Again FWIW, my experience is that Immutable.js is a decent library, though the documentation is not very helpful.<p>Storing application state within a single immutable tree might be counter-intuitive but doesn&#x27;t really make much difference to anything important. That data was going to be stored somewhere anyway, access is reasonably efficient in JS, and as you say, sometimes it has useful practical advantages.<p>One thing I would say is that nothing actually requires you to store all of your data in a single immutable tree like that. I&#x27;ve found some patterns tend to come up often when designing UIs with this sort of architecture, and in those patterns a small number of immutable data structures are used instead of one monolithic one.<p>For example, sometimes it&#x27;s advantageous to have one structure that contains strictly the underlying persistent state that you&#x27;ll ultimately store in some database on the back end. If you also need some supporting data derived from that first structure for presentation purposes, you can still create a second, also immutable structure, that is basically a clone of the first but then with the supplementary data added. Doing this sort of manipulation is usually reasonably efficient with Immutable.js because you basically have copy-on-write semantics for everything, and the logic for deriving the supplementary data tends to be easily tested.