localSync.js

toolbox.router.post(/graphql/, request => {
  return fetch(request).then(response => {
    let respClone = response.clone();
    setTimeout(() => {
      respClone.json().then(resp => {
        if (resp && resp.data && resp.data.updateBook && resp.data.updateBook.Book) {
          syncBook(resp.data.updateBook.Book);
        }
      }, 5);
    });
    return response;
  });
});

function syncBook(book) {
  let open = indexedDB.open("books", 1);

  open.onsuccess = evt => {
    let db = open.result;
    if (db.objectStoreNames.contains("books")) {
      let tran = db.transaction("books", "readwrite");
      let booksStore = tran.objectStore("books");
      booksStore.get(book._id).onsuccess = ({ target: { result: bookToUpdate } }) => {
        ["title", "authors", "isbn"].forEach(prop => (bookToUpdate[prop] = book[prop]));
        booksStore.put(bookToUpdate);
      };
    }
  };
}