Share css-modules compiled class names over pre-processers
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npm Travis

Share css-modules compiled class names over pre-processers!


npm i premodules-loader --save-dev

What's the problem

css-modules is a great workaround to generate scoped css. However since scoped css are tightly bound with js files that import them, when you are trying to write a reusable component, you're possibly getting a headache - how can other developers override the default styles in my components?


What if we can import class name maps into sass/less/stylus code?

How it works

As is explained in this article, this loader executed twice in the webpack workflow by the following order:

  1. Create directives:

  2. Use some directives to interpolate class names into source code - surely it should be converted to sass/less/stylus hashes.

  3. Use variable interpolation syntax to define class name, e.g. ${map-get($comp, 'main')} in sass.

  4. Process css-modules output:

  5. Restore twice-transformed class names to the previous one (both in stylesheet and class map).

  6. Save the class map for interpolation.



Webpack config (execute twice):

// webpack.config.js
module.exports = {
  module: {
    loaders: [{
      test: /\.scss/,
      loaders: [
        'premodules?restore', // restore here!
        'premodules?parse', // parse here!
  premodules: {
    transformer(varName, hash) {
      // custom code transformer

Components define:

// button.js
import React from 'react';
import styles from './button.scss';

const Button = () => (
  <button className={style.button}>click me</button>

export default Button;
// button.scss
.button {
  border-radius: 3px;

Your react app:

// app.js
import React from 'react';
import './app.scss';

const App = () => (
  <Button />
// app.scss
@module './button.scss' => $comp;

#{map-get($comp, 'button')} {
  background: #123;

Directive Syntax

// import node_modules
@module 'module_name/dir/file' => $var;

// or use relative/absolute path
@module '/path/of/module' => $var;

Customize Transformer

Default transform is designed for scss code transformation, write your own transformer to support your pre-processor.

Transformer function receives two parameters:

  1. varName:string: variable name defined in directive
  2. hash:object: css-modules exported class name map

A typical transformer may like this:

// scss transformer
// `@module './comp.scss' => $comp` -> `$comp: ( key: value, ... )`

const transformer = (varName, hash) => {
  if (!hash) return '';

  const main = Object.keys(hash).map(key => `${key}: '.${hash[key]}'`).join(',');

  return `${varName}: (${main});`;


  1. Must import all stylesheets you need into JavaScript code, otherwise it won't be passed through webpack.

  2. Importing files that declared with @module directives in your sass/less/stylus codes may cause breaks (pre-processors won't understand this directive).