@azure/functions@4.11.1 broken HttpResponseBodyInit typings

[sinedied]

Just updated from @azure/functions@4.11.0 to @azure/functions@4.11.1 and I get broken builds everywhere in all the samples using HTTP streaming:

src/functions/chats-post.ts:183:7 - error TS2322: Type 'Readable' is not assignable to type 'HttpResponseBodyInit'.
  Type 'Readable' is missing the following properties from type 'FormData': append, set, get, getAll, and 7 more.

183       body: jsonStream,
          ~~~~

  ../../node_modules/@azure/functions/types/http.d.ts:229:5
    229     body?: HttpResponseBodyInit;
            ~~~~
    The expected type comes from property 'body' which is declared here on type 'HttpResponseInit'

The HttpResponseBodyInit was previously defined as any, while on the new version it's:

export type HttpResponseBodyInit =
    | ReadableStream
    | Blob
    | ArrayBufferView
    | ArrayBuffer
    | FormData
    | URLSearchParams
    | string
    | null
    | undefined;

The type aliasing isn't working correctly in this case (not sure why) but this change is breaking many of our samples!

[TsuyoshiUshio]

E2E Test Report for PR #410

I've tested PR #410 against a real streaming scenario using the azure-functions-nodejs-stream sample app. Here are the results:

Test Results

Test Case	Result	Details
v4.11.1 type error occurs	✅ Pass	Confirmed Readable is not assignable to HttpResponseBodyInit
v4.11.2 (PR) type error resolved	✅ Pass	Type error eliminated, compilation succeeds
Runtime streaming works	✅ Pass	Successfully streamed a 100MB file response

Test Case 1: v4.11.1 produces type error

src/functions/streamResponse.ts:8:5 - error TS2322: Type '(request: HttpRequest, context: InvocationContext) => Promise<{ body: ReadStream; status: number; }>' is not assignable to type 'HttpHandler'.
  Type 'Promise<{ body: ReadStream; status: number; }>' is not assignable to type 'FunctionResult<HttpResponseInit | HttpResponse>'.

Test Case 2: v4.11.2 (PR branch) resolves type error

After installing the tarball built from PR branch:

• No HttpResponseBodyInit type errors on streamResponse.ts
• Only unrelated errors (missing undici types for test scripts)

Test Case 3: Runtime streaming works

> Invoke-WebRequest -Uri http://localhost:7073/api/streamResponse -UseBasicParsing

StatusCode ContentLength BodyLength
---------- ------------- ----------
       200               100663395

The 100MB file was successfully streamed through the Readable stream body.

Important Finding

v4.11.1 causes TypeScript compilation errors but runtime still works because:

1. TypeScript generates JS files even with type errors (default behavior)
2. Node.js 20+ Response constructor accepts Readable streams natively

This means Issue #409 is a build/CI pipeline failure, not a runtime failure. The PR correctly addresses this by fixing the type definition.

Conclusion

PR #410 correctly fixes this issue:

• ✅ Type safety: Readable streams can now be assigned to HttpResponseInit.body
• ✅ Backward compatibility: Existing code (string, Buffer, ReadableStream, etc.) continues to work
• ✅ Runtime behavior: Actual streaming scenarios work correctly

Recommendation: Ready to merge.