HAMT: Hash Array Mapped Trie. This data structure makes efficient immutable data possible. You can update a list of a million items, and keep a reference to the original list, by changing 3 or 4 references and some bytes. This should replace copy-on-write for scripting languages. I really want to see it in a JS spec soon. There are libraries that can do it, but they add translation penalties and extra steps. I’d comp…
That’s what Immutable.js used under the hood.
Given Node is mostly single threaded as well some of the advantages (not all!) of immutability diminish on that platform making the performance penalty harder to swallow, at least for my use case.
It of course may not matter to your case, but I find the benefits of these structures come from their quicker clone/copy time in many applications, snapshot like characteristics, easy large diff's, etc. For this to matter the collection has to be of a decent size where a linear copy/equals/etc is too slow to work for the use case. At that point speed starts to matter as the costs of random lookup start to grow/drift further away from a standard mutable map and for JS we deemed the penalty too high and decided on another backend platform amenable to writing generic data structures.