Mark endpoints as prerenderable

Describe the problem

If an endpoint loads data from the filesystem…

import fs from 'fs';
import marked from 'marked';

export function get({ params }) {
  const markdown = fs.readFileSync(`content/${params.slug}.md`, 'utf-8');

  return {
    body: marked(markdown)
  };
}

…then in most environments it will won’t work in production — it’s only suitable for prerendering. SvelteKit doesn’t know that, and will include the endpoint in the built app. This is bad for two reasons:

• it means deploying unnecessary code
• a mistyped URL will 500 instead of 404

We’re experiencing this right now with https://kit.svelte.dev/docs/nope.

Describe the proposed solution

If we could mark the endpoint with export const prerender = true, just like we do with pages, we could avoid including the endpoint in the deployed app.

The semantics are slightly different to export const prerender = true in pages, where it means ‘you can prerender this page’ as opposed to ‘you can ONLY prerender this endpoint’, but I think that’s probably fine.

Alternatives considered

No response

Importance

would make my life easier

Additional Information

No response

[Rich-Harris]

From #4942: we might also need a way to declare that an endpoint should never be prerendered (i.e. prerendering /foo doesn’t mean that /foo/__data.json is also prerendered, even though the data is inlined into the HTML)

[furudean]

This would be a huge performance boost for those managing content inside Svelte Kit (with e.g. MDSveX), where only a new build would actually require endpoints to update their content. Game changer for listing content, RSS feeds, sitemap, etc.