syncItem.js

function syncItem(item, table, transform = item => item) {
  let open = indexedDB.open("books", 1);

  return new Promise(res => {
    open.onsuccess = evt => {
      let db = open.result;
      let tran = db.transaction(table, "readwrite");
      let objStore = tran.objectStore(table);
      objStore.get(item._id).onsuccess = ({ target: { result: itemToUpdate } }) => {
        if (!itemToUpdate) {
          objStore.add(transform(item)).onsuccess = res;
        } else {
          Object.assign(itemToUpdate, transform(item));
          objStore.put(itemToUpdate).onsuccess = res;
        }
      };
    };
  });
}