Next.js + Turbopack + PostCSS example

next.config.ts

// next.config.mjs
import type { NextConfig } from 'next'

function getBabelLoader() {
  return {
    loader: 'babel-loader',
    options: {
      presets: [
        [
          'next/dist/compiled/babel/preset-typescript',
          {
            allowNamespaces: true,
          },
        ],
        [
          'react-strict-dom/babel-preset',
          {
            debug: true,
            dev: process.env.NODE_ENV === 'development',
            rootDir: process.cwd(),
          },
        ],
      ],
    },
  }
}

const nextConfig: NextConfig = {
  transpilePackages: ['react-strict-dom'],
  webpack: (config) => {
    config.module.rules.push({
      test: /\.(js|jsx|ts|tsx)$/,
      use: [getBabelLoader()],
    })
    return config
  },
  experimental: {
    turbo: {
      rules: {
        '*.{js,jsx,ts,tsx}': {
          loaders: [getBabelLoader()],
        },
      },
    },
  },
}

export default nextConfig

postcss.config.mjs

export default {
  plugins: {
    'postcss-react-strict-dom': {
      include: ['src/**/*.{js,jsx,ts,tsx}'],
      babelConfig: {
        babelrc: false,
        presets: [
          [
            'next/dist/compiled/babel/preset-typescript',
            {
              allowNamespaces: true,
            },
          ],
          [
            'react-strict-dom/babel-preset',
            {
              debug: true,
              dev: process.env.NODE_ENV === 'development',
              rootDir: process.cwd(),
            },
          ],
        ],
      },
    },
    autoprefixer: {},
  },
}

@MoOx

I am curious to understand why you need to pass babelconfig to postcss plugin.

StyleX Babel Plugin essentially does two things:

• It transforms your source code and replaces your style declarations to a set of minified class names
• It collects styles used in your application for plugins. Plugins will generate CSS using information collected by StyleX Babel Plugin.

PostCSS Plugin runs StyleX Babel Plugin and uses the extracted style information to generate CSS and replaces your placeholder @stylex directive. However, PostCSS is a CSS transformer and cannot affect your source code transformation.
Therefore, you must run StyleX Babel Plugin again in babel.config.js to transform your source code.

PostCSS and babel run by application run independently from each other, which is why you need to provide babel config for both.
More details in this comment:
facebook/stylex#842 (comment)

Also, why do you need to use "next compiled babel preset ts" ? Thanks !

Unlike babel-loader in webpack, which runs other nextjs preset transformations automatically, when using turbo.rules, files that match the rule do not run anything else other than the loader config you specify. Therefore, you need to run the Next.js default preset as well manually, which does framework transformations like converting RSC, Server Actions, use cache handling etc.

You can learn more about it from this article:
https://holocron.so/blog/migrate-next.js-plugins-to-turbopack-with-this-weird-little-trick

Thank you so much for all the explanations ! Some of this should be in RSD doc for sure !