Infinite loop of $effect when modifying states containing arrays then reading them

[GuillaumeDerval]

Describe the bug

Modifying non-trivial $state (i.e. not a number) and then reading it inside the same $effect cause an infinite loop of calls of the effect.

Reproduction

https://svelte.dev/playground/ec99f39270c6463b9162af3432c29298?version=5.34.7

This works:

<script>
	let mystate = $state(12);
	$effect(() => {
		mystate = 13;
		if(mystate == 21)
			alert("a message");
	});
</script>
<h1>{mystate}</h1>

This doesn't:

<script>
	let mystate = $state(12);
	$effect(() => {
		mystate = [];
		if(mystate == 21)
			alert("a message");
	});
</script>
<h1>{mystate}</h1>

(the difference is that I assign [] to mystate in the $effect)

Logs

Last ten effects were:  
Array(10) [ inner(), App(), inner(), App(), inner(), App(), inner(), App(), inner(), App() ]

Uncaught Svelte error: effect_update_depth_exceeded
Maximum update depth exceeded. This can happen when a reactive block or effect repeatedly sets a new value. Svelte limits the number of nested updates to prevent infinite loops
https://svelte.dev/e/effect_update_depth_exceeded [srcdoc line 45 > eval:191:18](about:srcdoc line 45 > eval)

System Info

Tested online on the Playground with svelte 5.34.7 and Firefox 139.0.4 on macOS 15.5

Severity

annoyance

[GuillaumeDerval]

This can be trivially bypassed by storing the new value inside a non-state variable temporarily:

<script>
	let mystate = $state(12);
	$effect(() => {
                const newValue = [];
		mystate = newValue;
		if(newValue == 21)
			alert("a message");
	});
</script>
<h1>{mystate}</h1>

[paoloricciuti]

This is expected: the effect doesn't rerun if the value is the same so when you set the value to 1 it rerun once, it's set back to 1 and the cycle stops. If you do the same with an Object each time is a new reference so the effects never stop. Thanks for reporting but I'm gonna close this!