SvelteKit Core Knowledge

Full Reference: See advanced.md for WebSocket integration, Server-Sent Events, Socket.IO, hybrid form actions, room management, and streaming patterns.

Deep Knowledge: Use mcp__documentation__fetch_docs with technology: sveltekit for comprehensive documentation.

Route Structure

src/routes/
├── +page.svelte      → / (page UI)
├── +page.server.ts   → / (server load/actions)
├── +layout.svelte    → (shared layout)
├── users/
│   ├── +page.svelte  → /users
│   └── [id]/
│       └── +page.svelte → /users/:id

Key Concepts

• +page.svelte → UI component
• +page.server.ts → Server-only code
• +page.ts → Universal (runs both)
• +layout → Nested layouts
• +error.svelte → Error pages

Load Functions

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

export const load: PageServerLoad = async ({ params, fetch }) => {
  const user = await db.users.find(params.id);
  return { user };
};

<!-- +page.svelte -->
<script lang="ts">
  import type { PageData } from './$types';
  export let data: PageData;
</script>

<h1>{data.user.name}</h1>

Form Actions

// +page.server.ts
import type { Actions } from './$types';

export const actions: Actions = {
  default: async ({ request }) => {
    const data = await request.formData();
    await db.users.create({ name: data.get('name') });
    return { success: true };
  },
  delete: async ({ params }) => {
    await db.users.delete(params.id);
  }
};

<form method="POST">
  <input name="name" />
  <button>Create</button>
</form>

<form method="POST" action="?/delete">
  <button>Delete</button>
</form>

Form Validation with Zod

// src/routes/users/+page.server.ts
import { fail } from '@sveltejs/kit';
import { z } from 'zod';

const UserSchema = z.object({
  email: z.string().email('Invalid email'),
  name: z.string().min(2, 'Name too short'),
});

export const actions = {
  create: async ({ request }) => {
    const formData = await request.formData();
    const data = Object.fromEntries(formData);

    const result = UserSchema.safeParse(data);
    if (!result.success) {
      return fail(400, {
        errors: result.error.flatten().fieldErrors,
        data,
      });
    }

    await createUser(result.data);
    return { success: true };
  },
};

<!-- +page.svelte -->
<script lang="ts">
  import { enhance } from '$app/forms';
  export let form;
</script>

<form method="POST" action="?/create" use:enhance>
  <input name="email" value={form?.data?.email ?? ''} />
  {#if form?.errors?.email}
    <span class="error">{form.errors.email[0]}</span>
  {/if}
  <button>Create</button>
</form>

Security Configuration

// svelte.config.js
export default {
  kit: {
    csrf: { checkOrigin: true },
    csp: {
      directives: {
        'default-src': ['self'],
        'script-src': ['self'],
      },
    },
  },
};

// src/hooks.server.ts
import type { Handle } from '@sveltejs/kit';

export const handle: Handle = async ({ event, resolve }) => {
  const response = await resolve(event);
  response.headers.set('X-Frame-Options', 'DENY');
  response.headers.set('X-Content-Type-Options', 'nosniff');
  return response;
};

Authentication

// src/hooks.server.ts
import type { Handle } from '@sveltejs/kit';
import { redirect } from '@sveltejs/kit';

export const handle: Handle = async ({ event, resolve }) => {
  const session = event.cookies.get('session');

  if (session) {
    event.locals.user = await verifySession(session);
  }

  // Protect routes
  if (event.url.pathname.startsWith('/admin')) {
    if (!event.locals.user?.isAdmin) {
      throw redirect(303, '/login');
    }
  }

  return resolve(event);
};

Error Handling

<!-- src/routes/+error.svelte -->
<script lang="ts">
  import { page } from '$app/stores';
</script>

<div class="error">
  <h1>{$page.status}</h1>
  <p>{$page.error?.message}</p>
  <a href="/">Go home</a>
</div>

// src/hooks.server.ts
import type { HandleServerError } from '@sveltejs/kit';

export const handleError: HandleServerError = async ({ error, event }) => {
  const errorId = crypto.randomUUID();
  console.error('Server error:', { errorId, error, url: event.url });

  return {
    message: 'An unexpected error occurred',
    errorId,
  };
};

Performance

// Prerendering
export const prerender = true;

export const entries = async () => {
  const posts = await getPosts();
  return posts.map((post) => ({ slug: post.slug }));
};

// Streaming
export const load: PageServerLoad = async () => {
  return {
    critical: await getCriticalData(),
    streamed: {
      slow: getSlowData(), // Not awaited
    },
  };
};

{#await data.streamed.slow}
  <Skeleton />
{:then slowData}
  <SlowContent data={slowData} />
{/await}

When NOT to Use This Skill

• Svelte without SvelteKit: Use frontend-svelte skill
• Next.js: Use nextjs-app-router skill
• Nuxt: Use nuxt3 skill
• Remix: Use remix skill

Anti-Patterns

Quick Troubleshooting

Production Checklist

• CSP headers configured
• CSRF protection enabled
• Error handling (+error.svelte, hooks)
• Auth in hooks.server.ts
• Form validation with Zod
• use:enhance for progressive forms
• Prerendering static routes
• Streaming for slow data

Reference Documentation

• Route Structure
• Load Functions