如何检查IndexedDB数据库是否已经存在？

var request = indexedDB.open("myDatabase", 1);

request.onupgradeneeded = function(event) {
  var db = event.target.result;
  if (!db.objectStoreNames.contains("myObjectStore")) {
    // 数据库不存在，可以在此处创建对象存储空间
    var objectStore = db.createObjectStore("myObjectStore", { keyPath: "id" });
  }
};

request.onsuccess = function(event) {
  var db = event.target.result;
  if (db.objectStoreNames.contains("myObjectStore")) {
    // 数据库已经存在，可以在此处执行需要的操作
    var transaction = db.transaction(["myObjectStore"], "readwrite");
    var objectStore = transaction.objectStore("myObjectStore");
    // ...
  }
};