SvelteKit Complete Guide 2026: Svelte 5 Runes, Routing und Full-Stack

npm create svelte@latest my-app
cd my-app
npm install

# Choose: SvelteKit, TypeScript, ESLint, Prettier, Playwright, Vitest

<!-- Counter.svelte — Svelte 5 runes syntax -->
<script lang="ts">
  // $state — reactive state (replaces let)
  let count = $state(0);
  let name = $state('World');

  // $derived — computed values (replaces $: derived)
  const doubled = $derived(count * 2);
  const greeting = $derived(`Hello, ${name}! Count: ${count}`);

  // $effect — side effects (replaces $: with side effects)
  $effect(() => {
    document.title = `Count: ${count}`;
    return () => {
      document.title = 'App'; // cleanup
    };
  });

  function increment() {
    count++;
  }
</script>

<div class="counter">
  <h2>{greeting}</h2>
  <p>Doubled: {doubled}</p>

  <!-- Svelte 5: onclick not on:click -->
  <button onclick={increment}>+1</button>
  <button onclick={() => count = 0}>Reset</button>

  <input bind:value={name} placeholder="Your name" />
</div>

<style>
  .counter {
    padding: 1rem;
    border: 1px solid var(--border);
    border-radius: 8px;
  }
</style>

<!-- Card.svelte -->
<script lang="ts">
  interface Props {
    title: string;
    description?: string;
    variant?: 'primary' | 'secondary';
    onclick?: () => void;
    children?: import('svelte').Snippet;
  }

  // $props replaces export let
  const { title, description = '', variant = 'primary', onclick, children }: Props = $props();
</script>

<article class="card card--{variant}" onclick={onclick}>
  <h3>{title}</h3>
  {#if description}
    <p>{description}</p>
  {/if}
  {#if children}
    {@render children()}
  {/if}
</article>

src/routes/
  +page.svelte          — home page
  +layout.svelte        — shared layout
  +error.svelte         — error page
  blog/
    +page.svelte        — /blog
    +page.server.ts     — server-side load
    [slug]/
      +page.svelte      — /blog/my-post
      +page.server.ts
  api/
    users/
      +server.ts        — REST API routes

// src/routes/blog/+page.server.ts
import type { PageServerLoad } from './$types';

export const load: PageServerLoad = async ({ fetch, url }) => {
  const page = url.searchParams.get('page') ?? '1';
  const response = await fetch(`/api/posts?page=${page}&limit=10`);

  if (!response.ok) {
    throw error(500, 'Failed to load posts');
  }

  const { posts, total } = await response.json();
  return { posts, total, page: parseInt(page) };
};

<!-- src/routes/blog/+page.svelte -->
<script lang="ts">
  import type { PageData } from './$types';

  const { data } = $props<{ data: PageData }>();

  const { posts, total, page } = $derived(data);
</script>

<svelte:head>
  <title>Blog | TechPulse</title>
  <meta name="description" content="Latest tech articles" />
</svelte:head>

<main>
  <h1>Blog</h1>
  {#each posts as post (post.id)}
    <article>
      <h2><a href="/blog/{post.slug}">{post.title}</a></h2>
      <p>{post.excerpt}</p>
      <time>{new Date(post.publishedAt).toLocaleDateString()}</time>
    </article>
  {/each}

  <nav>
    {#if page > 1}
      <a href="?page={page - 1}">Previous</a>
    {/if}
    {#if posts.length === 10}
      <a href="?page={page + 1}">Next</a>
    {/if}
  </nav>
</main>

// src/routes/api/users/+server.ts
import { json, error } from '@sveltejs/kit';
import type { RequestHandler } from './$types';

export const GET: RequestHandler = async ({ url, locals }) => {
  const page = parseInt(url.searchParams.get('page') ?? '1');
  const limit = Math.min(parseInt(url.searchParams.get('limit') ?? '20'), 100);

  const { users, total } = await locals.db.users.findMany({ page, limit });
  return json({ users, total, page, limit });
};

export const POST: RequestHandler = async ({ request, locals }) => {
  const body = await request.json();

  // Validate with Zod
  const result = createUserSchema.safeParse(body);
  if (!result.success) {
    throw error(422, { message: 'Validation failed', errors: result.error.errors });
  }

  const user = await locals.db.users.create(result.data);
  return json(user, { status: 201 });
};

// src/routes/contact/+page.server.ts
import { fail, redirect } from '@sveltejs/kit';
import type { Actions } from './$types';

export const actions: Actions = {
  default: async ({ request }) => {
    const data = await request.formData();
    const name = data.get('name') as string;
    const email = data.get('email') as string;
    const message = data.get('message') as string;

    if (!name || !email || !message) {
      return fail(422, { name, email, message, error: 'All fields required' });
    }

    await sendContactEmail({ name, email, message });
    throw redirect(303, '/contact?success=true');
  }
};

<!-- src/routes/contact/+page.svelte -->
<script lang="ts">
  import { enhance } from '$app/forms';
  const { form } = $props();
</script>

<form method="POST" use:enhance>
  {#if form?.error}
    <p class="error">{form.error}</p>
  {/if}

  <input name="name" value={form?.name ?? ''} required placeholder="Name" />
  <input name="email" type="email" value={form?.email ?? ''} required placeholder="Email" />
  <textarea name="message" required placeholder="Message">{form?.message ?? ''}</textarea>
  <button type="submit">Send</button>
</form>

# Vercel (adapter-vercel)
npm install --save-dev @sveltejs/adapter-vercel

# Netlify (adapter-netlify)
npm install --save-dev @sveltejs/adapter-netlify

# Node.js server (adapter-node)
npm install --save-dev @sveltejs/adapter-node

# Cloudflare Workers (adapter-cloudflare)
npm install --save-dev @sveltejs/adapter-cloudflare

# svelte.config.js
import adapter from '@sveltejs/adapter-vercel';
export default { kit: { adapter: adapter() } };