index.js

let db;
let version = 4;
const request = indexedDB.open("database", version);
request.onerror = function(event) {
  alert("IndexedDB が使えません");
};
request.onupgradeneeded = function(event) {
  db = event.target.result;
  let txn = event.target.transaction;
  console.log(txn.objectStoreNames);
  // -> DOMStringList {0: "items", 1: "products", length: 2}
};