This preprocessor converts css files in modules like Webpack. If we have css files

.class1, .class2, .class3 {
    display: block;
}

Webpack will transfrom it to object.

{
    class1: 'class1', //value may be different. It depends of localIndentName property
    class2: 'class2',
    class3: 'class3',
}

In testing you need to mock all css modules to aviod exception. But use pure object {} for mocking is bad idea, because it makes wrong classNames in components. This preprocessor makes correct modules as if Webpack did.

In Jest config add option

"transform": {
  ".+\\.(css|styl|less|sass|scss)$": "jest-css-modules-transform"
},

or

"transform": {
  ".+\\.(css|styl|less|sass|scss)$": "<rootDir>/node_modules/jest-css-modules-transform"
},

It supports pure CSS, SCSS, SASS, STYLUS and LESS.

For .css, .pcss, .postcss files used postcss.config.js from root folder(if file exist of course).
For tests you can rewrite your existed postcss.config.js. See options.

You can save preproccessor options in file jest-css-modules-transform-config.js in root of your project(Where is the file package.json). You can pass options for your preprocessors.

const path = require('path');  
const additionalResolvePath = path.resolve(__dirname, 'src', 'additional_modules');

module.exports = {
    sassConfig: {
        includePaths: [additionalResolvePath],
        precision: 5,
    },
    lessConfig: {
        paths: [additionalResolvePath],
    },
    stylusConfig: {
        paths: [additionalResolvePath],
    },
    postcssConfig: {
        plugins: [
            require('autoprefixer')({
                browsers: ['Chrome 68', 'Firefox 62', 'Safari 12']
            })
        ]
    },
};

For all preprocessor options see offical documentations for Sass, Less, Stylus.

Webpack's cssLoader has option exportLocalsStyle.

// file jest-css-modules-transform-config.js
module.exports = {
   cssLoaderConfig: {
        exportLocalsStyle: 'camelCase'
   }
};

then this css code

    .foo__bar{color: red;}
    .foo__bar2{color: red;}

converted to

    {
        foo__bar: 'foo__bar',
        fooBar: 'foo__bar',
        foo__bar2: 'foo__bar2',
        fooBar: 'foo__bar2',
    }

Available values camelCase, camelCaseOnly, dashes, dashesOnly, asIs(by default)

Type: string, Function, Array<string | Function>
Default: null

Pass urls for prepend(before file content) to transformed files.
Useful in a situation where the module uses variables or mixins without explicit import.

For example you have vars in file, but without implicit import. You can prepend file with sass variables before convert module.

// file jest-css-modules-transform-config.js
module.exports = {
   prepend: [
       'some/url/vars.scss',
       'some/url/theme.scss',
   ],
};

// file jest-css-modules-transform-config.js
module.exports = {
   prepend: 'some/url/vars.scss',
};

// file jest-css-modules-transform-config.js
module.exports = {
   prepend: (filepath) => {
       if (filepath.includes('components')) {
           return [
               'components/vars.scss',
               'components2/vars.scss',
           ];
       }

       return [
            'common/vars.scss',
            'common/vars2.scss',
       ];
   },
};

It works with any preprocessors(Sass, Less, Styles, PostCSS, pure CSS)

By default option plugin's config file is jest-css-modules-transform-config.js from root. For specify custom path just add env JEST_CSS_MODULES_TRANSFORM_CONFIG. For example:

JEST_CSS_MODULES_TRANSFORM_CONFIG=custom-config-path npm test

or

JEST_CSS_MODULES_TRANSFORM_CONFIG=custom-config-path yarn test

npm i jest-css-modules-transform