svelte5-best-practices

name: svelte5-best-practices description: "Svelte 5 runes, snippets, SvelteKit patterns, and modern best practices for TypeScript and component development. Use when writing, reviewing, or refactoring Svelte 5 components and SvelteKit applications. Triggers on: Svelte components, runes ($state, $derived, $effect, $props, $bindable, $inspect), snippets ({#snippet}, {@render}), event handling, SvelteKit data loading, form actions, Svelte 4 to Svelte 5 migration, store to rune migration, slots to snippets migration, TypeScript props typing, generic components, SSR state isolation, performance optimization, or component testing." license: MIT metadata: author: ejirocodes version: '1.0.0'

Svelte 5 Best Practices

Quick Reference

Essential Patterns

Reactive State

<script>
  let count = $state(0);           // Reactive state
  let doubled = $derived(count * 2); // Computed value
</script>

Component Props

<script>
  let { name, count = 0 } = $props();
  let { value = $bindable() } = $props(); // Two-way binding
</script>

Snippets (replacing slots)

<script>
  let { children, header } = $props();
</script>

{@render header?.()}
{@render children()}

Event Handlers

<!-- Svelte 5: use onclick, not on:click -->
<button onclick={() => count++}>Click</button>

Callback Props (replacing createEventDispatcher)

<script>
  let { onclick } = $props();
</script>

<button onclick={() => onclick?.({ data })}>Click</button>

Common Mistakes

1. Using let without $state - Variables are not reactive without $state()
2. Using $effect for derived values - Use $derived instead
3. Using on:click syntax - Use onclick in Svelte 5
4. Using createEventDispatcher - Use callback props instead
5. Using <slot> - Use snippets with {@render}
6. Forgetting $bindable() - Required for bind: to work
7. Setting module-level state in SSR - Causes cross-request leaks
8. Sequential awaits in load functions - Use Promise.all for parallel requests