[Feature] Pages guards?

[gustaveWPM]

Goals

Currently, I'm putting some "Guards" in the generateMetadata function.

Something like:

export async function generateMetadata({ params }: BlogPostPageProps) {
  await blogPostGuard({ params });
  const blogPostMetadatas = await getBlogPostMetadatas({ params });
  return blogPostMetadatas;
}

Where blogPostGuard is something like:

export async function blogPostGuard({ params }: BlogPostPageProps) {
  const category = params[CATEGORY];
  const subcategory = params[SUBCATEGORY];
  const validCombination: boolean = await isValidBlogCategoryAndSubcategoryPair(category, subcategory);

  const slug = params[SLUG];
  const lang = params[LANG_FLAG];
  const post = validCombination ? await getBlogPost(category, subcategory, slug, lang) : undefined;

  if (!post && validCombination) {
    redirectToBlogCategoryAndSubcategoryPairPage(category, subcategory);
  } else if (!post && isValidBlogCategory(category)) {
    redirectToBlogCategoryPage(category);
  } else if (!post) {
    redirect('/' + category);
  }
}

I think it could be a good feature to have declareGuard function?

Something like:

export async function declareGuard({ params }: BlogPostPageProps) {
  await blogPostGuard({ params });
}

export async function generateMetadata({ params }: BlogPostPageProps) {
  const blogPostMetadatas = await getBlogPostMetadatas({ params });
  return blogPostMetadatas;
}

So that declareGuard would be executed BEFORE generateMetadata and generateStaticParams.

Non-Goals

No response

Background

The current alternative is to put the guard in the generateMetadata.
But I think this is not the most expressive way to achieve this.

Even though the documentation says:

The redirect() and notFound() Next.js methods can also be used inside generateMetadata.

• https://nextjs.org/docs/app/api-reference/functions/generate-metadata#returns

I think it could be proper to define guards separately and document them.

Proposal

How should this feature be implemented

Idk. Maybe as generateMetadata, but as a function with a void return type?
And then add it to the Next.js magic which is handling generateMetadata and generateStaticParams, in order to make it run at first?

Are you interested in contributing

I could be, if you think it is a good first contribution, simple enough for me to implement correctly with the right hints.