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Part 3 / Transitions / Custom JS transitions

While you should generally use CSS for transitions as much as possible, there are some effects that can't be achieved without JavaScript, such as a typewriter effect:

App.svelte
function typewriter(node, { speed = 1 }) {
	const valid = node.childNodes.length === 1 && node.childNodes[0].nodeType === Node.TEXT_NODE;

	if (!valid) {
		throw new Error(`This transition only works on elements with a single text node child`);
	}

	const text = node.textContent;
	const duration = text.length / (speed * 0.01);

	return {
		duration,
		tick: (t) => {
			const i = Math.trunc(text.length * t);
			node.textContent = text.slice(0, i);
		}
	};
}

Next: Transition events

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<script>
	let visible = false;
 
	function typewriter(node, { speed = 1 }) {
		const valid = node.childNodes.length === 1 && node.childNodes[0].nodeType === Node.TEXT_NODE;
 
		if (!valid) {
			throw new Error(`This transition only works on elements with a single text node child`);
		}
 
		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}
initialising