I'm pretty sure that the concept (or implementation) of virtual dom wasn't invented as a part of React. IANAL but I don't think they could patent it. Ignoring specific optimizations, a virtual dom algorithm is fairly simple.
Correct, there is known prior art relating to vdoms, so any such patent would be fairly easy to defeat, IF it exists...and patents are public, and people have looked, and no such patent seems to exist.
The patent issue is really not a big deal with React; the real concern is philosophical.
The patent issue is really not a big deal with React; the real concern is philosophical.