Select on infinite query can't return a different type

[agos]

Describe the bug
It appears that using select on an InfiniteQuery is limited in potential if using Typescript, as the return type of the select function can't be different from the structure of the passed parameter. This would be useful, for instance to flatMap the pages onto a single list - in issue #1187 it was hinted so ("it should be easy to map/filter/flatten here")

To Reproduce
Steps to reproduce the behaviour:

1. Create an infinite query
2. Add a select function
3. Make the select function return something different from { pages, pageParams }
4. If not returning page & pageParams, type check will fail. Returning an additional field will work, but trying to use it will make type checking fail.

Expected behavior
It should be either possible to

1. return additional fields and have them accessible where you use the Query
2. return a completely different structure from the select function

[boschni]

A flatmap should be possible like this:

useInfiniteQuery('key', fetchPages, {
  select: data => ({
    pages: data.pages.flatMap(x => x),
    pageParams: data.pageParams
  })
})

[agos]

@boschni that works for flattening, but I have to keep the pages name even if it's not descriptive anymore. It's also not possible to return anything else than an array, so it's still not possible to add more fields

[boschni]

I don't think it would be easy to implement something like that but PRs are welcome!

[AndreyChernykh]

I've recently had the same issue. In JavaScript I have no problem doing this:

useInfiniteQuery('key', fetchPages, {
  select: data => ({
    return data.pages.flatMap(x => x)
  })
})

In TypeScript we had to use useMemo as a clunky alternative to select to achieve the similar result.

The problem is that select wraps anything that is returned from there into InfiniteData<T>, and there is no way to override this type (at least I couldn't find one)

https://github.com/tannerlinsley/react-query/blob/52854795b62946b4a4b73e4bddc6954db77e2058/src/core/types.ts#L225

not sure how to fix that properly, though

@tannerlinsley cc

[TkDodo]

There should be a way to get this done on type level as it works at runtime, too, but the last time I attempted it, I didn't get very far :/

[TkDodo]

we're now tracking this here: #3065