// => imported resources are handled this way:
require("css-loader!...")
This may change in the future, when the module system (i. e. webpack) supports loader matching by origin.
By default the css-loader minimizes the css if specified by the module system.
In some cases the minification is destructive to the css, so you can provide some options to it. clean-css is used for minification and you find a list of options here. Just provide them as query parameter: i. e. require("css-loader?-restructuring&compatibility")
to disable restructuring and enable compatibility mode.
You can also disable or enforce minification with the minimize
query parameter.
require("css-loader?minimize!./file.css")
(enforced)
require("css-loader?-minimize!./file.css")
(disabled)
FAQs
css loader module for webpack
The npm package css-loader receives a total of 18,417,627 weekly downloads. As such, css-loader popularity was classified as popular.
We found that css-loader demonstrated a healthy version release cadence and project activity because the last version was released less than a year ago. It has 3 open source maintainers collaborating on the project.
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