Better support for custom fetch implementations

[u873838]

The current TypeScript typing information makes it difficult to use a custom fetch implementation with certain libraries e.g. undici.

import { fetch as undiciFetch } from "undici";
// ...
const signedFetch = createSignedFetcher({
    region,
    credentials,
    service: "service",
    fetch: (input, init) => {
        return undiciFetch(
            input as unknown as UndiciRequestInfo,
            init as unknown as UndiciRequestInit
        );
    },
}) as typeof undiciFetch;

The createSignedFetcher shouldn't be overly strict with what the arguments and return value the custom implementation has. It should also be generic so that the return type of createSignedFetcher is the same type as SignedFetcherOptions["fetch"]:

const signedFetch = createSignedFetcher({
    region,
    credentials,
    service: "service",
    fetch: undiciFetch,
});
// signedFetch is typeof undiciFetch

[zirkelc]

Hey @u873838 thanks for bringing this to my attention. I actually didn't think there would be any type issue, because this library is using the official fetch types from TypeScript DOM lib and unidici implements the Fetch standard.

I'll see what I can do and will come back to you soon!

[u873838]

You would think, but this is the error that I get when trying to use undici as the fetch option:

file.ts:10:7 - error TS2322: Type '(input: RequestInfo, init?: RequestInit | undefined) => Promise<Response>' is not assignable to type '(input: string | URL | Request, init?: RequestInit | undefined) => Promise<Response>'.
  Types of parameters 'input' and 'input' are incompatible.
    Type 'string | URL | Request' is not assignable to type 'RequestInfo'.
      Type 'Request' is not assignable to type 'RequestInfo'.
        Property 'referrer' is missing in type 'import("<project>/node_modules/undici-types/fetch").Request' but required in type 'import("<project>/node_modules/undici/types/fetch").Request'.

64       fetch: undiciFetch,
         ~~~~~

  node_modules/undici/types/fetch.d.ts:166:12
    166   readonly referrer: string
                   ~~~~~~~~
    'referrer' is declared here.
  node_modules/aws-sigv4-fetch/dist/index.d.cts:7:5
    7     fetch?: typeof fetch;
          ~~~~~
    The expected type comes from property 'fetch' which is declared here on type 'SignedFetcherOptions'