I recently ran into an issue with race conditions between the main thread and a worker thread clobbering each other's writes using @jakearchibald 's idb-keyval, and decided I needed a function like:

let oldValue: any = await idb.swap(
  key: string,
  expect: (oldValue: any) => boolean,
  value: any
);

as a primitive in order to implement a mutex. Unfortunately, even after forking idb-keyval and hacking on it for a bit, I wasn't able to code such a function correctly, AFAICT. 😭

I know that thread-safety is probably beyond the scope of this module (see #5) but I think it would be nice if a future version could add support for atomic operations.