fix: add error result type to preloadData

[teemingc]

fixes #12399

This PR changes the preloadData function so that it returns the 'error' type instead of 'loaded' when the page fails to load. It also returns the error that caused the loading to fail and the correct status (previously it was always 200).

EDIT: changed it to also return status for redirects

TODO:

• test
• changeset

---

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 pnpm changeset and following the prompts. Changesets that add features should be minor and those that fix bugs should be patch. Please prefix changeset messages with feat:, fix:, or chore:.

Edits

• Please ensure that 'Allow edits from maintainers' is checked. PRs without this option may be closed.

[changeset-bot]

🦋 Changeset detected

Latest commit: 2e08d47

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	Major

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

[PavelNuzhin]

Hello! Thank you for that change!

I'm currently testing changes and noticed that the error I throw from the page is always returned as an Internal Error (500) from preloadData(). However, it seems logical that I would see, for example, my error(401, "Test error") from the load() function.

This is because when SvelteKit encounters my error in load_route and passes it to handle_error, this check happens:

if (error instanceof HttpError) {
    return error.body;
}

The error I threw isn't recognized as an HttpError or a SvelteKitError. So the kit can't get its status and always returns Internal Error (500).

Here, for example, get_status function will never return my 401 status

export function get_status(error) {
	return error instanceof HttpError || error instanceof SvelteKitError ? error.status : 500;
}

Is it intended behavior?

Here`s whats happening in handle_error():

Actually i think only problem is that get_status and get_message return default values

[PavelNuzhin]

I fixed it for my project. It may be bad but it works for me :)
I really appreciate it if you could make some changes to the preloadData() error handling.

kit/src/utils/error.js

/**
 * @param {unknown} error
 * @returns {number}
 */
export function get_status(error) {
	if(error instanceof HttpError || error instanceof SvelteKitError)
		return error.status;

	if(error instanceof Object && 'status' in error && typeof error.status === 'number')
		return error.status;

	return 500;
}

/**
 * @param {unknown} error
 * @returns {string}
 */
export function get_message(error) {
	if(error instanceof SvelteKitError)
		return error.text;

	if(error instanceof Object && 'error' in error)
		error = error.error;
	
	if(error instanceof Object && 'message' in error && typeof error.message === 'string')
		return error.message;

	return 'Internal Error';
}

[teemingc]

Hi @PavelNuzhin , can you provide a minimal reproduction along with an expected output compared to actual output?

[svelte-docs-bot]

Preview: https://svelte-dev-git-preview-kit-12579-svelte.vercel.app/

[elliott-with-the-longest-name-on-github]

@teemingc want to bring this back up to date so we can review it? Is it ready?

[teemingc]

Done! I've no idea how to reproduce the issue reported above though. Going to assume it was some bundling error that caused instanceof HttpError not to work.

EDIT: it was a server load that caused it because we were rethrowing a plain object rather than HttpError

[dummdidumm]

nice!