storybook/addons/docs/common/makePreset.js

const fs = require('fs');
const common = require('./preset');

module.exports = framework => {
  const frameworkConfig = `${__dirname}/../${framework}/config.js`;
  const preconfig = fs.existsSync(frameworkConfig) ? [frameworkConfig] : [];

  function config(entry = []) {
    return [...preconfig, ...entry];
  }

  return {
    ...common,
    config,
  };
};