sveltekit-data-flow

name	sveltekit-data-flow
description	SvelteKit data flow guidance. Use when working with load functions, form actions, server/client boundaries, serialization, or invalidation.
metadata	{"last_updated":"2026-06-08","verified_against":"Svelte 5/Kit docs and sveltejs/kit#15934"}

SvelteKit Data Flow

Quick Start

Environment variables: Prefer explicit env vars (src/env.ts + $app/env/private|public) when the experimental flag is enabled.

Which file? Server-only (DB/secrets): +page.server.ts | Universal (runs both): +page.ts | API: +server.ts

Load decision: Need server resources? → server load | Need client APIs? → universal load

Form actions: Always +page.server.ts. Return fail() for errors, throw redirect() to navigate, throw error() for failures.

Example

// +page.server.ts
import { fail, redirect } from '@sveltejs/kit';

export const load = async ({ locals }) => {
	const user = await db.users.get(locals.userId);
	return { user }; // Must be JSON-serializable
};

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

		if (!email) return fail(400, { email, missing: true });

		await updateEmail(email);
		throw redirect(303, '/success');
	},
};

Reference Files

• load-functions.md - Server vs universal
• form-actions.md - Form handling patterns
• serialization.md - What can/can't serialize
• error-redirect-handling.md - fail/redirect/error
• client-auth-invalidation.md - invalidateAll() after client-side auth
• explicit-environment-variables.md - typed, validated env vars

Notes

• Server load → universal load via data param | ALWAYS throw redirect()/error()
• No class instances/functions from server load (not serializable)
• $app/env/private is server-only; only mark genuinely safe values as public
• Last verified: 2026-06-08