Server Side to Client Store Rehydration in SvelteKit (kinda)

hooks.client.js

import { user } from './store.js'

user.set(window__SK__USER__)

hooks.server.js

/** @type {import('@sveltejs/kit').Handle} */
export async function handle({ event, resolve }) {
  const user = await getUserInformation(event.cookies.get('sessionid'));

  return resolve(event, {
    transformPageChunk({ html }) {
      const scriptContent = `<script>window.__SK_USER__ = ${JSON.stringify(user)}</script>`;
      return insertScriptBeforeHeadEnd(html, scriptContent)
    }
  })
}

function insertScriptBeforeHeadEnd(htmlString, scriptContent) {
  const headEndIndex = htmlString.toLowerCase().indexOf('</head>');

  return htmlString.slice(0, headEndIndex) + scriptContent + htmlString.slice(headEndIndex);
}