fix(rest-api-sdk): preserve middleware instances in app-scoped client

Author: roncohen

packages/rest-api-sdk/src/api.ts

@@ -95,6 +95,17 @@ export class Api extends DefaultApi {
   }
 }
 
+function isPlainObject(
+  value: unknown,
+): value is Record<string, unknown> | Record<string, never> {
+  if (!value || typeof value !== "object" || Array.isArray(value)) {
+    return false;
+  }
+
+  const prototype = Object.getPrototypeOf(value);
+  return prototype === Object.prototype || prototype === null;
+}
+
 function scopeApiToAppId<T extends object>(
   api: T,
   appId: string,
@@ -118,9 +129,11 @@ function scopeApiToAppId<T extends object>(
       };
 
       return (arg1: unknown, ...rest: unknown[]) => {
-        if (arg1 && typeof arg1 === "object" && !Array.isArray(arg1)) {
+        const isWithMethod =
+          typeof prop === "string" && prop.startsWith("with");
+        if (!isWithMethod && isPlainObject(arg1)) {
           const args =
-            "appId" in (arg1 as object) ? arg1 : { ...(arg1 as object), appId };
+            "appId" in arg1 ? arg1 : { ...arg1, appId };
           const result = (value as (...args: unknown[]) => unknown).call(
             target,
             args,

packages/rest-api-sdk/test/createAppClient.test.ts

@@ -71,4 +71,36 @@ describe("createAppClient", () => {
       "https://example.test/apps/app-123/flags",
     );
   });
+
+  it("preserves class-based middleware hooks after withMiddleware", async () => {
+    const fetchApi = vi.fn(
+      async (_input: RequestInfo | URL, _init?: RequestInit) =>
+        new Response(JSON.stringify(listFlagsResponse), {
+          status: 200,
+          headers: { "Content-Type": "application/json" },
+        }),
+    );
+    const client = createAppClient("app-123", {
+      basePath: "https://example.test",
+      fetchApi,
+    });
+
+    class PrototypePreMiddleware implements Middleware {
+      preCalls = 0;
+
+      async pre({ url, init }: { url: string; init: RequestInit }) {
+        this.preCalls += 1;
+        return { url, init };
+      }
+    }
+
+    const middleware = new PrototypePreMiddleware();
+    const withMiddleware = client.withMiddleware(middleware);
+
+    await expect(
+      (withMiddleware as unknown as ScopedListFlagsClient).listFlags({}),
+    ).resolves.toEqual(listFlagsResponse);
+    expect(middleware.preCalls).toBe(1);
+    expect(fetchApi).toHaveBeenCalledTimes(1);
+  });
 });