LJSON VS monocle-ts

That actually exists and was one of inspirations: https://github.com/MaiaVictor/LJSON

Instead of the "deep path properties" syntax, you might want to take the opportunity to learn about functional lenses with monocle-ts

https://gcanti.github.io/monocle-ts/

You may be interested also in checking out the implementation of Lens in monocle-ts, one of the libraries in the fp-ts ecosystem

For anyone - anyone who's faffed around with JavaScript's spread syntax when working with nested immutable data, anyway - struggling to understand the general concept or utility of lenses, I'd encourage looking into monocle-ts. The motivation [0] that kicks off the README instantly made me a believer, and TypeScript's idiosyncratic type system makes using the library remarkably natural; it fits right in without needing to deal with anything analogous to Template Haskell.

[0] https://github.com/gcanti/monocle-ts#motivation

You could have a look at the monocle-ts library. It was designed to deal with this kind of problem.

spectacles-ts integrates seamlessly with the fp-ts ecosystem (it's built on top of the excellent monocle-ts library)

lenses help with this quite a bit, check out monocle-ts

You notice that Getters are composable as they are just pure functions, but the fact that setters are mutating values is not that nice. Tho, at this point we have invented quasi mutable lenses and If you really want you might invent actual purely functional lenses as well, but that’s for another day. Meanwhile you can take a look at an actual typescript lesses library from @gcanti / monocle-ts.