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PostCSS Plugin Guidelines

A PostCSS plugin is a function that receives and, usually, transforms a CSS AST from the PostCSS parser.

The rules below are mandatory for all PostCSS plugins.

See also ClojureWerkz’s recommendations for open source projects.

1. API

1.1 Clear name with postcss- prefix

The plugin’s purpose should be clear just by reading its name. If you wrote a transpiler for CSS 4 Custom Media, postcss-custom-media would be a good name. If you wrote a plugin to support mixins, postcss-mixins would be a good name.

The prefix postcss- shows that the plugin is part of the PostCSS ecosystem.

This rule is not mandatory for plugins that can run as independent tools, without the user necessarily knowing that it is powered by PostCSS — for example, RTLCSS and Autoprefixer.

1.2. Do one thing, and do it well

Do not create multitool plugins. Several small, one-purpose plugins bundled into a plugin pack is usually a better solution.

For example, postcss-preset-env contains many small plugins, one for each W3C specification. And cssnano contains a separate plugin for each of its optimization.

1.3. Do not use mixins

Preprocessors libraries like Compass provide an API with mixins.

PostCSS plugins are different. A plugin cannot be just a set of mixins for postcss-mixins.

To achieve your goal, consider transforming valid CSS or using custom at-rules and custom properties.

1.4. Keep postcss to peerDependencies

AST can be broken because of different postcss version in different plugins. Different plugins could use a different node creators (like postcss.decl()).

{
  "peerDependencies": {
    "postcss": "^8.0.0"
  }
}

It is better even not to import postcss.

- const { list, decl } = require('postcss')
  module.exports = opts => {
    postcssPlugin: 'postcss-name',
-   Once (root) {
+   Once (root, { list, decl }) {
      // Plugin code
    }
  }
  module.exports.postcss = true

1.5. Set plugin.postcssPlugin with plugin name

Plugin name will be used in error messages and warnings.

module.exports = opts => {
  return {
    postcssPlugin: 'postcss-name',
    Once (root) {
      // Plugin code
    }
  }
}
module.exports.postcss = true

2. Processing

2.1. Plugin must be tested

A CI service like Travis is also recommended for testing code in different environments. You should test in (at least) Node.js active LTS and current stable version.

2.2. Use asynchronous methods whenever possible

For example, use fs.writeFile instead of fs.writeFileSync:

let { readFile } = require('fs').promises

module.exports = opts => {
  return {
    postcssPlugin: 'plugin-inline',
    async Decl (decl) {
      const imagePath = findImage(decl)
      if (imagePath) {
        let imageFile = await readFile(imagePath)
        decl.value = replaceUrl(decl.value, imageFile)
      }
    }
  }
}
module.exports.postcss = true

2.3. Use fast node’s scanning

Subscribing for specific node type is much faster, than calling walk* method:

  module.exports = {
    postcssPlugin: 'postcss-example',
-   Once (root) {
-     root.walkDecls(decl => {
-       // Slow
-     })
-   }
+   Declaration (decl) {
+     // Faster
+   }
  }
  module.exports.postcss = true

But you can make scanning even faster, if you know, what declaration’s property or at-rule’s name do you need:

  module.exports = {
    postcssPlugin: 'postcss-example',
-   Declaration (decl) {
-     if (decl.prop === 'color') {
-       // Faster
-     }
-   }
+   Declaration: {
+     color: decl => {
+       // The fastest
+     }
+   }
  }
  module.exports.postcss = true

2.4. Set node.source for new nodes

Every node must have a relevant source so PostCSS can generate an accurate source map.

So if you add a new declaration based on some existing declaration, you should clone the existing declaration in order to save that original source.

if (needPrefix(decl.prop)) {
  decl.cloneBefore({ prop: '-webkit-' + decl.prop })
}

You can also set source directly, copying from some existing node:

if (decl.prop === 'animation') {
  const keyframe = createAnimationByName(decl.value)
  keyframes.source = decl.source
  decl.root().append(keyframes)
}

2.5. Use only the public PostCSS API

PostCSS plugins must not rely on undocumented properties or methods, which may be subject to change in any minor release. The public API is described in API docs.

3. Errors

3.1. Use node.error on CSS relevant errors

If you have an error because of input CSS (like an unknown name in a mixin plugin) you should use node.error to create an error that includes source position:

if (typeof mixins[name] === 'undefined') {
  throw node.error('Unknown mixin ' + name)
}

3.2. Use result.warn for warnings

Do not print warnings with console.log or console.warn, because some PostCSS runner may not allow console output.

Declaration (decl, { result }) {
  if (outdated(decl.prop)) {
    result.warn(decl.prop + ' is outdated', { node: decl })
  }
}

If CSS input is a source of the warning, the plugin must set the node option.

4. Documentation

4.1. Document your plugin in English

PostCSS plugins must have their README.md wrote in English. Do not be afraid of your English skills, as the open source community will fix your errors.

Of course, you are welcome to write documentation in other languages; just name them appropriately (e.g. README.ja.md).

4.2. Include input and output examples

The plugin's README.md must contain example input and output CSS. A clear example is the best way to describe how your plugin works.

The first section of the README.md is a good place to put examples. See postcss-opacity for an example.

Of course, this guideline does not apply if your plugin does not transform the CSS.

4.3. Maintain a changelog

PostCSS plugins must describe the changes of all their releases in a separate file, such as CHANGELOG.md, History.md, or GitHub Releases. Visit Keep A Changelog for more information about how to write one of these.

Of course, you should be using SemVer.

4.4. Include postcss-plugin keyword in package.json

PostCSS plugins written for npm must have the postcss-plugin keyword in their package.json. This special keyword will be useful for feedback about the PostCSS ecosystem.

For packages not published to npm, this is not mandatory, but is recommended if the package format can contain keywords.

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