fix(fetch): improve Headers and Request type-compatibility

[kettanaito]

• Fixes Fetch types are incompatible with "lib/dom" definitions #1943

I'm using a simple reproduction example to test type congruence of Undici against lib/dom:

import { Request as UndiciRequest } from 'undici'

function transform(r: Request): void {}

transform(new UndiciRequest('/hello'))
// ^^^ This won't compile in TypeScript!

[kettanaito]

It appears that the iterator symbol on Headers must be iterable.

[kettanaito]

Also narrowing down the referrer policy as we already have the ReferrerPolicy type used for RequestInit. It cannot really be anything else, to my best knowledge.

[kettanaito]

Adding the missing referrer property.

[kettanaito]

With these changes, I can eliminate most of the type-related errors in my usage project. However, there seems to be an incongruence between the Request.body types. The ReadableStream<R> type from stream/web is incompatible with the ReadableStream<T> type from lib/dom. Here, I'm hesitant to call stream/web the issue because it looks like lib/dom contains outdated types and has issues raised regarding this (DefinitelyTyped/DefinitelyTyped#58252). Sadly, those were never addressed.

There are a few waves of errors that happen:

1. ReadableStream<any> (Undici) is incompatible with ReadableStream<Uint8Array> (lib/dom)

This can be rather simply addressed by:

-body: ReadableStream | null
+body: ReadableStream<Uint8Array> | null

However, another issues arises from this.

2. ReadableStream<Uint8Array> (Undici*) is not compatible with ReadableStream<Uint8Array>

Argument of type 'import("/undici/types/fetch").Request' is not assignable to parameter of type 'Request'.
  Types of property 'body' are incompatible.
    Type 'import("stream/web").ReadableStream<Uint8Array> | null' is not assignable to type 'ReadableStream<Uint8Array> | null'.
      Type 'import("stream/web").ReadableStream<Uint8Array>' is not assignable to type 'ReadableStream<Uint8Array>'.
        The types returned by 'getReader(...).read(...)' are incompatible between these types.
          Type 'Promise<ReadableStreamDefaultReadResult<Uint8Array>>' is not assignable to type 'Promise<ReadableStreamReadResult<T>>'.
            Type 'ReadableStreamDefaultReadResult<Uint8Array>' is not assignable to type 'ReadableStreamReadResult<T>'.
              Type 'ReadableStreamDefaultReadValueResult<Uint8Array>' is not assignable to type 'ReadableStreamReadResult<T>'.
                Type 'ReadableStreamDefaultReadValueResult<Uint8Array>' is not assignable to type 'ReadableStreamReadValueResult<T>'.
                  Types of property 'value' are incompatible.
                    Type 'Uint8Array' is not assignable to type 'T'.
                      'Uint8Array' is assignable to the constraint of type 'T', but 'T' could be instantiated with a different subtype of constraint 'ArrayBufferView'.

This has to do with proper generics usage, my head is too cloudy right now to remember how to resolve it. Will circle back to it later.

[KhafraDev]

changes look fine, the body type fix can wait for another PR (although ReadableStream<Uint8Array> is correct)

[kettanaito]

By the way, I don't mind lending a hand with type testing in this repo. I've worked some time with it and it has proven rather useful. You can see an example of it in here. Basically, with a bit of a configuration, we can automate the type-checking of Undici examples against different versions of TypeScript even, and use those as tests to catch type-related regressions. I think in the light that Undici is not authored in TypeScript, type tests stand twice as useful.

[mcollina]

By the way, I don't mind lending a hand with type testing in this repo. I've worked some time with it and it has proven rather useful. You can see an example of it in here. Basically, with a bit of a configuration, we can automate the type-checking of Undici examples against different versions of TypeScript even, and use those as tests to catch type-related regressions. I think in the light that Undici is not authored in TypeScript, type tests stand twice as useful.

Any help is appreciated! Go for it.

---

Can this PR land? It's still marked as draft.

[kettanaito]

Hey, @mcollina. It's ready at its current state. Is it okay that some tests are failing on CI? They also fail for me locally (on main) so I assume that's not related to the changes.

[mcollina]

I think there was a bug in one of the deps that was fixed.

[mcollina]

Could you add a test for the types? We use tsd.

[kettanaito]

I'm a bit overwhelmed with work, I will add the test this/next week.

[kettanaito]

I love GitHub. Let's put two buttons with totally opposite actions 3px away from each other. Sorry, will re-open.