Svelte Patterns

Svelte 5 Runes

Runes replace Svelte 4's let reactivity and $: labels with explicit primitives.

<script lang="ts">
  // Reactive state
  let count = $state(0)
  let items = $state<string[]>([])

  // Derived values (replaces $: derived = ...)
  let doubled = $derived(count * 2)
  let total = $derived.by(() => {
    return items.reduce((sum, item) => sum + item.length, 0)
  })

  // Side effects (replaces $: { ... })
  $effect(() => {
    console.log(`count is ${count}`)
    return () => console.log('cleanup')  // cleanup function
  })
</script>

<button onclick={() => count++}>{count} (doubled: {doubled})</button>

Component Props (Svelte 5)

$props() replaces export let. Use Snippet type for children (replaces <slot>). Named snippets take typed args.

SvelteKit Routing

src/routes/
  +page.svelte          # /
  +layout.svelte        # shared layout
  +page.server.ts       # server load function
  blog/
    +page.svelte        # /blog
    [slug]/
      +page.svelte      # /blog/:slug
      +page.ts           # universal load
      +page.server.ts    # server-only load
  api/
    posts/
      +server.ts         # API route: GET, POST, etc.

Load Functions

Server load (+page.server.ts) has DB/secret access. Universal load (+page.ts) runs both sides. Return typed data props.

Form Actions

Define named actions in +page.server.ts. Use fail() for validation errors, throw redirect() for redirects. Client-side use:enhance for progressive enhancement.

Stores

Svelte 5 runes preferred. Stores (writable, derived) still useful for cross-component shared state. Access with $ prefix.

Transitions

Built-in transition:fade, in:fly, out:slide. Use animate:flip for list reordering within {#each}.

Gotchas

• Svelte 5 migration: let x = 0 is no longer reactive at top level -- must use $state(0). $: labels are deprecated; use $derived and $effect
• $effect vs $derived: use $derived for computed values, $effect only for side effects. $effect does NOT return a value
• Object/array reactivity: $state uses proxies; direct property mutation works (obj.key = val), but reassigning nested arrays needs care: items = [...items, newItem] or mutate in place
• Stores to runes: Svelte 5 still supports stores but runes are preferred. $state replaces writable for component-local state; stores still useful for cross-component shared state
• Hydration mismatch: SvelteKit SSR means server/client must render identically. Guard browser APIs with browser from $app/environment or onMount
• Form action gotcha: throw redirect() (not return redirect()) -- redirect is implemented as a thrown response in SvelteKit
• use:enhance: without it, form submissions trigger full page reload. With it, SvelteKit progressively enhances to fetch

Cross-References

• frontend:form-patterns -- advanced form validation and multi-step wizard patterns
• frontend:react-state-management -- compare Svelte stores/runes with React state solutions