SvelteKit Data Flow

Quick Start

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

• references/load-functions.md - Server vs universal loads
• references/form-actions.md - Form handling patterns
• references/serialization.md - What can/can't serialize
• references/error-redirect-handling.md - fail(), redirect(), error()

Notes

• Server load output is automatically passed to universal load as data parameter
• ALWAYS rethrow redirects/errors: throw redirect(), throw error()
• Don't return class instances or functions from server load (not serializable)
• Last verified: 2025-01-11