astro/examples/framework-multiple/src/components/SvelteCounter.svelte

<script>
  let children;
  let count = 0;

  function add() {
		count += 1;
	}

  function subtract() {
		count -= 1;
	}
</script>

<div class="counter">
    <button on:click={subtract}>-</button>
    <pre>{ count }</pre>
    <button on:click={add}>+</button>
</div>
<div class="children">
    <slot />
</div>