postsのmdxファイルを階層深くしたところからも取り出す[Nextjs]

export const getStaticProps = async ({ params }) => {
  const globalData = getGlobalData();
  // const { mdxSource, data } = await getPostBySlug(params.slug);
  // const prevPost = getPreviousPostBySlug(params.slug);
  // const nextPost = getNextPostBySlug(params.slug);

  ↓
  配列から文字列に直して引数に入れる
  const slug = params.slug.join('/');

  const { mdxSource, data } = await getPostBySlug(slug);
  const prevPost = getPreviousPostBySlug(slug);
  const nextPost = getNextPostBySlug(slug);

  return {
    props: {
      globalData,
      source: mdxSource,
      frontMatter: data,
      prevPost,
      nextPost,
    },
  };
};

export const getStaticPaths = async () => {
  const paths = postFilePaths
    .map((path) => path.replace(/\.mdx?$/, ''))
    .map((slug) => {
      // return { params: { slug } }
      ↓
      return { params: { slug: [slug] } }
    });

  return {
    paths,
    // fallback: false,
    ↓
    fallback: true,
  };
};

/* 追加 */
const readSubDirSync = (folderPath) => {
  let result = [];
  const readTopDirSync = ((folderPath) => {
    let items = fs.readdirSync(folderPath)
    items = items.map((itemName) => (path.join(folderPath, itemName)))
    items.forEach((itemPath) => {
      const relativePath = itemPath.match(/.+\/posts\/(.+)/)
      result.push(relativePath[1])
      //再帰処理
      if (fs.statSync(itemPath).isDirectory()) readTopDirSync(itemPath)
    })
  })
  readTopDirSync(folderPath)
  // mdxだけ抽出
  const onlyMdx = result.filter((path) => /\.mdx?$/.test(path))
  return onlyMdx
}

/* 変更 */
// export const postFilePaths = fs
//   .readdirSync(POSTS_PATH)
//   // Only include md(x) files
//   .filter((path) => /\.mdx?$/.test(path));

↓
// 深い海藻のファイルも取り出せる
export const postFilePaths = readSubDirSync(POSTS_PATH).filter((path) => /\.mdx?$/.test(path));

<Link
  as={`/posts/${post.filePath.replace(/\.mdx?$/, '')}`}
  // href={`/posts/[slug]`}
  ↓
  href={`/posts/[[...slug]]`}
>

export default function PostPage({
  source,
  frontMatter,
  prevPost,
  nextPost,
  globalData,
}) {
  ↓ 追加
  if (!frontMatter) return ''
  return (
    <Layout>
      <SEO
        title={`${frontMatter.title} - ${globalData.name}`}
        description={frontMatter.description}
      />

...