SvelteKit: Definition, Features, Routing, Deployment Guide

What is SvelteKit?

SvelteKit is the official application framework for Svelte. Where Svelte is the UI component compiler, SvelteKit is everything else needed to build a full-stack web app: routing, server-side rendering (SSR), static site generation (SSG), API routes, build tooling, and deployment adapters. It's analogous to Next.js for React or Nuxt for Vue.

SvelteKit reached 1.0 in December 2022 and is the recommended way to build any non-trivial Svelte app. It uses Vite for dev/build and ships with first-class TypeScript support.

Why SvelteKit?

• Tiny bundles. Svelte compiles components to vanilla JS — no runtime overhead.
• Fast page loads. SSR + minimal JS = fast LCP, fast INP.
• Flexible rendering. SSR, SSG, SPA, ISR, or hybrid — per route.
• File-based routing. Routes mirror folder structure.
• Adapters for any host. Vercel, Netlify, Cloudflare, Node, static — one config change.
• Form actions. Progressive-enhancement form handling without JS frameworks.
• TypeScript everywhere. First-class, not a retrofit.
• Vite dev server. Fast HMR, modern ESM.

SvelteKit project structure

my-app/
├── src/
│   ├── routes/                    # File-based routing
│   │   ├── +page.svelte           # / page
│   │   ├── +layout.svelte         # Shared layout
│   │   ├── about/
│   │   │   └── +page.svelte       # /about
│   │   ├── blog/
│   │   │   ├── +page.svelte       # /blog (list)
│   │   │   └── [slug]/
│   │   │       ├── +page.svelte   # /blog/:slug
│   │   │       └── +page.server.ts # Server-side load
│   │   └── api/
│   │       └── posts/
│   │           └── +server.ts     # API route GET/POST
│   ├── lib/                       # Shared modules
│   └── app.html                   # HTML shell
├── static/                        # Static assets
├── svelte.config.js
└── vite.config.ts

Routing in SvelteKit

File-based routing maps directly to URL paths. Special files prefixed with +:

Data loading example

// src/routes/blog/[slug]/+page.server.ts
import type { PageServerLoad } from './$types';
import { error } from '@sveltejs/kit';

export const load: PageServerLoad = async ({ params }) => {
  const post = await db.post.findUnique({ where: { slug: params.slug } });
  if (!post) throw error(404, 'Post not found');
  return { post };
};

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

<article>
  <h1>{data.post.title}</h1>
  <div>{@html data.post.body}</div>
</article>

API routes

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

export const GET: RequestHandler = async () => {
  const posts = await db.post.findMany();
  return json(posts);
};

export const POST: RequestHandler = async ({ request }) => {
  const data = await request.json();
  const post = await db.post.create({ data });
  return json(post, { status: 201 });
};

Form actions: progressive enhancement

// +page.server.ts
export const actions: Actions = {
  default: async ({ request }) => {
    const data = await request.formData();
    await db.post.create({ data: { title: data.get('title') as string } });
    return { success: true };
  }
};

<!-- +page.svelte -->
<form method="POST" use:enhance>
  <input name="title" />
  <button>Save</button>
</form>

Works without JS (form posts normally). With JS, use:enhance upgrades to AJAX without page reload.

Adapters: deploy anywhere

SvelteKit vs Next.js vs Nuxt

SvelteKit best practices

• Use +page.server.ts for sensitive data. Server-only code never reaches client.
• Use form actions. Progressive enhancement; works without JS.
• Type everything. SvelteKit auto-generates types in ./$types.
• Choose the right adapter. Static for pure-content sites; node/serverless for dynamic.
• SSR by default; opt out where needed. Add export const ssr = false in +page.ts for client-only routes.
• Streaming with defer. Stream slow data after fast paint.
• Use $state and $derived (Svelte 5 runes). New reactivity model.

FAQ: SvelteKit

Is SvelteKit production-ready?

Yes — 1.0 released Dec 2022. Used by NY Times, Apple Music, Spotify Tech Blog, and many others.

SvelteKit or Next.js?

SvelteKit if you want smaller bundles and simpler syntax. Next.js if you need React's massive ecosystem.

Does SvelteKit support TypeScript?

Yes — first-class. Auto-generated types per route in ./$types.

Can I use SvelteKit for a static site?

Yes — adapter-static generates a fully static site. SSG mode, no Node required.

How does SvelteKit handle authentication?

Hooks (src/hooks.server.ts) intercept all requests; set user in event.locals; access in load functions.

Does SvelteKit work with my CSS framework?

Yes — Tailwind, UnoCSS, vanilla CSS, CSS modules all work. Svelte has built-in scoped styles.

What's the difference between Svelte and SvelteKit?

Svelte = UI library / compiler. SvelteKit = full-stack framework using Svelte. Like React vs Next.js.

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