communicate-with-sw.js

//in web app
if ("serviceWorker" in navigator) {
  try {
    navigator.serviceWorker.controller.postMessage({ command: "sync-images" });
  } catch (er) {}
}

//in sw-manual.js
self.addEventListener("message", evt => {
  if (evt.data && evt.data.command == "sync-images") {
    let open = indexedDB.open("books", 1);

    open.onsuccess = evt => {
      let db = open.result;
      if (db.objectStoreNames.contains("books")) {
        syncImages(db);
      }
    };
  }
});