The svelte/transition module has a handful of built-in transitions, but it’s very easy to create your own. By way of example, this is the source of the fade transition:

function fade(node, { delay = 0, duration = 400 }) {
	const o = +getComputedStyle(node).opacity;

	return {
		delay,
		duration,
		css: (t) => `opacity: ${t * o}`
	};
}

The function takes two arguments — the node to which the transition is applied, and any parameters that were passed in — and returns a transition object which can have the following properties:

• delay — milliseconds before the transition begins
• duration — length of the transition in milliseconds
• easing — a p => t easing function (see the chapter on tweening)
• css — a (t, u) => css function, where u === 1 - t
• tick — a (t, u) => {...} function that has some effect on the node

The t value is 0 at the beginning of an intro or the end of an outro, and 1 at the end of an intro or beginning of an outro.

Most of the time you should return the css property and not the tick property, as CSS animations run off the main thread to prevent jank where possible. Svelte ‘simulates’ the transition and constructs a CSS animation, then lets it run.

For example, the fade transition generates a CSS animation somewhat like this:

0% { opacity: 0 }
10% { opacity: 0.1 }
20% { opacity: 0.2 }
/* ... */
100% { opacity: 1 }

We can get a lot more creative though. Let’s make something truly gratuitous:

<script>
	import { fade } from 'svelte/transition';
	import { elasticOut } from 'svelte/easing';

	let visible = $state(true);

	function spin(node, { duration }) {
		return {
			duration,
			css: t => {
				const eased = elasticOut(t);

				return `
					transform: scale(${eased}) rotate(${eased * 1080}deg);
					color: hsl(
						${Math.trunc(t * 360)},
						${Math.min(100, 1000 * (1 - t))}%,
						${Math.min(50, 500 * (1 - t))}%
					);`
			}
		};
	}
</script>

Remember: with great power comes great responsibility.