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I think you would use discrimated unions.

const MyResult = z.discriminatedUnion("status", [ z.object({ status: z.literal("success"), data: z.string() }), z.object({ status: z.literal("failed"), error: z.string() }), ]);

You can define passthrough behavior if there are a bunch of special attributes for a moderator but you don't want to list/check them all.

With different methods that have different schema- If they share part of the same schema with alterations, you can define an object for the shared part and create objects that contain the shared object schema with additional fields.

If you have a lot of different possibilities, it will be messy, but it sounds like it already is for you, so validators could still at least validate the messines.

Hmm, this is pretty interesting, but I still have issues. While Zod seems to be able to transform my server-returned object into a discriminated union, it doesn't seem to be able to tell which of the unions it is. For instance, say that api/user/mod-panel (e.g.) returns only moderator user objects. My API endpoint already knows that it will have moderator-related fields, but Zod forgets that when I parse it.

Then you don't use the discriminated union for that endpoint, but the schema that only accepts moderator user objects.

That's not surprising, "this endpoint will only return moderator user objects" is a bit of knowledge that has to be represented in code somehow.

You can use an ‘is’ method to pick out a type from union. Although, if union is discriminated, it’s discriminated with a ‘kind’ so you should be able to know which kind you received.

I am not totally sure what your API's look like but if you know the API only has a certain type (or subset of types), you would only validate that type on the method. Don't use a single union for everything.

Here is some pseudocode.

Person = { name: string, height: number }

Animal = {name: string, capability: string}

A = { post: object, methodType: string, person: Person }

ModeratorA = { post: object, moderatorField1: string, moderatorField2: string, person: Person }

UnionA = A && ModeratorA (There's probably a better way of defining A and ModeratorA to share the shared fields)

B = { post: object, animal: Animal }

endpoint person parses UnionA

endpoint animal parses B

You don't put all of your types in one big Union.

You can define all the different sub types separately and make the discriminated union using them. The use only a single type for an endpoint if it only uses a single type. Only use the discriminated union where you actually might be handling multiple types.

In an ideal world you'd have one source of truth for what the shape of a User could be (which may well be a discriminated union of User and AnonymousUser or similar).

Without fullstack TS this could look something like: (for a Python backend) Pydantic models+union for the various shapes of `User`, and then OpenAPI/GraphQL schema generation+codegen for the TS client.

The problem with this is that your One True User shape tends to have a bunch of optional properties on it. e.g., in the user profile you fetch Post[], but in the user directory you don't, and so on with other joined properties. If every endpoint returns the One True User, then you end up needing to write conditional logic to guard against (say) `.posts` when you fetch users in the profile, even though you know that `.posts` exists.

In Typescript at least, if the discriminated union is set up correctly, you just need a single check of the type field. That lets TS narrow the type so it knows whether e.g. `.posts` is present or not.

I think I can do this, yes, but it becomes a very large amount of repetitive work.

Let's say I have api/profile (which has `.posts`) and api/user-directory (which does not). I define User as a discriminated union - one has type `user-with-posts` and one has type `user-no-posts`. OK, good so far. But now say I have a photo gallery, which returns photos on user. Now I have to add that into my discriminated union type, e.g. `user-with-posts-and-photos`, `user-with-posts-but-not-photos`, `user-with-no-posts-but-yes-photos`... and it gets worse if I also have another page with DMs...

You can use a string union to discriminate when it makes sense, but that's not the only way to discriminate and in this case you'd instead use the presence of the items themselves (essentially duck-typing with strong type guarantees)

Typescript playground: https://www.typescriptlang.org/play/?#code/C4TwDgpgBACg9gZ2F...

Now you run into the issue I mentioned in GP, where you end up writing `if (blah)` everywhere, even though you know that `blah` is definitely present.

Change the design of the API so that the posts and the photos and so on are not returned inside the user object but on the same level as it.

So {user: {...}, photos: [...]}, not {user: {..., photos: [...]}}.

Alternatively define separate schemas for each endpoint that extend the base User schema. But I'd prefer having the same structure everywhere as much as possible.

Not sure how other stacks solve this, but with GraphQL the backend defines a `User` type with a full set of fields, and the client specifies only the fields it wants to query. And with codegen you get type safety.

So on the /posts page the client asks for `{ user: { id, posts: { id, content }[] } }`, and gets a generated properly-typed function for making the query.

It's hard to unpack without knowing more about the use case, but adding discriminant properties (e.g. "user_type") to all the types in the union can make it easier to handle the general and specific case.

E.g.

if (user.user_type === 'authenticated') {

  // do something with user.name because the type system knows we have that now

}

This doesn't help in your case, but this is what GraphQL was invented for.

TS libraries for GQL queries can dynamically infer the response shape from the shape of the selected fields.

Such that the query

    query {
        user {
            username
            posts {
                text
            }
        }
    }

Would be

    type Response = {
        user: {
            username: string
            posts: Array<{
                text: string
            }>
        }
    }

And a query with just { user { username } } would have the posts property omitted entirely.

Your server should export the types. Don't write types by hand in your client, that makes no sense. The server knows what data it is sending and should provide that information as metadata to the client. Practically this can mean that a Python backend uses Pydantic schemas which can be used to automatically generate an OpenAPI specification that your client can use to generate types.

This answer is how I wish the world would work, but I am stuck with a server with a poor type system that can't refine the types nearly as accurately as TS can. :(

Either Unions, or optional fields