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). Don't mix Svelte 4/5 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](references/reactivity-patterns.md) -

When to use each rune

• [references/migration-gotchas.md](references/migration-gotchas.md) -

Svelte 4 → 5 translation

• [references/component-api.md](references/component-api.md) -

$props, $bindable patterns

• [references/snippets-vs-slots.md](references/snippets-vs-slots.md) -

New snippet syntax

• [references/common-mistakes.md](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

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