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Key blocks destroy and recreate their contents when the value of an expression changes. This is useful if you want an element to play its transition whenever a value changes instead of only when the element enters or leaves the DOM.

Here, for example, we’d like to play the typewriter transition from transition.js whenever the loading message, i.e. i changes. Wrap the <p> element in a key block:

App
{#key i}
	<p in:typewriter={{ speed: 10 }}>
		{messages[i] || ''}
	</p>
{/key}

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<script>
	import { typewriter } from './transition.js';
	import { messages } from './loading-messages.js';
 
	let i = $state(-1);
 
	$effect(() => {
		const interval = setInterval(() => {
			i += 1;
			i %= messages.length;
		}, 2500);
 
		return () => {
			clearInterval(interval);
		};
	});
</script>
 
<h1>loading...</h1>
 
<p in:typewriter={{ speed: 10 }}>
	{messages[i] || ''}
</p>