postcss-modules

What is postcss-modules?

The postcss-modules package is a PostCSS plugin that enables CSS Modules, a technique for locally scoping CSS by automatically creating a unique classname of the format [filename]__[local classname]__[hash]. This helps in avoiding global scope pollution in CSS and makes component-based development easier and more maintainable.

What are postcss-modules's main functionalities?

Local Scope for CSS Classes

This feature automatically scopes class names locally rather than globally. This is done by appending a unique hash to each class name, which makes them unique across the project. This prevents styles from leaking into other parts of the application unintentionally.

/* Input CSS */
.className {
  color: red;
}

/* Output CSS */
.fileName__className__hash {
  color: red;
}

Composition of Styles

This feature allows CSS classes to compose from other classes, potentially from different files. This promotes reusability of CSS styles and helps in maintaining a cleaner and more organized stylesheet architecture.

/* Input CSS */
.className {
  composes: anotherClass from './anotherStyle.css';
  color: red;
}

/* Output CSS */
.fileName__className__hash {
  color: red;
}
.fileName__anotherClass__hash {
  /* styles from anotherClass */
}

Other packages similar to postcss-modules

css-loader

css-loader is a webpack loader that interprets @import and url() like import/require() and will resolve them. It also offers modules functionality similar to postcss-modules, where it localizes CSS by default using similar scoping rules. Compared to postcss-modules, css-loader is tightly integrated with webpack's module system, which might be preferable in webpack-based projects.

styled-components

styled-components is a library for React and React Native that allows CSS to be written inside JavaScript files by tagging template literals. This approach scopes styles by generating unique class names at runtime. Unlike postcss-modules, styled-components embeds styles directly into components, which can enhance runtime performance and offers enhanced theming capabilities.

README

postcss-modules

A PostCSS plugin to use CSS Modules everywhere. Not only at the client side.

What is this? For example, you have the following CSS:

/* styles.css */
:global .page {
  padding: 20px;
}

.title {
  composes: title from "./mixins.css";
  color: green;
}

.article {
  font-size: 16px;
}

/* mixins.css */
.title {
  color: black;
  font-size: 40px;
}

.title:hover {
  color: red;
}

After the transformation it will become like this:

._title_116zl_1 {
  color: black;
  font-size: 40px;
}

._title_116zl_1:hover {
  color: red;
}

.page {
  padding: 20px;
}

._title_xkpkl_5 {
  color: green;
}

._article_xkpkl_10 {
  font-size: 16px;
}

And the plugin will give you a JSON object for transformed classes:

{
  "title": "_title_xkpkl_5 _title_116zl_1",
  "article": "_article_xkpkl_10"
}

Usage

Saving exported classes

By default, a JSON file with exported classes will be placed next to corresponding CSS. But you have a freedom to make everything you want with exported classes, just use the getJSON callback. For example, save data about classes into a corresponding JSON file:

postcss([
  require("postcss-modules")({
    getJSON: function (cssFileName, json, outputFileName) {
      var path = require("path");
      var cssName = path.basename(cssFileName, ".css");
      var jsonFileName = path.resolve("./build/" + cssName + ".json");
      fs.writeFileSync(jsonFileName, JSON.stringify(json));
    },
  }),
]);

getJSON may also return a Promise.

Generating scoped names

By default, the plugin assumes that all the classes are local. You can change this behaviour using the scopeBehaviour option:

postcss([
  require("postcss-modules")({
    scopeBehaviour: "global", // can be 'global' or 'local',
  }),
]);

To define paths for global modules, use the globalModulePaths option. It is an array with regular expressions defining the paths:

postcss([
  require('postcss-modules')({
    globalModulePaths: [/path\/to\/legacy-styles/, /another\/paths/],
  });
]);

To generate custom classes, use the generateScopedName callback:

postcss([
  require("postcss-modules")({
    generateScopedName: function (name, filename, css) {
      var path = require("path");
      var i = css.indexOf("." + name);
      var line = css.substr(0, i).split(/[\r\n]/).length;
      var file = path.basename(filename, ".css");

      return "_" + file + "_" + line + "_" + name;
    },
  }),
]);

Or just pass an interpolated string to the generateScopedName option (More details here):

postcss([
  require("postcss-modules")({
    generateScopedName: "[name]__[local]___[hash:base64:5]",
  }),
]);

It's possible to add custom hash to generate more unique classes using the hashPrefix option (like in css-loader):

postcss([
  require("postcss-modules")({
    generateScopedName: "[name]__[local]___[hash:base64:5]",
    hashPrefix: "prefix",
  }),
]);

Exporting globals

If you need to export global names via the JSON object along with the local ones, add the exportGlobals option:

postcss([
  require("postcss-modules")({
    exportGlobals: true,
  }),
]);

Loading source files

If you need, you can pass a custom loader (see FileSystemLoader for example):

postcss([
  require("postcss-modules")({
    Loader: CustomLoader,
  }),
]);

You can also pass any needed root path:

postcss([
  require('postcss-modules')({
    root: 'C:\\',
  });
]);

localsConvention

Type: String | (originalClassName: string, generatedClassName: string, inputFile: string) => className: string Default: null

Style of exported classnames, the keys in your json.

Name	Type	Description
'camelCase'	{String}	Class names will be camelized, the original class name will not to be removed from the locals
'camelCaseOnly'	{String}	Class names will be camelized, the original class name will be removed from the locals
'dashes'	{String}	Only dashes in class names will be camelized
'dashesOnly'	{String}	Dashes in class names will be camelized, the original class name will be removed from the locals

In lieu of a string, a custom function can generate the exported class names.

Integration with templates

The plugin only transforms CSS classes to CSS modules. But you probably want to integrate these CSS modules with your templates. Here are some examples.

Assume you've saved project's CSS modules in cssModules.json:

{
  "title": "_title_xkpkl_5 _title_116zl_1",
  "article": "_article_xkpkl_10"
}

Pug (ex-Jade)

Let's say you have a Pug template about.jade:

h1(class=css.title) postcss-modules
article(class=css.article) A PostCSS plugin to use CSS Modules everywhere

Render it:

var jade = require("jade");
var cssModules = require("./cssModules.json");
var html = jade.compileFile("about.jade")({ css: cssModules });
console.log(html);

And you'll get the following HTML:

<h1 class="_title_xkpkl_5 _title_116zl_1">postcss-modules</h1>
<article class="_article_xkpkl_10">
  A PostCSS plugin to use CSS Modules everywhere
</article>

HTML

For HTML transformation we'll use PostHTML and PostHTML CSS Modules:

npm install --save posthtml posthtml-css-modules

Here is our template about.html:

<h1 css-module="title">postcss-modules</h1>
<article css-module="article">
  A PostCSS plugin to use CSS Modules everywhere
</article>

Transform it:

var fs = require("fs");
var posthtml = require("posthtml");
var posthtmlCssModules = require("posthtml-css-modules");
var template = fs.readFileSync("./about.html", "utf8");

posthtml([posthtmlCssModules("./cssModules.json")])
  .process(template)
  .then(function (result) {
    console.log(result.html);
  });

(for using other build systems please check the documentation of PostHTML)

And you'll get the following HTML:

<h1 class="_title_xkpkl_5 _title_116zl_1">postcss-modules</h1>
<article class="_article_xkpkl_10">
  A PostCSS plugin to use CSS Modules everywhere
</article>

May I see the plugin in action?

Sure! Take a look at the example.

See PostCSS docs for examples for your environment and don't forget to run

npm install --save-dev postcss postcss-modules

Changelog

4.1.3

• Fixed package contents