index.js

let db;
let version = 2;
const request = indexedDB.open("database", version);
request.onerror = function(event) {
  alert("IndexedDB が使えません");
};
request.onsuccess = function(event) {
  db = event.target.result;
  db.onclose = function(event) {
    console.log('IndexeDBを閉じました');
  }
}