allow to type Params of RequestHandler

[ivanhofer]

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Currently it is not possible to type Params of a RequestHandler handler function

import type { RequestHandler } from '@sveltejs/kit'

type ReturnType = { id: string }

// current
export const get: RequestHandler<ReturnType> = ({ params }) => {
   return {
      body: {
         // `params.id` is of type `string | undefined`
         id: params.id
      },
   }
}

// with the changes of this PR
type ParamsType = { id: string }

export const get: RequestHandler<ParamsType, ReturnType> = ({ params }) => {
   return {
      body: {
         // `params.id` is of type `string`
         id: params.id
      },
   }
}

[changeset-bot]

🦋 Changeset detected

Latest commit: 44ad923

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@sveltejs/kit	Patch

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[dummdidumm]

If we add this, I feel like we should make this a breaking change and switch the order of generics. Feels strange to me to have the input generic be the second generic, but maybe that's just me

[ivanhofer]

I was also thinking about this, but didn't want to introduce a breaking change. I'll update it

[benmccann]

this PR would need to be rebased

[ivanhofer]

this PR would need to be rebased

done

[Rich-Harris]

agree that input should go first, though i'll note that if we manage to pull off #647 we'll have to revert this breaking change with another. i don't think that's a reason not to do it though