src/hooks.js

import cookie from 'cookie';
import { v4 as uuid } from '@lukeed/uuid';

export const handle = async ({ request, resolve }) => {
	const cookies = cookie.parse(request.headers.cookie || '');
	request.locals.userid = cookies.userid || uuid();

	// TODO https://github.com/sveltejs/kit/issues/1046
	const method = request.url.searchParams.get('_method');
	if (method) {
		request.method = method.toUpperCase();
	}

	const response = await resolve(request);

	if (!cookies.userid) {
		// if this is the first time the user has visited this app,
		// set a cookie so that we recognise them when they return
		response.headers['set-cookie'] = cookie.serialize('userid', request.locals.userid, {
			path: '/',
			httpOnly: true
		});
	}

	return response;
};