fix(core): correct useCustom return type during loading

- Changed result.data to return undefined instead of empty object while loading
- Updated type to CustomResponse<TData>[data] | undefined
- Added test to verify undefined data during loading state

Resolves #7088

Author: alicanerdurmaz

.changeset/four-ads-sin.md

@@ -0,0 +1,9 @@
+---
+"@refinedev/core": patch
+---
+
+fix(core): correct `useCustom` return type to reflect undefined data during loading #7088
+
+Fixed a type safety issue where `useCustom` returned an empty object (`{}`) while loading but TypeScript type was `CustomResponse<TData>["data"]`. This caused runtime errors like `result.data.map is not a function` when accessing data without proper checks. Now `result.data` returns `undefined` while loading with type `CustomResponse<TData>["data"] | undefined`, requiring proper null checks like `result.data?.map()`.
+
+Resolves #7088

packages/core/src/hooks/data/useCustom.spec.tsx

@@ -27,9 +27,47 @@ describe("useCustom Hook", () => {
 
     const { data } = result.current.result;
 
+    expect(data).toBeDefined();
     expect(data).toHaveLength(2);
   });
 
+  it("should return undefined data while loading", async () => {
+    const { result } = renderHook(
+      () =>
+        useCustom({
+          url: "remoteUrl",
+          method: "get",
+        }),
+      {
+        wrapper: TestWrapper({
+          dataProvider: {
+            default: {
+              ...MockJSONServer.default,
+              custom: () => {
+                return new Promise((res) => {
+                  setTimeout(() => res({ data: [1, 2, 3] } as any), 100);
+                });
+              },
+            },
+          },
+          resources: [{ name: "posts" }],
+        }),
+      },
+    );
+
+    // While loading, data should be undefined
+    expect(result.current.query.isPending).toBeTruthy();
+    expect(result.current.result.data).toBeUndefined();
+
+    await waitFor(() => {
+      expect(result.current.query.isSuccess).toBeTruthy();
+    });
+
+    // After loading, data should be defined
+    expect(result.current.result.data).toBeDefined();
+    expect(result.current.result.data).toEqual([1, 2, 3]);
+  });
+
   describe("without custom query key", () => {
     const config = { sorters: [{ field: "id", order: "desc" }] } as any;
     const meta = { meta: "meta" };

packages/core/src/hooks/data/useCustom.ts

@@ -110,12 +110,10 @@ export type UseCustomProps<TQueryFnData, TError, TQuery, TPayload, TData> = {
 export type UseCustomReturnType<TData, TError> = {
   query: QueryObserverResult<CustomResponse<TData>, TError>;
   result: {
-    data: CustomResponse<TData>["data"];
+    data: CustomResponse<TData>["data"] | undefined;
   };
 } & UseLoadingOvertimeReturnType;
 
-const EMPTY_OBJECT = Object.freeze({}) as any;
-
 export const useCustom = <
   TQueryFnData extends BaseRecord = BaseRecord,
   TError extends HttpError = HttpError,
@@ -233,7 +231,7 @@ export const useCustom = <
     return {
       query: queryResponse,
       result: {
-        data: queryResponse.data?.data || EMPTY_OBJECT,
+        data: queryResponse.data?.data,
       },
       overtime: { elapsedTime },
     };