loadBookProgress.js

import cab from './cab.js'; // the file from above

const getOrCreateProgressData = async () => {
  // Get the ID from the URL
  const url = new URL(document.location);
  const existingId = url.searchParams.get('id');

  // If there was an ID, fetch the associated data
  if (existingId) {
    const readResponse = await cab.read(existingId);
    return {id: existingId, data: readResponse.data};
  }

  // If there was no ID, create a new record
  const initialData = {scrollPos: 0};
  const createResponse = await cab.create(initialData);

  // Put the new ID in the URL
  url.searchParams.set('id', createResponse.id);
  window.history.replaceState('', null, url.href);

  return {id: createResponse.id, data: initialData};
};

// Kickoff
(async () => {
  const {id, data} = await getOrCreateProgressData();

  // If there was a scroll position, apply that now
  if (data.scrollPos !== 0) window.scrollY = data.scrollPos;

  // And keep the database up to date with the scroll position
  window.addEventListener('scroll', () => {
      cab.update(id, {scrollPos: window.scrollY});
  });
})();