Modularize next config

lib__next-extendable-config__index.mjs

import fs from 'fs';
import path from 'path';

const loadNextConfigs = async (rootFolder) => {
  return (
    await Promise.all(
      fs
        .readdirSync(rootFolder, { withFileTypes: true })
        .filter((dirent) => dirent.isDirectory())
        .map(async ({ name }) => {
          const nextConfigPath = path.join(rootFolder, name, 'next.config.mjs');
          if (!fs.existsSync(nextConfigPath)) {
            return undefined;
          }
          console.info('->', nextConfigPath);
          const { default: withConfig } = await import(
            `../../${nextConfigPath}`
          );
          return withConfig;
        })
    )
  ).filter(Boolean);
};

const compose =
  (...fns) =>
  (arg) =>
    fns.reduceRight((res, fn) => fn(res), arg);

const withConfig = async (
  /** @type {import('next').NextConfig} */
  nextConfig,
  { folders = ['./lib', './app'] } = {}
) => {
  console.info('Loading next.config.mjs from', folders.join(', '));
  const configFunctions = (
    await Promise.all(folders.map(loadNextConfigs))
  ).flat();

  return compose(...configFunctions)(nextConfig);
};

export default withConfig;

lib__upload__next.config.mjs

const withUpload = (
  /** @type {import('next').NextConfig} */
  nextConfig
) => {
  return {
    ...nextConfig,
    env: {
      R2_ENDPOINT: process.env.R2_ENDPOINT,
      R2_BUCKET: process.env.R2_BUCKET,
      R2_PUBLIC_URL: process.env.R2_PUBLIC_URL,
      ...nextConfig.env,
    },
    images: {
      ...(nextConfig?.images || {}),
      remotePatterns: [
        ...(nextConfig?.images?.remotePatterns || []),
        {
          protocol: 'https',
          hostname: process.env.R2_PUBLIC_URL.replace('https://', ''),
        },
      ],
    },
  };
};

export default withUpload;

next.config.mjs

import nextExtendableConfig from './lib/next-extendable-config/index.mjs';

/** @type {import('next').NextConfig} */
const nextConfig = {
  env: {
    SITE_NAME: process.env.SITE_NAME,
  },
};

export default nextExtendableConfig(nextConfig, {
  folders: ['./lib', './app'],
});