As I understand it, immutability makes management of state on complex objects easier to deal with. Whereas ImmutableJS the library makes immutability performant in JS. Someone please correct me if I'm wrong because I cannot seem to locate my source anymore, but I believe what ImmutabilityJS does is not actually deep-copy complex objects, but reuse references to nested objects that have not changed. Such that you get a new object, but not everything in the object is new, which is what keeps things fast.