(types) undici.Response cannot be assigned to Response

[panva]

Bug Description

type undici.Response cannot be assigned to type Response so that module authors can use globally available Response as a type but users may use undici as the implementation to obtain one.

Reproducible By

https://gist.github.com/panva/a350238b643aef7a77b29e6db395c3b9

git clone https://gist.github.com/panva/a350238b643aef7a77b29e6db395c3b9 response-type-reproduction
cd response-type-reproduction
npm i
npx tsc

Expected Behavior

The types should be compatible.

Logs & Screenshots

index.ts:4:3 - error TS2322: Type 'Promise<import("./node_modules/undici/types/fetch").Response>' is not assignable to type 'Promise<Response>'.
  Type 'import("./node_modules/undici/types/fetch").Response' is not assignable to type 'Response'.
    The types returned by 'headers.entries()' are incompatible between these types.
      Type 'SpecIterableIterator<[string, string]>' is missing the following properties from type 'HeadersIterator<[string, string]>': map, filter, take, drop, and 9 more.

4   return undici.fetch(url)
    ~~~~~~


Found 1 error in index.ts:4

[panva]

Okay, regardless of the "target" set in tsconfig.json there's always an error, but the above one is for "ESNext" and there is a different one otherwise, both probably need to be dealt with.

[KhafraDev]

I think the types are correct here. undici.fetch doesn't return a global response.

[panva]

I think the types are correct here. undici.fetch doesn't return a global response.

It's not the global that i'm questioning here, it's the type (well its typescript so really just the TS interfaces), and the those must conform (or extend) the global one's otherwise users cannot use an undici Response in places where the type (interface) of standard Response is expected.

Developers can opt-in to allow global-like objects by not doing an instanceof check and in those cases they will still refer to the TS type Response. But then undici Response cannot be passed solely because of types, not because the runtime not being ready for it.

[panva]

It's these helpers typings that are available in node's V8 already that are missing.

[KhafraDev]

The issue afaict is that the DOM types replaced the old iterator(s) types with a "HeadersIterator". All we should need to do is implement something similar. Lmk if my understanding of the issue is wrong :)

[Uzlopak]

I investigated this and there are two issues.

1. webkitRelativePath is expected to be an attribute of File. This has to be implemented in nodejs and in the types/node package
2. the esnext iterator helpers are not implemented.

The second one has to be implemented in undici and probably added here:

undici/lib/web/fetch/util.js

Line 968 in 44a7e91

function iteratorMixin (name, object, kInternalIterator, keyIndex = 0, valueIndex = 1) {

I guess they are trivial changes, if you know how the iterable helper functions work...
Only after those functions are added, we should change the types.

[KhafraDev]

1. No it isn't.
2. They exist. new Headers()[Symbol.iterator]().xyz