chore(frontend): migrate utility JS files to TypeScript (#36721)

Co-authored-by: Claude Opus 4.5 <noreply@anthropic.com>

superset-frontend/src/explore/reducers/saveModalReducer.ts

@@ -20,8 +20,26 @@
 import * as actions from '../actions/saveModalActions';
 import { HYDRATE_EXPLORE } from '../actions/hydrateExplore';
 
-export default function saveModalReducer(state = {}, action) {
-  const actionHandlers = {
+interface SaveModalState {
+  isVisible?: boolean;
+  dashboards?: unknown[];
+  saveModalAlert?: string;
+  data?: unknown;
+}
+
+interface SaveModalAction {
+  type: string;
+  isVisible?: boolean;
+  choices?: unknown[];
+  userId?: string;
+  data?: unknown;
+}
+
+export default function saveModalReducer(
+  state: SaveModalState = {},
+  action: SaveModalAction,
+): SaveModalState {
+  const actionHandlers: Record<string, () => SaveModalState> = {
     [actions.SET_SAVE_CHART_MODAL_VISIBILITY]() {
       return { ...state, isVisible: action.isVisible };
     },
@@ -37,11 +55,12 @@ export default function saveModalReducer(state = {}, action) {
     [actions.SAVE_SLICE_FAILED]() {
       return { ...state, saveModalAlert: 'Failed to save slice' };
     },
-    [actions.SAVE_SLICE_SUCCESS](data) {
-      return { ...state, data };
+    [actions.SAVE_SLICE_SUCCESS]() {
+      return { ...state, data: action.data };
     },
     [HYDRATE_EXPLORE]() {
-      return { ...action.data.saveModal };
+      const payload = action.data as { saveModal?: SaveModalState } | undefined;
+      return { ...payload?.saveModal };
     },
   };