Setting a store to `undefined` and then trying to use it in a rune throws `state_unsafe_mutation` error

[ndom91]

Describe the bug

Previously, in Svelte 4, you used to be able to set a variable to undefined and later set it to a store and meanwhile use it in reactive statements. While not ideal, this used to work.

In svelte 5 if you do something similar it'll throw the state_unsafe_mutation error. See reproduction below. It's probably right to error in this case, but the error is very misleading we spent hours trying to track it down yesterday 😅

Maybe for some context, our before / after:

Before

const prs = $derived($hostedListingServiceStore?.prs);
const listedPr = $derived($prs?.find((pr) => pr.sourceBranch === branch.upstreamName));

After

const prStore = $derived($hostedListingServiceStore?.prs);
const prs = $derived(prStore ? $prStore : undefined);
const listedPr = $derived(prs?.find((pr) => pr.sourceBranch === branch.upstreamName));

Reproduction

https://svelte-5-preview.vercel.app/#H4sIAAAAAAAAE1WMzQqDMBCEXyUsHloQeven0GfosenBmrFdMJuQrIKI7148FOlxZr5vVhp4RKbqsZJ0HlTRLUYqSZe4hzxjVFBJOUyp35sm94mjXq1YHaHGd8sLdw0JpjWTOAwscPU-90HyDwge-mF5m9YUDolnuFNxuOfaSnM5rsXKWvybG5Xkg-OB4ajSNGF7bl-Xn5hLwAAAAA==

Logs

No response

System Info

`svelte@5.0.0-next.191`

Severity

annoyance

[paoloricciuti]

There's something definitely wrong in your reproduction: you are accessing the store inside the derived which means you get back the value but then you are trying to access the value from $derived with $ like if it was a store...was the intended reproduction something like this

<script>
	let maybeStore = undefined;
	const maybeSomething = $derived($maybeStore);
</script>

{maybeSomething}

instead?

[paoloricciuti]

Regardless i think this is probably something worth exploring...deriving a store works fine so it's probably a correct bug. Will try to investigate later

[dummdidumm]

I know where the problem lies, PR soon

[paoloricciuti]

I know where the problem lies, PR soon

Oh was about to comment...just to confirm...i fixed it by changing this

export function store_get(store, store_name, stores) {
	const entry = (stores[store_name] ??= {
		store: null,
		source: mutable_source(undefined),
		unsubscribe: noop
	});

	if (entry.store !== store) {
		entry.unsubscribe();
		entry.store = store ?? null;

		if (store == null) {
-			set(entry.source, undefined);
+			entry.source.v = undefined;
			entry.unsubscribe = noop;
		} else {
			var is_synchronous_callback = true;

			entry.unsubscribe = subscribe_to_store(store, (v) => {
				if (is_synchronous_callback) {
					// If the first updates to the store value (possibly multiple of them) are synchronously
					// inside a derived, we will hit the `state_unsafe_mutation` error if we `set` the value
					entry.source.v = v;
				} else {
					set(entry.source, v);
				}
			});

			is_synchronous_callback = false;
		}
	}

	return get(entry.source);
}

but you still need to wrap the actual store definition in $state or else the derived wont rerun. Did i miss something?

[dummdidumm]

No that's exactly what I did as well 👍 And yes, the example needs $state or it won't rerun, and the additional $ in front of the derived value is wrong.

[paoloricciuti]

No that's exactly what I did as well 👍 And yes, the example needs $state or it won't rerun, and the additional $ in front of the derived value is wrong.

I will tell my grandkid i had the same fix as dummdidumm 😜

[ndom91]

Awesome thanks a lot for the quick turn around! 😊