[graphql-client] Improves error handling for missing Response.body in…

… multipart requests

.changeset/funny-mangos-study.md

@@ -0,0 +1,5 @@
+---
+'@shopify/graphql-client': patch
+---
+
+Better error handling for missing Response.body in multipart requests. Instead of being "Cannot read properties of undefined (reading 'Symbol(Symbol.asyncIterator)')", it will now be the more useful and accurate message "API multipart response did not return an iterable body".

packages/api-clients/graphql-client/src/graphql-client/graphql-client.ts

@@ -365,7 +365,7 @@ function createMultipartResponseAsyncInterator(
 
   if (
     !response.body?.getReader &&
-    !(response.body as any)![Symbol.asyncIterator]
+    !(response.body as any)?.[Symbol.asyncIterator]
   ) {
     throw new Error('API multipart response did not return an iterable body', {
       cause: response,

packages/api-clients/graphql-client/src/graphql-client/tests/graphql-client/client-requestStream.test.ts

@@ -712,6 +712,36 @@ describe('GraphQL Client', () => {
                   });
                 });
 
+                it('returns an async iterator that returns a response object with an error when Response.body is undefined', async () => {
+                  const mockedResponse = new Response(undefined, {
+                    status: 200,
+                    headers: new Headers({
+                      'Content-Type': 'multipart/mixed; boundary=graphql',
+                    }),
+                  });
+                  Object.defineProperty(mockedResponse, 'body', {
+                    value: undefined,
+                    writable: true,
+                  });
+
+                  fetchMock.mockResolvedValue(mockedResponse);
+
+                  const responseStream = await client.requestStream(operation);
+
+                  const results: any = [];
+                  for await (const response of responseStream) {
+                    results.push(response);
+                  }
+
+                  expect(results[0].errors).toEqual({
+                    message:
+                      'GraphQL Client: API multipart response did not return an iterable body',
+                    networkStatusCode: 200,
+                    response: mockedResponse,
+                  });
+                  expect(results[0].data).toBeUndefined();
+                });
+
                 it('returns an async iterator that returns a response object with no data value and a JSON parsing error if the returned data is a malformed JSON', async () => {
                   const multipleResponsesArray = [
                     `