I've somehow missed this until now, but apparently IDBFactory has a .databases property.

While I could see the use for this, it can't be implemented in a non-racy way. The problem is that other processes or threads can create and delete databases at any time, so there is no way to guarantee that a database which existed when .databases is checked, will exist a few milliseconds later when the knowledge of a database's existence is used. For example

if (indexedDB.databases.contains("hello")) {
  indexedDB.open("hello").onsuccess = ...;
}

has a race condition.

Another problem is that it can't be implemented without blocking the main thread while going off to another thread or process where the indexedDB implementation lives. It'll likely also require synchronous IO to get the list of databases from file.

I suggest we simply remove it for now.