Edit File: plugin.md
# 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. [ClojureWerkz’s recommendations]: http://blog.clojurewerkz.org/blog/2013/04/20/how-to-make-your-open-source-project-really-awesome/ ## 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, [cssnext] and [Autoprefixer]. [Autoprefixer]: https://github.com/postcss/autoprefixer [cssnext]: http://cssnext.io/ ### 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, [cssnext] contains many small plugins, one for each W3C specification. And [cssnano] contains a separate plugin for each of its optimization. [cssnext]: http://cssnext.io/ [cssnano]: https://github.com/ben-eb/cssnano ### 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. [postcss-mixins]: https://github.com/postcss/postcss-mixins ### 1.4. Create plugin by `postcss.plugin` By wrapping your function in this method, you are hooking into a common plugin API: ```js module.exports = postcss.plugin('plugin-name', function (opts) { return function (root, result) { // Plugin code }; }); ``` ## 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](https://github.com/nodejs/LTS) and current stable version. [Travis]: https://travis-ci.org/ ### 2.2. Use asynchronous methods whenever possible For example, use `fs.writeFile` instead of `fs.writeFileSync`: ```js postcss.plugin('plugin-sprite', function (opts) { return function (root, result) { return new Promise(function (resolve, reject) { var sprite = makeSprite(); fs.writeFile(opts.file, function (err) { if ( err ) return reject(err); resolve(); }) }); }; }); ``` ### 2.3. 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 new declaration based on some existing declaration, you should clone the existing declaration in order to save that original `source`. ```js if ( needPrefix(decl.prop) ) { decl.cloneBefore({ prop: '-webkit-' + decl.prop }); } ``` You can also set `source` directly, copying from some existing node: ```js if ( decl.prop === 'animation' ) { var keyframe = createAnimationByName(decl.value); keyframes.source = decl.source; decl.root().append(keyframes); } ``` ### 2.4. 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]. [API docs]: http://api.postcss.org/ ## 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: ```js if ( typeof mixins[name] === 'undefined' ) { throw decl.error('Unknown mixin ' + name, { plugin: 'postcss-mixins' }); } ``` ### 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. ```js 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` written 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](https://github.com/iamvdo/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]. [Keep A Changelog]: http://keepachangelog.com/ [GitHub Releases]: https://help.github.com/articles/creating-releases/ [SemVer]: http://semver.org/ ### 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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