I've spent a lot of time searching for this API, and only found it in thedeclaration file.
If I'm mistaking for it's need, I would be more then grateful to learn.

I believe there's another example that shows the getState() version - perhaps we'd be better off showing a synchronous function for this example? I just wanted to have a way to show both.

Well, my difficulty was that I just could not understand how one can manipulate the state object after an async operation happened, and expect the results to be valid, as the state could change during that period, but the state object being manipulated is the old state. If you think there is no need in this example I understand, I just though it will save some people from having bugs caused by an asynchronous behavior. Thanks!