Using a zod object as a generic argument in a nested function to narrow input types

[lediur]

Hey folks. I've recently introduced zod into my codebase, and now I'm trying to implement something to help with validating some paginated input data. I looked through the existing issues and discussions but didn't come across anything that might help me work through this.

I have various payloads of this shape from different endpoints that return paginated lists:

{
  "object": "list",
  "url": "/some/url",
  "has_more": false,
  "data": [
    // different data types here depending on the endpoint
  ]
}

Because the endpoint pagination data in the Links header, I wanted to incorporate it into the zod model to avoid having to plumb arbitrary header info through my application. I represented this in zod as:

const PaginatedList = z.object({
  object: z.literal('list'),
  url: z.string(),
  has_more: z.boolean(),
  data: z.array(z.any()),
  links: z  // <-- Note the additional links object
    .object({
      next: PaginationLinkArgs.optional(),
      prev: PaginationLinkArgs.optional()
    })
    .optional()
})

const PaginatedListSomeDataType = PaginatedList.extend({
  data: z.array(SomeDataType)
})

I'm trying to write a transformResponse function to inject the pagination info into the payload, and then parse it using the zod model. I was hoping to use generic types to work through this, but it's not quite working out.

What I have so far:

const parsePaginatedHeaders = <T extends typeof PaginatedList>(zodModel: T) => (
  elem: z.infer<T>,
  meta: FetchBaseQueryMeta | undefined
) => {
  const linkHeaderStr = meta?.response?.headers.get('Link')
  if (linkHeaderStr != null) {
    const parsedHeaders = parseLinkHeader(linkHeaderStr)
    if (parsedHeaders != null) {
      elem.links = {
        next: parsedHeaders['next'],
        prev: parsedHeaders['prev']
      }
    }
  }
  const parsed = zodModel.parse(elem)
  return parsed
}

This function itself typechecks, but the usage of the function transformResponse: parsePaginatedHeaders(PaginatedListSomeDataType) doesn't (the types of _shape don't match up).

I took a look through the type definitions and ZodObject.extend, and I believe I need to represent the merged types somehow. Thought I'd ask here first since the type definition are quite complex.

[lediur]

After working on this some more I believe this typechecks, though it seems kinda verbose:

const parsePaginatedHeaders = <
  T extends z.ZodObject<
    z.extendShape<typeof PaginatedList._shape, any>,
    "strip" | "passthrough" | "strict",  // HACK: UnknownKeysParam is not exported
    z.ZodTypeAny,
    any,
    any
  >
>(
  zodModel: T
) => (elem: z.infer<T>, meta: FetchBaseQueryMeta | undefined) => {
  const linkHeaderStr = meta?.response?.headers.get('Link')
  if (linkHeaderStr != null) {
    const parsedHeaders = parseLinkHeader(linkHeaderStr)
    if (parsedHeaders != null) {
      elem.links = {
        next: parsedHeaders['next'],
        prev: parsedHeaders['prev']
      }
    }
  }
  const parsed = zodModel.parse(elem)
  return parsed
}

Edit: ended up extracting this out into:

export type ZodExtends<
  A extends z.SomeZodObject,
  B extends z.SomeZodObject
> = z.ZodObject<
  z.extendShape<A['_shape'], B['_shape']>,
  'strip' | 'passthrough' | 'strict', // HACK: UnknownKeysParam is not exported
  z.ZodTypeAny,
  any,
  any
>

[Tanimodori]

The proper use of generic types in zod is somehow unclear in document. I've managed to work it out as follows:

// The zod model with generics type in function form
const PaginatedList = <T extends z.ZodTypeAny>(elemType: T) =>
  z.object({
    // some properties omitted
    url: z.string(),
    data: z.array(elemType),
  });

// The "dynamic type" of model "PaginatedList"
// Which extends z.ZodTypeAny
// The generic param T here is dynamic type
// Note: require typescript@^4.7 for instantiation expressions
type PaginatedListModel<T extends z.ZodTypeAny> = ReturnType<typeof PaginatedList<T>>;

// The "static type" of model "PaginatedList"
// Which is like z.infer<z.ZodTypeAny>
// The generic param T here is static type
type PaginatedList<T> = z.infer<PaginatedListModel<z.ZodType<T>>>;
/*
  = {
      url: string;
      data: T[];
  }
 */

// The parser
const definePaginatedListParser =
  <T extends z.ZodTypeAny>(zodModel: PaginatedListModel<T>) =>
  (data: z.infer<typeof zodModel>): PaginatedList<T> => {
    return zodModel.parse(data);
  };

// Using verified data in codebase
const extractData = (list: PaginatedList<string>): string[] => {
  return list.data;
};