The IndexedDB’s syncronous API is intented to be used inside a Web worker:
The synchronous API is intended to be used only inside of Web Workers.
But since theres an ansyncronous API, whats the point of using the synchronous API in a web worker. The async API wont affect the UI thread anyway?
Synchronous API has not been implemented in any browser so far and it is marked as a risky part of the IndexedDB standard and it might be removed. And currently only Google Chrome has implemented IndexedDB access inside web workers with async api.
References W3C, Mozilla: