If Facebook can has patents that cover Preact, they likely cover parts of every JavaScript framework out there.

React is a functional component library that uses the virtual DOM. It's well executed, but it's a simple concept.

Yep. And if Facebook has such patents, I wouldn't be surprised if Google, Microsoft, Oracle, etc. do too.

The more you think about the ramifications, the more I come to the conclusion that software patents are an existential threat.

Tip: move to the EU.

The EU does not recognise software patents.

This is not true, since Preact only implements the same software interface. Its internals are fundamentally different and not a derivative work.

Software interfaces cannot be patented - this has been upheld in every court, most recently in Google V Oracle.

> Its internals are fundamentally different and not a derivative work.

That would be a strong defence to a copyright claim. We're talking about patents, where there is no concept of "derivative works", and what matters isn't implementations but broad concepts.

> Software interfaces cannot be patented

Right, but what makes Preact concerning isn't that it offers the same API, but that it is built using the same underlying concepts.

Preact renders independent, encapsulated, nested, functional-ish components, usually but not always written in JSX, into a virtual DOM. Components are rerendered to the virtual DOM when their inputs change, and then diffing is used to efficiently sync the virtual DOM with the browser DOM.

If Facebook has some sort of patent covering the virtual DOM (although thankfully as far as I know, they don't), it's hard to see how it would not also cover Preact. And this is true even if, as I assume is the case, Preact shares 0 lines of code with React.

Edit: To be clear, I don't believe the patent situation is concerning, I think Preact is a great project, and I may well be using it in production soon. But if I was worried about Facebook's patent portfolio, I would be worried about using Preact.

Understandable, and agreed about my point being for copyright as opposed to patents. It's interesting to note that, since the majority of DOM rendering libraries now employ some variant of Virtual DOM (react, preact, mithril, glimmer/ember, angular, even polymer!), a patent claim covering the concept of Virtual DOM itself would likely be invalid - the presence of so many prior and contemporary implementations of that paradigm make would invalidate it on the grounds that it cannot be "Non-obvious" or "non-novel". My knowledge of this stuff is largely gleaned from Wikipedia and thus not to be trusted, but it seems like there would be little grounds for a patent suit over VDOM. Also interesting to note: there are few (if any?) other companies directly associated with any of the other VDOM implementations - can a patent claim even be made against an Open Source project?