I considered ImmutableJS a little while back. It's a great library but it makes you to a lot of work to go back and forth between native JS objects, and just abstracts you from the real objects underneath.

Instead I ended up just deepFreezing my data (only in development) and wrote a bunch of pure functions which perform all the important array/object functions but return a new array/object instead of mutating. It was a really great exercise in functional programming and I'd recommend it to anyone wanting immutable data and to understand immutable programming styles.