diff --git a/packages/devextreme/js/__internal/data/data_source/data_source.ts b/packages/devextreme/js/__internal/data/data_source/data_source.ts
index 79350d094d78..72aae3df3fab 100644
--- a/packages/devextreme/js/__internal/data/data_source/data_source.ts
+++ b/packages/devextreme/js/__internal/data/data_source/data_source.ts
@@ -788,21 +788,3 @@ export class DataSource {
     return this;
   }
 }
-
-/*
- * `Class.inherit()` defined prototype members as enumerable, and consumers still rely on
- * that: the grid's DataSourceAdapter copies a data source's members with a `for…in` loop
- * (see its "remove copying dataSource's members" TODO). ES6 class methods are not
- * enumerable, so restore the descriptors a data source used to expose.
- */
-Object.getOwnPropertyNames(DataSource.prototype).forEach((memberName) => {
-  if (memberName === 'constructor') {
-    return;
-  }
-
-  const descriptor = Object.getOwnPropertyDescriptor(DataSource.prototype, memberName);
-
-  if (descriptor) {
-    Object.defineProperty(DataSource.prototype, memberName, { ...descriptor, enumerable: true });
-  }
-});
diff --git a/packages/devextreme/js/__internal/grids/data_grid/export/m_export.ts b/packages/devextreme/js/__internal/grids/data_grid/export/m_export.ts
index 3cbdec732579..4a8226c4538c 100644
--- a/packages/devextreme/js/__internal/grids/data_grid/export/m_export.ts
+++ b/packages/devextreme/js/__internal/grids/data_grid/export/m_export.ts
@@ -701,7 +701,7 @@ export class ExportController extends dataGridCore.ViewController {
 
   private needLoadItemsOnExportingSelectedItems(): boolean {
     return this.option('loadItemsOnExportingSelectedItems')
-      ?? this._dataController._dataSource.remoteOperations().filtering;
+      ?? (this._dataController._dataSource?.remoteOperations().filtering ?? false);
   }
 }
 
diff --git a/packages/devextreme/js/__internal/grids/data_grid/focus/m_focus.ts b/packages/devextreme/js/__internal/grids/data_grid/focus/m_focus.ts
index 95ad327d2018..3aa4315f72de 100644
--- a/packages/devextreme/js/__internal/grids/data_grid/focus/m_focus.ts
+++ b/packages/devextreme/js/__internal/grids/data_grid/focus/m_focus.ts
@@ -93,12 +93,14 @@ const data = (Base: DataControllerBase) => class FocusDataControllerExtender ext
     // @ts-expect-error
     const deferred = new Deferred();
     const isGroupKey = Array.isArray(key);
-    const group = dataSource.group();
 
-    if (isGroupKey) {
+    if (isGroupKey || !dataSource) {
       return deferred.resolve(-1).promise();
     }
 
+    const group = dataSource.group();
+
+    // @ts-expect-error badly typed DataSourceAdapter
     if (!dataSource._grouping._updatePagingOptions) {
       this._calculateGlobalRowIndexByFlatData(key, null, true)
         .done(deferred.resolve)
@@ -110,12 +112,14 @@ const data = (Base: DataControllerBase) => class FocusDataControllerExtender ext
       filter: this._concatWithCombinedFilter(filter),
       group,
     }).done((data) => {
+      // @ts-expect-error badly typed DataSourceAdapter
       const hasData = isDefined(data) && data.length > 0;
 
       if (this._dataSource !== dataSource || !hasData) {
         return deferred.resolve(-1).promise();
       }
 
+      // @ts-expect-error badly typed DataSourceAdapter
       const groupPath = this._getGroupPath(data, group.length);
 
       this._expandGroupByPath(this, groupPath, 0).done(() => {
diff --git a/packages/devextreme/js/__internal/grids/data_grid/grouping/extenders/grouping_data_controller.ts b/packages/devextreme/js/__internal/grids/data_grid/grouping/extenders/grouping_data_controller.ts
index 82d91ef7dcb8..916507cb79dc 100644
--- a/packages/devextreme/js/__internal/grids/data_grid/grouping/extenders/grouping_data_controller.ts
+++ b/packages/devextreme/js/__internal/grids/data_grid/grouping/extenders/grouping_data_controller.ts
@@ -148,17 +148,19 @@ export const groupingDataControllerExtender = (
 
