svelte/documentation/tutorial/10-transitions/05-custom-js-transitions/app-a/App.svelte

<script>
	let visible = false;

	function typewriter(node, { speed = 50 }) {
		// implementation goes here

		return {};
	}
</script>

<label>
	<input type="checkbox" bind:checked={visible} />
	visible
</label>

{#if visible}
	<p transition:typewriter>The quick brown fox jumps over the lazy dog</p>
{/if}