We use ExtJS a lot. But I think the exact opposite is the case. With ExtJS, you have the usual double-binding, e.g. if you set this property of that Model somewhere in your view hierarchy some little snippet of HTML are inserted/removed from the DOM. It's basically side effects only.

If I understood it correctly, one can think of react app/page as a pure function taking data and producing a DOM.

Reasoning and testing pure functions should be much easier than a set of components which partly rely on side effects.

I've used both React and ExtJS. Overall React is much easier. ExtJS becomes a mess of event handlers, just take a look at all the possible events when working with their trees. In theory it looks great, in practice it's hard to follow the code and performance is terrible. The two way binding causes all sorts of headaches including the usual remove listener, update model, add listener type code. It's difficult to follow what code updates what part of the DOM. With React I only need to look at a component's render function. Same with the component's state.