Slot renders 'undefined' when passed an undefined variable

Describe the bug Passing an undefined variable to a component that uses <slot/> renders undefined

To Reproduce https://svelte.dev/repl/d2fd61d90bca4e6f870950f9eb29b76b?version=3.14.0

Expected behavior Nothing is render in the slot. This matches the behaviour of passing nothing like <Component></Component>

Severity Not critical but it forces you user of the component to ensure the property passed is initialised rather than the component internally controlling this.

[ghost]

@nitro52,

I do this in the component to avoid having checks all over the place:

<script>
export let foo = undefined;
const optional = x => x || '';
</script>
{optional(foo)}

Adding this functionality to the core would only muddy the waters since there’s situations where you actually want a canary “undefined” to show up so that you know right away you’ve done something wrong.

To me, the power of Svelte is that it doesn’t “help” any more than I need it to.

[ryburn52]

Thinking about it there is already a similar syntax that i think would solve the issue but it does not work on slots. But if you could get a reference to the slot it would open a lot of doors

<script> 
	let description;
</script>
{#if description}
<article >
	<h1>
		Hot Offer!
	</h1>
	<div>
		<slot bind:this={description}></slot> 
	</div>
</article>
{/if}