[mlajtos]

In JavaScript world, there is ImmutableJS, which uses this style of updates. However, there is better approach which is what ImmerJS uses – you write a function where you can do mutation to the target data structure, wrap it in a "produce" function, and library will pick on those mutations and create a new copy of the data that is structurally shared with the original.

https://immerjs.github.io/immer/

[simongray]

Yeah, that is basically what that Clojure code does. All of the data structures in Clojure are persistent and use structural sharing.

[dwohnitmok]

That Clojure code isn't quite grasping the essence of ImmerJS. The whole point of ImmerJS is that JS has nice, built-in syntax for in-place mutation and we can reuse that syntax to generate updates to an immutable data structure so long as we scope the mutation syntax so that outside of an individual block of mutation syntax everything stays immutable. That it is implemented with JS proxies is something of an implementation detail (it could e.g. be implemented with linear or affine types or something like Haskell's ST type).

In this sense it's closer to Clojure's transients, if Clojure came prepackaged with special mutation syntax (notably assignment via =) and if transient-ness could be propagated transitively to all nested structures in a data structure.