Error on structuredClone() + Svelte 5 store

[ginpei]

I'm trying to migrate my project to Svelte 5, and encountered an error by structuredClone() when used with store content. Here is the code:

const obj = writable({ user: { name: 'one' } });
console.log(1, $obj); // Object
const obj2 = structuredClone($obj); // fine

$obj = { user: { name: 'two'} };
console.log(2, $obj); // Proxy
const obj3 = structuredClone($obj); // DataCloneError

DataCloneError: Failed to execute 'structuredClone' on 'Window': #<Object> could not be cloned.

https://svelte.dev/playground/66ed5c5340a44a2da72514f21dc150b8?version=5.20.0

The Svelte 5 migration guide mentions Proxy, and the HTML Standard spec explains a Proxy instance results this error. This doesn't seems to be a bug.

But how should I proceed with my migration? Does anyone have any suggestions on how to deal with this?

[tomoam]

please use $state.snapshot.

<script lang="ts">
	import { writable } from 'svelte/store';
	
	const props = $props(); // !?
	
	const obj = writable({ name: 'Yo' });
	console.log(1, $obj); // Object
	const obj2 = structuredClone($obj); // fine
	
	$obj = { name: 'Hey' };
	
	const snapshot = $state.snapshot($obj);

	console.log(2, snapshot);
	const obj3 = structuredClone(snapshot);
</script>

[tomoam]

At my second thought, this behaviour seems a little strange.
Using set to update the store value is a workaround for once.

<script lang="ts">
	import { writable } from 'svelte/store';
	
	const props = $props(); // !?
	
	const obj = writable({ name: 'Yo' });
	console.log(1, $obj); // Object
	const obj2 = structuredClone($obj); // fine

	obj.set({ name: 'Hey' });
	
	console.log(2, $obj);
	const obj3 = structuredClone($obj);
</script>

[ginpei]

Thank you so much. The API explanation is perfect for my case!

This is handy when you want to pass some state to an external library or API that doesn’t expect a proxy, such as structuredClone.

Right, the behavior is strange, but it's Svelte internal. I understand that I need to ensure $obj returns either the original object or a proxy.

Thanks again

[tomoam]

I suspect that this behavior might be a bug, though I'm not entirely sure. I opened #15281.