Svelte 5 Runes

Quick Start

Which rune? Props: $props() | Bindable: $bindable() | Computed: $derived() | Side effect: $effect() | State: $state()

Key rules: Runes are top-level only. $derived can be overridden (use const for read-only). Use consistent Svelte 5 syntax. Objects/arrays are deeply reactive by default.

Example

<script>
	let count = $state(0); // Mutable state
	const doubled = $derived(count * 2); // Computed (const = read-only)

	$effect(() => {
		console.log(`Count is ${count}`); // Side effect
	});
</script>

<button onclick={() => count++}>
	{count} (doubled: {doubled})
</button>

Reference Files

Before suggesting code, check these:

• •references/reactivity-patterns.md - When to use each rune
• •references/component-api.md - $props, $bindable patterns
• •references/snippets-vs-slots.md - New snippet syntax
• •references/common-mistakes.md - Anti-patterns with fixes

Notes

• •Event handlers: Use onclick not on:click in Svelte 5
• •Children: Use {@render children()} in layouts
• •Check Svelte version before suggesting syntax
• •Svelte 5.25+ breaking change: $derived can now be reassigned (use const for read-only)
• •Last verified: 2025-01-11