prerender external links

[Rich-Harris]

In #1679 we stopped crawling links with rel="external" during prerendering. rel="external" is often used to force a full page navigation (instead of a client-side navigation), and in those cases is orthogonal to the question of whether something should be prerendered (see e.g. #3331).

It seems like the simplest thing to do would simply be to follow those links, but @v1ack perhaps you can elaborate on the motivation for #1679?

The alternative would be to introduce a new construct for disabling client-side navigation on a link (i.e. a new sveltekit:blah attribute) but it would be better to avoid that unless we really need it.

Please don't delete this checklist! Before submitting the PR, please make sure you do the following:

• It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC: https://github.com/sveltejs/rfcs
• This message body should clearly illustrate what problems it solves.
• Ideally, include a test that fails without this PR but passes with it.

Tests

• Run the tests with pnpm test and lint the project with pnpm lint and pnpm check

Changesets

• If your PR makes a change that should be noted in one or more packages' changelogs, generate a changeset by running pnpx changeset and following the prompts. All changesets should be patch until SvelteKit 1.0

[changeset-bot]

🦋 Changeset detected

Latest commit: 6a7f51b

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@sveltejs/kit	Patch

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[netlify]

✔️ Deploy Preview for kit-demo canceled.

🔨 Explore the source changes: 6a7f51b

🔍 Inspect the deploy log: https://app.netlify.com/sites/kit-demo/deploys/620bd81c681cf0000734b9bb

[v1ack]

External needs when another page on the same host handled by another service.

For example, my case is

• main site on GitHub pages (v1ack.github.io) running on svelte kit
• docs site for lib (https://v1ack.github.io/svelte-virtual-scroll-list/) running from another repository

If I place link to lib site on main site without #1679 - crawler fails with 404.

Maybe the logic of crawler can be changed to something like this:

• if we faced to external - try to crawl
• if external page doesn't exist - make a warning (not error) and continue crawling

[Conduitry]

You can control what happens when the crawler runs into a missing page with https://kit.svelte.dev/docs/configuration#prerender

[Rich-Harris]

Yeah, I think I'm inclined to say that this can be handled via the prerender.onError option:

// svelte.config.js
export default {
  kit: {
    prerender: {
      onError: ({ status, path }) => {
        if (path.startsWith('/svelte-virtual-scroll-list')) return;
        throw new Error(`${status} ${path}`);
      }
    }
  }
};

[v1ack]

Ok, it looks good
But I think it's better to be mentioned in docs, near externaloption, maybe as a link to FAQ

[Rich-Harris]

Added some docs: 6a7f51b

[babichjacob]

Partially related discussion on Discord for those curious: https://discord.com/channels/457912077277855764/750401468569354431/943224194634764369