Svelte 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). Don't mix Svelte 4/5 syntax. Objects/arrays are deeply reactive by default.

Example

svelte
1<script>
2	let count = $state(0); // Mutable state
3	const doubled = $derived(count * 2); // Computed (const = read-only)
4
5	$effect(() => {
6		console.log(`Count is ${count}`); // Side effect
7	});
8</script>
9
10<button onclick={() => count++}>
11	{count} (doubled: {doubled})
12</button>

Reference Files

• reactivity-patterns.md - When to use each rune
• migration-gotchas.md - Svelte 4→5 translation
• component-api.md - $props, $bindable patterns
• snippets-vs-slots.md - New snippet syntax
• common-mistakes.md - Anti-patterns with fixes

For @attach and other template directives, see the svelte-template-directives skill.

Notes

• Use onclick not on:click, {@render children()} in layouts
• $derived can be reassigned (5.25+) - use const for read-only
• Use createContext over setContext/getContext for type safety
• Use $inspect.trace to debug reactivity issues
• Last verified: 2026-03-12