npm install css-loader --save-dev

Documentation: Using loaders

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 @imported 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. cssnano is used for minification and you find a list of options here. Just provide them as query parameter: i. e. require("css-loader?-autoprefixer") to disable removing of deprecated vendor prefixes.

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)

css-loader?-url disables url(...) handling.

css-loader?-import disables @import handling.

MIT (https://www.opensource.org/licenses/mit-license.php)