Svelte runes

<script>
	let { optional = 'default value',  required, catch: theCatch, ..rest } = $props();
	
	let count = $state(0);
	let doubled = $derived(count * 2);
	
	$effect(() => {
		console.log(count);
	});
</script>

Signals are used under the hood.

References

• Svelte blog: Introducing runes