sveltekit

SvelteKit

SvelteKit is the meta-framework for Svelte, similar to Next.js for React. It uses standard Web APIs and provides routing, server-side rendering, and API endpoints.

When to Use

• Svelte Apps: The standard framework for building full-stack Svelte applications.
• Full Stack: Unified backend and frontend.
• Edge Deployment: Runs on any platform via Adapters (Vercel, Cloudflare, Netlify, Node).

Quick Start

File-based routing in src/routes.

<!-- src/routes/+page.svelte -->
<script>
  let { data } = $props(); // Received from +page.server.js
</script>

// src/routes/+page.server.js
export function load() {
  return { title: "Hello from Server" };
}

Core Concepts

Load Functions

load functions in +page.server.js run before the page renders, fetching data. The data is typed automatically in the component.

Form Actions

Handle form submissions in +page.server.js exports named actions.

export const actions = {
  default: async ({ request }) => {
    const data = await request.formData();
    // save to db
  },
};

Adapters

Deployment targets are plugins. adapter-auto, adapter-node, adapter-cloudflare.

Common Patterns

Form Actions with Progressive Enhancement

Problem: Writing separate API endpoints and client fetch handlers for simple form submissions.

Solution: Use SvelteKit Form Actions with use:enhance:

// src/routes/login/+page.server.ts
import { fail, redirect, type Actions } from "@sveltejs/kit";

export const actions: Actions = {
  default: async ({ request }) => {
    const data = await request.formData();
    const email = data.get("email");
    if (!email) return fail(400, { email, missing: true });
    // Process login
    throw redirect(303, "/dashboard");
  },
};

<!-- src/routes/login/+page.svelte -->
<script>
  import { enhance } from '$app/forms';
  export let form;
</script>

<form method="POST" use:enhance>
  <input name="email" value={form?.email ?? ''} />
  <button type="submit">Log In</button>
</form>

Best Practices

Do:

• Use Svelte 5 Runes: SvelteKit 2+ fully supports Runes mode.
• Use App.State: New SvelteKit interface for typesafe global app state.
• Stream Data: Return promises in load functions to stream non-critical data.

Don't:

• Use store for server data: Use page.data (Context) for data passing down the tree to avoid state leakage on valid server execution.
• Perform sensitive database queries or secret token lookups in client-facing +page.ts files; place them in +page.server.ts instead.

Troubleshooting

Error	Cause	Solution
500 Internal Error: Cannot load module ... in SSR	Client-only library imported directly in SSR bundle.	Use dynamic import in onMount() or check browser in $app/environment.
Not found: /route	File naming convention mismatch.	Ensure route page is named +page.svelte (with leading plus sign).
Adapter not specified	svelte.config.js missing build adapter for target platform.	Install and configure @sveltejs/adapter-auto or @sveltejs/adapter-node.

References

• SvelteKit Documentation