storybook/examples/mithril-kitchen-sink/.storybook/webpack.config.js

const path = require('path');

module.exports = (storybookBaseConfig, configType, defaultConfig) => {
  defaultConfig.module.rules.push({
    test: [/\.stories\.js$/],
    loaders: [require.resolve('@storybook/addon-storysource/loader')],
    include: [path.resolve(__dirname, '../src')],
    enforce: 'pre',
  });

  return defaultConfig;
};