syncImages.js

function syncImages(db) {
  let tran = db.transaction("books");
  let booksStore = tran.objectStore("books");
  let idx = booksStore.index("imgSync");
  let booksCursor = idx.openCursor(0);
  let booksToUpdate = [];

  booksCursor.onsuccess = evt => {
    let cursor = evt.target.result;
    if (!cursor) return runIt();

    let book = cursor.value;
    booksToUpdate.push({ _id: book._id, smallImage: book.smallImage });
    cursor.continue();
  };

  async function runIt() {
    if (!booksToUpdate.length) return;

    for (let book of booksToUpdate) {
      try {
        await preCacheBookImage(book);
        let tran = db.transaction("books", "readwrite");
        let booksStore = tran.objectStore("books");
        await new Promise(res => {
          let req = booksStore.get(book._id);
          req.onsuccess = ({ target: { result: bookToUpdate } }) => {
            bookToUpdate.imgSync = 1;
            booksStore.put(bookToUpdate);
            res();
          };
          req.onerror = () => res();
        });
      } catch (er) {
        console.log("ERROR", er);
      }
    }
  }
}