svelte5-best-practices — ejirocodes/agent-skills…

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Svelte 5 Best Practices

Topic	When to Use	Reference
Runes	$state, $derived, $effect, $props, $bindable, $inspect	runes.md
Snippets	Replacing slots, {#snippet}, {@render}	snippets.md
Events	onclick handlers, callback props, context API	events.md
TypeScript	Props typing, generic components	typescript.md
Migration	Svelte 4 to 5, stores to runes	migration.md
SvelteKit	Load functions, form actions, SSR, page typing	sveltekit.md
Performance	Universal reactivity, avoiding over-reactivity, streaming	performance.md

svelte

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

svelte

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

svelte

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

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

svelte

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

svelte

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

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

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