storybook/code/lib/core-webpack/src/load-custom-webpack-config.ts

import path from 'path';
import { serverRequire } from '@storybook/core-common';

const webpackConfigs = ['webpack.config', 'webpackfile'];

export const loadCustomWebpackConfig = (configDir: string) =>
  serverRequire(webpackConfigs.map((configName) => path.resolve(configDir, configName)));