diff --git a/packages/core/src/data-editor/row-grouping-api.ts b/packages/core/src/data-editor/row-grouping-api.ts
index 988de9013..035fef94c 100644
--- a/packages/core/src/data-editor/row-grouping-api.ts
+++ b/packages/core/src/data-editor/row-grouping-api.ts
@@ -1,18 +1,31 @@
 import React from "react";
 import type { Item } from "../internal/data-grid/data-grid-types.js";
-import { flattenRowGroups, mapRowIndexToPath, type RowGroup, type RowGroupingOptions, type MapResult } from "./row-grouping.js";
+import { flattenRowGroups, mapRowIndexToPath, type FlattenedRowGroup, type RowGroup, type RowGroupingOptions } from "./row-grouping.js";
 
-export interface RowGroupingMapperResult<T> extends Omit<MapResult, 'originalIndex'> {
+export interface RowGroupingMapperResult<T> {
     path: readonly number[];
-    originalIndex: T;
     isGroupHeader: boolean;
     groupRows: number;
+    originalIndex: T;
+    readonly groupIndex: number;
+    readonly contentIndex: number;
 }
 
-export type RowGroupingMapper = {
-    (itemOrRow: number): RowGroupingMapperResult<number>;
-    (itemOrRow: Item): RowGroupingMapperResult<Item>;
-};
+export type RowGroupingMapper = (itemOrRow: number | Item) => RowGroupingMapperResult<Item | number>;
+
+export function rowGroupingMapper(itemOrRow: number, flattenedRowGroups?: readonly FlattenedRowGroup[] | undefined): RowGroupingMapperResult<number>;
+export function rowGroupingMapper(itemOrRow: Item, flattenedRowGroups?: readonly FlattenedRowGroup[] | undefined): RowGroupingMapperResult<Item>;
+export function rowGroupingMapper(itemOrRow: number | Item, flattenedRowGroups?: readonly FlattenedRowGroup[] | undefined): RowGroupingMapperResult<number> | RowGroupingMapperResult<Item>;
+export function rowGroupingMapper(itemOrRow: number | Item, flattenedRowGroups?: readonly FlattenedRowGroup[] | undefined): RowGroupingMapperResult<number> | RowGroupingMapperResult<Item> {
+    if (typeof itemOrRow === "number") {
+        return mapRowIndexToPath(itemOrRow, flattenedRowGroups);
+    }
+    const r = mapRowIndexToPath(itemOrRow[1], flattenedRowGroups);
+    return {
+        ...r,
+        originalIndex: [itemOrRow[0], r.originalIndex],
+    } as RowGroupingMapperResult<Item>;
+}
 
 export interface UseRowGroupingResult {
     readonly mapper: RowGroupingMapper;
@@ -34,16 +47,7 @@ export function useRowGrouping(options: RowGroupingOptions | undefined, rows: nu
         getRowGroupingForPath,
         updateRowGroupingByPath,
         mapper: React.useCallback<UseRowGroupingResult["mapper"]>(
-            (itemOrRow: number | Item) => {
-                if (typeof itemOrRow === "number") {
-                    return mapRowIndexToPath(itemOrRow, flattenedRowGroups);
-                }
-                const r = mapRowIndexToPath(itemOrRow[1], flattenedRowGroups);
-                return {
-                    ...r,
-                    originalIndex: [itemOrRow[0], r.originalIndex],
-                } as any; // FIXME
-            },
+            (itemOrRow: number | Item) => rowGroupingMapper(itemOrRow, flattenedRowGroups),
             [flattenedRowGroups]
         ),
     };
diff --git a/packages/core/src/data-editor/row-grouping.ts b/packages/core/src/data-editor/row-grouping.ts
index f226dd93b..d975fb7b5 100644
--- a/packages/core/src/data-editor/row-grouping.ts
+++ b/packages/core/src/data-editor/row-grouping.ts
@@ -3,6 +3,7 @@ import type { Theme } from "../common/styles.js";
 import type { DataEditorProps } from "./data-editor.js";
 import type { DataGridProps } from "../internal/data-grid/data-grid.js";
 import { whenDefined } from "../common/utils.js";
+import type { RowGroupingMapperResult } from "./row-grouping-api.js";
 
 type Mutable<T> = {
     -readonly [K in keyof T]: T[K];
@@ -177,17 +178,9 @@ export function flattenRowGroups(rowGrouping: RowGroupingOptions, rows: number):
         });
 }
 
-export interface MapResult {
-    readonly path: readonly number[];
-    readonly isGroupHeader: boolean;
-    readonly originalIndex: number;
-    readonly groupIndex: number;
-    readonly groupRows: number;
-    readonly contentIndex: number;
-}
 
 // grid relative index to path and other details
-export function mapRowIndexToPath(row: number, flattenedRowGroups?: readonly FlattenedRowGroup[]): MapResult {
+export function mapRowIndexToPath(row: number, flattenedRowGroups?: readonly FlattenedRowGroup[]): RowGroupingMapperResult<number> {
     if (flattenedRowGroups === undefined || flattenRowGroups.length === 0)
         return {
             path: [row],