This sounds like “persistent data structures,” which are offered in most modern FP environments (including JavaScript, if you use libraries.) Those structures

This sounds like “persistent data structures,” which are offered in most modern FP environments (including JavaScript, if you use libraries.) Those structures don’t behave as objects themselves, but it’s not uncommon for Haskell monads, for example, to have very object-like behavior when you build them up a high layer, while each step in the monad is still an immutable value with cheap mutation-copies.