Collection

---
name: svelte-runes
# IMPORTANT: Keep description on ONE line for Claude Code compatibility
# prettier-ignore
description: Svelte runes guidance. Use for reactive state, props, effects, attachments, or migration. Covers $state, $derived, $effect, @attach. Prevents reactivity mistakes.
---

# 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
<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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