sum-types VS mori

I entirely agree about using symbols. sum-types does that.

With fp-ts and sum-types you'd handle this functionally like so:

If you're looking to have discriminated unions like Rust, follow that section to roll your own or look for a library that's already implemented them. fp-ts has everything under the sun in a Haskell style, neverthrow has Rust-like Result and ResultAsync, and sum-types helps with the boilerplate of defining your own sum types.

I'd suggest sum-types. Instead of unsafe user-defined type guards you get safe pattern matching.

If you wanted to lean (more) into FP you could replace the Event type and buildPopup function with a sum type and pattern match respectively, as well as represent side effects like in popup with fp-ts' IO, make use of function composition, etc. The former in particular is great for readability:

You can test types, at least in terms of asserting happy path types and whether a piece of code, runtime or purely type-level, should trigger a tsc type error. Here's an example using eslint-plugin-expect-type.

Anywhere that overloads are needed will rely upon type assertions or the unsafety implicit in overloads, for example in fp-ts/function::pipe. It'll come up a lot with objects too when there isn't a preexisting primitive you can compose atop of, as in for example fp-ts-std/Record::pick. Something as generic as @unsplash/sum-types has assertions all over the place, though to be fair that's mostly again a case of struggles interacting with object types.

You might like @unsplash/sum-types also.

immutable-js or mori for immutable data structures

Use persistent immutable data structures (as implemented in, for instance, mori or Immutable.js) to represent the state. As much as possible, push calculations into referentially transparent functions (i.e., input depends purely on output) which take persistent data. Write the interactions with the real world in imperative style.

Mori

Always make immutable variables. You can achieve this by having a persistence data structure using Mori or immutable.js libraries to open the features such as linked list, trees, path dealing to help with efficiency. I will not be covering how to create a persistence data structure using those libraries as it’s not the scope of this post.