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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

<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

• [reactivity-patterns.md](references/reactivity-patterns.md) - When

to use each rune

• [migration-gotchas.md](references/migration-gotchas.md) - Svelte 4→5

translation

• [component-api.md](references/component-api.md) - $props, $bindable

patterns

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

snippet syntax

• [common-mistakes.md](references/common-mistakes.md) - Anti-patterns

with fixes

• [attachments.md](references/attachments.md) - @attach replaces use:

actions

Notes

• Use onclick not on:click, {@render children()} in layouts
• $derived can be reassigned (5.25+) - use const for read-only
• Last verified: 2025-01-11

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