Preact, React alternative that almost certainly implements many of React's patents, but without a license to do so. Doesn't sound like a great alternative to me.
"To determine patent liability asserted by Facebook, we need to assess Facebook’s various patents and their claims. Or more specifically, which Facebook patents claim any underlying React technology? As the above illustrates, this is not a trivial question. But in my cursory search, that answer is zero. (Again, if you have examples, send them over.) Moreover, it would suprise me given the USPTO and courts general disfavor of software patents if Facebook had anything patentable in React, much less actually patented. The prior art of componetization, diffing, and virtual doms circa 2013 is very well known, and I don’t see any applicable continuation patent applications."
Do you really think they don't have a pending patent for some part of React? I bet they've got at least one covering virtual DOM.
And don't worry about prior art. As long as some part of it can weasel through a patent it's enough. Doesn't have to be the whole thing.