Quick-n-dirty IDBFactory.prototype.databases (a.k.a. indexedDB.databases) polyfill for Firefox to get all the IndexedDB database names

IDBFactory.databases.js

if (!globalThis.IDBFactory.prototype.databases) {
  const KEY = 'IDB_DATABASES_POLYFILL';

  globalThis.IDBFactory.prototype.databases = function databases() {
    const serialized = globalThis.localStorage.getItem(KEY);
    return Promise.resolve(serialized ? JSON.parse(serialized) : []);
  };

  const listener = async (event) => {
    const { name, version } = event.target.result;
    const entries = await globalThis.indexedDB.databases();

    const foundEntry = entries.find((entry) => entry.name === name);
    if (!foundEntry) entries.push({ name, version });
    else foundEntry.version = version;

    globalThis.localStorage.setItem(KEY, JSON.stringify(entries));
  };

  const boundOpen = globalThis.indexedDB.open.bind(globalThis.indexedDB);
  globalThis.indexedDB.open = function open(name, version) {
    const request = boundOpen(name, version);
    request.addEventListener('success', listener);
    return request;
  };

  const boundDeleteDatabase = globalThis.indexedDB.deleteDatabase.bind(globalThis.indexedDB);
  globalThis.indexedDB.deleteDatabase = function deleteDatabase(name) {
    const request = boundDeleteDatabase(name);

    request.addEventListener('success', async () => {
      const entries = await globalThis.indexedDB.databases();

      const idx = entries.findIndex((entry) => entry.name === name);
      if (idx === -1) return;
      entries.splice(idx, 1);
      
      globalThis.localStorage.setItem(KEY, JSON.stringify(entries));
    });

    return request;
  };
}

IDBFactory.databases.ts

if (!globalThis.IDBFactory.prototype.databases) {
  const KEY = 'IDB_DATABASES_POLYFILL';

  globalThis.IDBFactory.prototype.databases = function databases(): Promise<IDBDatabaseInfo[]> {
    const serialized = globalThis.localStorage.getItem(KEY);
    return Promise.resolve(serialized ? JSON.parse(serialized) : []);
  };

  const listener = (async (event: Event & { target: IDBOpenDBRequest }) => {
    const { name, version } = event.target.result;
    const entries = await globalThis.indexedDB.databases();

    const foundEntry = entries.find((entry) => entry.name === name);
    if (!foundEntry) entries.push({ name, version });
    else foundEntry.version = version;

    globalThis.localStorage.setItem(KEY, JSON.stringify(entries));
  }) as (event: Event) => Promise<void>;

  const boundOpen = globalThis.indexedDB.open.bind(globalThis.indexedDB);
  globalThis.indexedDB.open = function open(name: string, version?: number): IDBOpenDBRequest {
    const request = boundOpen(name, version);
    request.addEventListener('success', listener);
    return request;
  };

  const boundDeleteDatabase = globalThis.indexedDB.deleteDatabase.bind(globalThis.indexedDB);
  globalThis.indexedDB.deleteDatabase = function deleteDatabase(name: string): IDBOpenDBRequest {
    const request = boundDeleteDatabase(name);

    request.addEventListener('success', async () => {
      const entries = await globalThis.indexedDB.databases();

      const idx = entries.findIndex((entry) => entry.name === name);
      if (idx === -1) return;
      entries.splice(idx, 1);
      
      globalThis.localStorage.setItem(KEY, JSON.stringify(entries));
    });

    return request;
  };
}