Svelte 5 - $dervied causes svelte-check errors that do not occur with $derived.by

[eighty4]

Svelte 5 - $dervied causes svelte-check errors that do not occur with $derived.by

[7nik]

Obviously, the concept of reactive expressions doesn't even exist in TS. So TS thinks that when buildBoundingBoxes(inspectResult?.elements)) is executed, inspectResult can only be null. And then it seems to face this issue microsoft/TypeScript#41766.

It seems another workaround is inspectResult!?.elements.

[dummdidumm]

Yeah this is a type narrowing issue, and the type is widened in the lambda.
Another way to get around it is to type the $state generic: let foo = $state<boolean | null>(null)

Giving this the documentation label because there's not much we can do.

[eighty4]

Why wouldn't derived.by be the API style of derived and drop derived.by? I'm not sure why compiler magic that necessitates a duplicate rune to work around trumps having a natural API.

[7nik]

@eighty4 it was discussed multiple times, including in the initial #10240