$derived(await ...) is a symbol

[dylan1951]

Describe the bug

Since 5.43.0, sometimes the value of a $derived(await ...) is unexpectedly a symbol which can cause errors.

Reproduction

REPL

<script>
    let foo = $state(null);

    $effect(() => {
        foo = 69;
    });

    let bar = $derived(await 1);

    let baz = $derived(foo ? foo * bar : null);

    const qux = "qux";
</script>

<p>baz: {baz}</p>

<svelte:boundary>
    {#snippet pending()}
        <p>Loading...</p>
    {/snippet}

    {#if qux}
        <p></p>
    {/if}
</svelte:boundary>

Logs

Unhandled Promise Rejection: TypeError: Cannot convert a symbol to a number

	in <unknown>
	in __wrapper.svelte



TypeError
@
update_reaction@
execute_derived@
update_derived@
is_dirty@
flush_queued_effects@
process@
flush_effects@
flush@
revive@
decrement@
@
@[native code]

System Info

Svelte playground

Severity

blocking an upgrade

[sillvva]

I think this one is getting me too when I update to 5.43+.

In this code, _tdata is an array of data coming from a remote function.

if (!Array.isArray(this._tdata)) console.error("EntitySearchFactory: _tdata is not an array:", this._tdata);
return (Array.isArray(this._tdata) ? this._tdata : []).map(...)

The log returns:

EntitySearchFactory: _tdata is not an array: Symbol()

I've had to do a type check here to ensure it's an array and return an empty array if not to "fix" the issue.

For me, this only happens with data preloading turned on.

[hmnd]

@dylan1951 @sillvva this should be addressed in #17373 now. Could you confirm if it's still reproducible on your end?

[sillvva]

Can't test in my project as I've reverted to 5.41.4 due to too many bugs still.

In dylan's example, it does fix the Symbol issue, but the $effect is not working.

[hmnd]

@sillvva I believe the PR for this specific issue combined with #17362 would show the correct result. I'll double check later