Allow using prefixed relative paths in metadata, especially in alternates.languages

[cprussin]

Goals

Allow syntax like this:

export const metadata = {
  metadataBase = new URL("http://example.com"),
  alternates: {
    canonical: "./",
    languages: {
      es: "/es/./",
      fr: "/fr/./"
    }
  }
};

With this syntax, es would then expand to http://example.com/es/the/current/pathname.

Non-Goals

No response

Background

This would enable configuring alternates in a layout without having to propagate the configuration to each page file and manually expand pathnames. Without such syntax it's nearly impossible to reliably configure alternates across a large app since doing so would require that developers remember to inject the metadata on every page they create which is super error prone.

Proposal

In

next.js/packages/next/src/lib/metadata/resolvers/resolve-url.ts

Lines 75 to 80 in e838f26

	function resolveRelativeUrl(url: string | URL, pathname: string): string | URL {
	if (typeof url === 'string' && url.startsWith('./')) {
	return path.posix.resolve(pathname, url)
	}
	return url
	}

, we could change the function body to something like:

function resolveRelativeUrl(url: string | URL, pathname: string): string | URL {
  if (typeof url === 'string') {
    const match = /(^|.*\/)(\.\/.*)/.exec(url)
    if (match) {
      return path.posix.resolve(match[1] + pathname, match[2])
    }
  }
  return url
}

And yes I'm happy to contribute if this contribution would be accepted.

[cprussin]

@huozhi @shuding @timneutkens from commit logs it looks like you three are the most active folks working on metadata who have approval / merge access -- if one of you can give me a sign-off on accepting this feature I'll happily put in a PR to implement it!

[lednhatkhanh]

+1 for this as well!

[neo-elgiz-abbasov]

+1 here 🥺

[eason-dev]

+1 need this!