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

const path = require('path');
const webpack = require('webpack');

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',
  });

  defaultConfig.plugins.push(
    new webpack.optimize.CommonsChunkPlugin({
      name: 'vendor',
      chunks: ['preview'],
      minChunks(module) {
        // this assumes your vendor imports exist in the node_modules directory
        return module.context && module.context.indexOf('node_modules') !== -1;
      },
    })
  );

  return defaultConfig;
};