[libdom] Allow Response / Request to be generics

[karlhorky]

Similar to the FormData request by @wesbos in microsoft/TypeScript#43797, it would be great if the Request and Response globals could also accept generic type parameters.

This will become more relevant as more frameworks continue to increase their usage of these standard globals, also in Node.js.

Consider the following example of an App Route handler in Next.js (handling a POST request to an API route):

export async function POST(request: Request): Response {
  const body = await request.json();
     // ^? const body: any ❌
  return new Response(JSON.stringify({
    abc: 123, // ❌ how can the POST function return type check this object?
  }))
}

What would be amazing would be something similar to this:

type RequestBody = { zzz: number };
type ResponseBody = { abc: number };

export async function POST(request: Request<RequestBody>): Response<ResponseBody> {
  const body = await request.json();
     // ^? const body: { zzz: number } ✅
  return new Response(JSON.stringify({
    abc: 123, // ✅ type checked
  }))
}

Workaround

Response

Wasn't able to find a nice workaround for typing the body of a Response yet.

Request

Using Type Assertion / casting via as:

type RequestBody = { zzz: string };

export async function POST(request: Request) {
  const body = await request.json() as RequestBody;

Caveats

1. Arguably, the Request is unknown until runtime, so maybe this should be handled instead with Zod or some other runtime validation

[karlhorky]

Oops, somehow reported this in the wrong repo, moved over here: microsoft/TypeScript#52777