Last updated a year ago by darajava .
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klass-loader is an improvement to CSS workflow and encapsulation in React. It enforces an opinionated, yet simple and non-obtrusive structure of presentation components and corresponding stylesheets. klass-loader utilises css-modules to scope style rules and gives warnings when attempting to use a non-existent rule.


Once the loader is correctly installed, there is no need to import any libraries or reference CSS files. klass attributes are evaluated as vanilla CSS selector strings, similar to the class tag in HTML. This attribute can be treated as a language feature in the same way class was pre modern Javascript. Usage is as follows:


import React from 'react';

let variableName = 'child';

const Component = (props) => {
  return (
    <div klass='component'>
      <div klass={variableName}>

export default Component;


.component {
  padding: 10px;

.child {
  background-color: red; 

This minimal setup will apply classes in a corresponding file called styles.css in the same directory as the component. (LESS might work too, but it's untested so far). An example hierarchy is shown here:

└── App.js
    ├── NoStyleButton/
    │   └── NoStyleButton.js
    ├── Progress/
    │   ├── Progress.js
    │   └── styles.css
    └── Question/
        ├── Question.js
        ├── Answer.js
        └── styles.css

A component requires a corresponding stylesheet in the same directory only if the component declares a klass attribute as shown above. If there are multiple components in the same folder, they will all be based on the same stylesheet.

Using global styles

In the styles.css file, the webpack implementation of css-modules allows us compose selectors, import selectors from other stylesheets, and more! Further info on their README. Alternatively, className can still be used alongside klass to reference global CSS files.

Install & Configuration


npm i --save klass-loader


Set up and install style-loader and css-loader for your CSS files (and less-loader too if you use less):

  loader: require.resolve('css-loader'),
  options: {
    importLoaders: 1,
    modules: true,
    localIdentName: "[name]__[local]___[hash:base64:5]",  
}, // TODO: Update example to use newer webpack syntax

Make sure to set the localIdentName to something like the above to enable encapsulation. More details on how to install can be found on their README.

Next, set up klass-loader itself. To do this, place the loader just before babel-loader in the webpack config:

  test: /\.(js|jsx|mjs)$/,
  include: paths.appSrc,
  use: [
    // other loaders
      loader: require.resolve('klass-loader'),
      loader: require.resolve('babel-loader'),
      options: {
        // babel options

Example repo

An example of a project using klass-loader can be found here.

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