postcss-loader

What is postcss-loader?

The postcss-loader npm package is a loader for webpack that allows you to use PostCSS to process CSS with JavaScript. It enables the use of PostCSS plugins to perform various operations on CSS files, such as autoprefixing, minification, and custom transformations.

What are postcss-loader's main functionalities?

Autoprefixing

Automatically adds vendor prefixes to CSS rules using values from Can I Use. It is useful for supporting multiple browser versions.

module.exports = {
  module: {
    rules: [
      {
        test: /\.css$/,
        use: [
          'style-loader',
          'css-loader',
          {
            loader: 'postcss-loader',
            options: {
              postcssOptions: {
                plugins: [
                  require('autoprefixer')
                ]
              }
            }
          }
        ]
      }
    ]
  }
};

CSS Minification

Optimizes and minifies CSS files to reduce file size and improve load times.

module.exports = {
  module: {
    rules: [
      {
        test: /\.css$/,
        use: [
          'style-loader',
          'css-loader',
          {
            loader: 'postcss-loader',
            options: {
              postcssOptions: {
                plugins: [
                  require('cssnano')()
                ]
              }
            }
          }
        ]
      }
    ]
  }
};

Custom Transformations

Applies custom transformations or future CSS features using PostCSS plugins.

module.exports = {
  module: {
    rules: [
      {
        test: /\.css$/,
        use: [
          'style-loader',
          'css-loader',
          {
            loader: 'postcss-loader',
            options: {
              postcssOptions: {
                plugins: [
                  require('postcss-custom-properties')()
                ]
              }
            }
          }
        ]
      }
    ]
  }
};

Other packages similar to postcss-loader

sass-loader

The sass-loader compiles Sass/SCSS files to CSS. It requires Node.js-style .sass/.scss files. Unlike postcss-loader, it's specifically designed for Sass pre-processing.

less-loader

The less-loader processes .less files and compiles them to CSS. It's similar to postcss-loader in that it transforms styles, but it's tailored for the Less pre-processor.

stylus-loader

This package is a webpack loader that compiles Stylus files to CSS. It's a pre-processor loader like sass-loader and less-loader, but for Stylus syntax.

README

PostCSS for Webpack

PostCSS loader for webpack to postprocesses your CSS with PostCSS plugins.

Install

npm install postcss-loader --save-dev

Usage

Add PostCSS Loader to webpack.config.js. Put it after css-loader and style-loader. But before sass-loader, if you use it.

module.exports = {
  module: {
    loaders: [
      {
        test: /\.css$/,
        loaders: [
          'style-loader',
          'css-loader?importLoaders=1',
          'postcss-loader'
        ]
      }
    ]
  }
}

Then create postcss.config.js:

module.exports = {
  plugins: [
    require('postcss-smart-import')({ /* ...options */ }),
    require('precss')({ /* ...options */ }),
    require('autoprefixer')({ /* ...options */ })
  ]
}

You could put different configs in different directories. For example, global config in project/postcss.config.js and override its plugins in project/src/legacy/postcss.config.js.

You can read more about common PostCSS config in postcss-load-config.

Options

Syntaxes

PostCSS can transforms styles in any syntax, not only in CSS. There are 3 parameters to control syntax:

• syntax accepts module name with parse and stringify function.
• parser accepts module name with input parser function.
• stringifier accepts module name with output stringifier function.

module.exports = {
  module: {
    loaders: [
      {
        test:   /\.sss/,
        loaders: [
          'style-loader',
          'css-loader?importLoaders=1',
          'postcss-loader?parser=sugarss'
        ]
      }
    ]
  }
}

SourceMaps

Loader will use source map settings from previous loader.

You can set this sourceMap parameter to inline value to put source maps into CSS annotation comment:

module.exports = {
  module: {
    loaders: [
      {
        test: '\/.css',
        loaders: [
          'style-loader',
          'css-loader?importLoaders=1',
          'postcss-loader?sourceMap=inline'
        ]
      }
    ]
  }
}

Examples

CSS Modules

This loader cannot be used with CSS Modules out of the box due to the way css-loader processes file imports. To make them work properly, either add the css-loader’s importLoaders option

…
      {
        test: /\.css$/,
        loaders: [
          'style-loader',
          'css-loader?modules&importLoaders=1',
          'postcss-loader'
        ]
      }
…

or use postcss-modules plugin instead of css-loader.

JS Styles

If you want to process styles written in JavaScript you can use the postcss-js parser.

…
      {
        test: /\.style.js$/,
        loaders: [
          'style-loader',
          'css-loader?modules&importLoaders=1',
          'postcss-loader?parser=postcss-js',
          'babel'
        ]
      }
…

As result you will be able to write styles as:

import colors from './config/colors'

export default {
  '.menu': {
    color: colors.main,
    height: 25,
    '&_link': {
      color: 'white'
    }
  }
}

If you use JS styles without postcss-js parser, you can add exec parameter:

…
      {
        test:   /\.style.xyz$/,
        loaders: [
          'style-loader',
          'css-loader?modules&importLoaders=1',
          'postcss-loader?parser=custom-parser&exec'
        ]
      }
…

Dynamic Config

PostCSS loader sends a loaded instance to PostCSS common config. You can use it to do some real magic:

module.exports = function (ctx) {
    if (check(ctx.webpack.resourcePath)) {
        return plugins1;
    } else {
        return plugins2;
    }
}

Webpack Events

Webpack provides webpack plugin developers a convenient way to hook into the build pipeline. The postcss-loader makes use of this event system to allow building integrated postcss-webpack tools.

See the example implementation.

Event postcss-loader-before-processing is fired before processing and allows to add or remove postcss plugins.

Changelog

1.1.1

• Fix this in options function (by Jeff Escalante).

1.1

• PostCSS common config could be placed to subdirs.
• Add webpack instance to PostCSS common config context.

1.0

• Add common PostCSS config support (by Mateusz Derks).
• Add Webpack 2 support with plugins query option (by Izaak Schroeder).
• Add dependency message support.
• Rewrite docs (by Michael Ciniawsky).