webpack.babel.js

module: {
  rules: [
    // ...
      // Rules for Style Sheets
      {
        test: /\.(scss|sass)$/,
        include: [path.resolve(__dirname, 'src'), path.resolve(__dirname, 'assets/scss')],
        use: extractSass.extract({
          use: [
            // Process internal/project styles (from assets/scss folder)
            {
              loader: 'css-loader',
              options: {
                // CSS Loader https://github.com/webpack/css-loader
                importLoaders: 1,
                sourceMap: isDebug,
                // CSS Modules https://github.com/css-modules/css-modules
                modules: true,
                localIdentName: isDebug
                  ? '[name]-[local]-[hash:base64:5]'
                  : '[hash:base64:5]',
                // CSS Nano http://cssnano.co/
                minimize: isDebug ? false : minimizeCssOptions,
              },
            },
            { loader: 'postcss-loader', options: { sourceMap: isDebug } },
            { loader: 'sass-loader', options: { sourceMap: isDebug } },
          ],
          fallback: 'style-loader',
        }),
      },
   ]
}