Redux requires that reducers not mutate the state object in any way. Object.freeze() only works at the top level so there are a large variety of utilities to enable deeper protection. They can also offer some performance improvements based on their immutability guarantees.

The most commonly adopted implementation (and the one endorsed at the talk I went to last week) is ImmutableJS by Facebook. However, it doesn't actually guarantee immutability:

https://github.com/facebook/immutable-js/issues/546

And the syntax is... weird. Very many of the React related tools have special support for it. But perhaps not everything we may one day want to use. There are several alternatives, one that I'm leaning towards is seamless-immutable, they make a good case for it here:

http://tech.noredink.com/post/107617838018/switching-from-immutablejs-to-seamless-immutable

But it seems like that company is moving to use Elm for everything rather than Javascript so I wonder how well it will be maintained going forward? OTOH, not exactly an OS, fairly simple thing and has enough traction that it probably is okay?

https://github.com/rtfeldman/seamless-immutable
http://tech.noredink.com/post/136615783598/welcome-evan

At the moment I'm playing with it on the experiments repo as that's what the redux-crud package uses for its reducers. However it's very likely we'll reimplement that for OKAPI-specific crud anyway (OLE-89)