fix: prevent JSON parse error when loading data

[teemingc]

fixes #8751

This PR handles the edge case where loading the data of a non-existent prerendered dynamic route returns a 404 HTML page instead of JSON data. No test because our /basics test app has a root server layout load function, which causes it to do a native navigation instead, returning a 404 SSR response. The error only occurs when there is no root server layout load.

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[dummdidumm]

Why is the response to the non-existent file returned as HTML and not as json? Would it be possible to solve this at the prerendering stage instead by creating a __data.json that tells the client "this is a 404" somehow?
Alternatively, if we detect that the return type is HTML, should we do a full page navigation instead? These cases are probably rare enough (and something's went wrong badly) that a full page reload is fine.

[teemingc]

Why is the response to the non-existent file returned as HTML and not as json?

My guess is it's because the route doesn't exist in the server manifest (due to it being prerendered). So a request to /doesnt-exist/__data.json returns a 404 page. In contrast, a non-prerendered dynamic page will always return JSON when requesting the __data.json because the route exists in the server manifest.

Would it be possible to solve this at the prerendering stage instead by creating a __data.json that tells the client "this is a 404" somehow?

Right now we always fetch the __data.json relative to the missing route such as /a/missing/__data.json and /a/non-existent/__data.json. Maybe when the data fetch fails we fetch again from a special 404 data route?

kit/packages/kit/src/runtime/client/client.js

Lines 1847 to 1858 in 87a18b7

	async function load_data(url, invalid) {
	const data_url = new URL(url);
	data_url.pathname = add_data_suffix(url.pathname);
	if (url.pathname.endsWith('/')) {
	data_url.searchParams.append(TRAILING_SLASH_PARAM, '1');
	}
	if (DEV && url.searchParams.has(INVALIDATED_PARAM)) {
	throw new Error(`Cannot used reserved query parameter "${INVALIDATED_PARAM}"`);
	}
	data_url.searchParams.append(INVALIDATED_PARAM, invalid.map((i) => (i ? '1' : '0')).join(''));
	const res = await native_fetch(data_url.href);

Alternatively, if we detect that the return type is HTML, should we do a full page navigation instead? These cases are probably rare enough (and something's went wrong badly) that a full page reload is fine.

Yes, this is another good alternative solution.

Another alternative, is to always return JSON when __data.json is requested from the server. But I'm not sure what the negative effects of this could be. EDIT: Thinking about it again, this alternative would not work for a CSR only application.

[teemingc]

updated to fallback to native navigation when receiving a HTML response instead of JSON when loading __data.json

[dummdidumm]

What's this change about btw?

[teemingc]

ah I was trying to resolve to the TODO about the eslint error https://eslint.org/docs/latest/rules/no-async-promise-executor I can revert it though

[dummdidumm]

I don't think that lint rule applies here, so we can revert it