贴心的Svelte还提供了些响应式类（reactive classes), 对应JavaScript中的相应类：Map, Set, Date, URL, URLSearchParams. 比如我们可能会使用$state(new Date())声明一个date变量，并在setInterval中重新赋值。还可以使用svelte/reactivity提供的SvelteDate:

Svelte ships with several reactive classes that you can use in place of JavaScript built-in objects — namely Map, Set, Date, URL and URLSearchParams. In this exercise, we could declare date using $state(new Date()) and reassign it inside the setInterval. But a nicer alternative is to use SvelteDate from svelte/reactivity:

<script>
	import { SvelteDate } from 'svelte/reactivity';

	let date = new SvelteDate();

	// ...
</script>

<script lang="ts">
	import { SvelteDate } from 'svelte/reactivity';

	let date = new SvelteDate();

	// ...
</script>