No error for `@render undefined` if snippet function is state

[rChaoz]

Describe the bug

Currently, @render has no problem if it the snippet function is undefined if it's a state/prop, which is weird since it is a function call. Considering the {@render func?.() syntax is supported, it should throw.

Case 1:

<script>
  let func = $state()
  $effect(() => {
    console.log(func === undefined) // true
  });
</script>

This doesn't throw: {@render func()}

Case 2:

<script>
  let func
  console.log(func === undefined) // true
</script>

This is an error: {@render func()}

Severity

annoyance

[rChaoz]

Is there a chance this can get looked at / added to the milestone? Fixing this would be a breaking change.

[paoloricciuti]

To me it feels like we should do the opposite: just render nothing if func is undefined even if it's not state.

[trueadm]

This is a bug, undefined or null should render nothing. Or it should always throw an exception.

[dummdidumm]

Should it? Shouldn't we error if it's called but is undefined, similar to how you get a JS error when invoking undefined as a function?

[trueadm]

Then case 1 above should also throw an exception? Either way it's a bug.

[paoloricciuti]

How do we want to fix it? I already have a fix locally in case we want to always render nothing...throwing should also be pretty straightforward

[Conduitry]

I'm torn. I know in other situations we are erring on the side of rendering nothing. I don't know whether we want to say that we should error here because the syntax looks like a function invocation.

In a similar vein, if Foo is undefined, then <svelte:component this={Foo}/> renders nothing (as does <svelte:element this={Foo}/>), while <Foo/> is a runtime error. I don't know. We have varying behavior here, but all of these things feel like the same sort of thing to me as rendering a snippet, regardless of whether it's a function-like syntax.

[dummdidumm]

For me it's more about the look - it looks like a function call, and a function call would error if you'd call a falsy value.

[trueadm]

Agreed that we should not allow state of undefined to render nothing, it should also throw an exception.

[trueadm]

Turns out that making it a runtime error for state is a bit tricker than expected due to how we compile the dynamic logic out.

@render optional?.()

and also

@render optional.()

Becomes this when compiled:

$.snippet(node_7, () => $$props.optional)

So I'm actually leaning towards all nullish entries being rendered as empty instead.