index.js

module.exports = (pluginOptions = {}) => (nextConfig = {}) => {
  const {
    extension = /\.mdx$/,
    MDXOptions = {},
  } = pluginOptions

  const MDXLoader = {
    loader: '@mdx-js/loader',
    options: MDXOptions,
  }
  const frontmatterLoader = {
    loader: require.resolve('./loader'),
  }

  return Object.assign({}, nextConfig, {
    webpack(config, options) {
      if (!options.defaultLoaders) {
        throw new Error(
          'This plugin is not compatible with Next.js versions below 5.0.0 https://err.sh/next-plugins/upgrade'
        )
      }

      config.module.rules.push({
        test: extension,
        use: [
          options.defaultLoaders.babel,
          MDXLoader,
          frontmatterLoader,
        ]
      })

      if (typeof nextConfig.webpack === 'function') {
        return nextConfig.webpack(config, options)
      }

      return config
    }
  })
}