   private collapseAll(groupIndex: number): void {
     const dataSource = this._dataSource;
+    // @ts-expect-error badly typed DataSourceAdapter
     if (dataSource?.collapseAll(groupIndex)) {
-      dataSource.pageIndex(0);
-      dataSource.reload();
+      dataSource?.pageIndex(0);
+      dataSource?.reload();
     }
   }
 
   private expandAll(groupIndex: number): void {
     const dataSource = this._dataSource;
+    // @ts-expect-error badly typed DataSourceAdapter
     if (dataSource?.expandAll(groupIndex)) {
-      dataSource.pageIndex(0);
-      dataSource.reload();
+      dataSource?.pageIndex(0);
+      dataSource?.reload();
     }
   }
 
@@ -199,6 +201,7 @@ export const groupingDataControllerExtender = (
   }
 
   private isRowExpanded(key: RowKey): boolean {
+    // @ts-expect-error badly typed DataSourceAdapter
     return !!this._dataSource?.isRowExpanded(key);
   }
 
diff --git a/packages/devextreme/js/__internal/grids/data_grid/grouping/m_grouping.ts b/packages/devextreme/js/__internal/grids/data_grid/grouping/m_grouping.ts
index 456cded7a7f6..815fc4e1ed7e 100644
--- a/packages/devextreme/js/__internal/grids/data_grid/grouping/m_grouping.ts
+++ b/packages/devextreme/js/__internal/grids/data_grid/grouping/m_grouping.ts
@@ -58,13 +58,13 @@ const dataSourceAdapterExtender = (Base: ModuleType<DataSourceAdapter>) => class
     }
   }
 
-  protected totalItemsCount() {
+  public totalItemsCount() {
     const totalCount = super.totalItemsCount();
 
     return totalCount > 0 && this._dataSource.group() && this._dataSource.requireTotalCount() ? totalCount + this._grouping.totalCountCorrection() : totalCount;
   }
 
-  protected itemsCount() {
+  public itemsCount() {
     return this._dataSource.group() ? this._grouping.itemsCount() || 0 : super.itemsCount.apply(this, arguments as any);
   }
 
@@ -121,7 +121,7 @@ const dataSourceAdapterExtender = (Base: ModuleType<DataSourceAdapter>) => class
     return this._grouping.refresh.apply(this._grouping, arguments);
   }
 
-  protected changeRowExpand(path) {
+  public changeRowExpand(path) {
     const that = this;
     const dataSource = that._dataSource;
 
@@ -183,7 +183,7 @@ const dataSourceAdapterExtender = (Base: ModuleType<DataSourceAdapter>) => class
     return this._grouping.handleDataLoading(options);
   }
 
-  protected customizeLoadResultHandler(options) {
+  public customizeLoadResultHandler(options) {
     return this._grouping.handleDataLoaded(options, super.customizeLoadResultHandler.bind(this));
   }
 
diff --git a/packages/devextreme/js/__internal/grids/data_grid/summary/extenders/summary_data_controller.ts b/packages/devextreme/js/__internal/grids/data_grid/summary/extenders/summary_data_controller.ts
index 7a3594a6b081..8cf597ad96f3 100644
--- a/packages/devextreme/js/__internal/grids/data_grid/summary/extenders/summary_data_controller.ts
+++ b/packages/devextreme/js/__internal/grids/data_grid/summary/extenders/summary_data_controller.ts
@@ -43,6 +43,7 @@ export const summaryDataControllerExtender = (
 
   public getTotalSummaryValue(summaryItemName?: string | number | null): unknown {
     const summaryItemIndex = getSummaryItemIndex(this.option('summary.totalItems'), summaryItemName);
+    // @ts-expect-error badly typed DataSourceAdapter
     const aggregates = this._dataSource.totalAggregates();
 
     if (aggregates.length && summaryItemIndex > -1) {
@@ -293,6 +294,7 @@ export const summaryDataControllerExtender = (
     this._footerItems = [];
 
     if (dataSource && summaryTotalItems?.length) {
+      // @ts-expect-error badly typed DataSourceAdapter
       const totalAggregates = dataSource.totalAggregates();
       const summaryCells = this._getSummaryCells(summaryTotalItems, totalAggregates);
 
diff --git a/packages/devextreme/js/__internal/grids/grid_core/data_controller/data_controller.ts b/packages/devextreme/js/__internal/grids/grid_core/data_controller/data_controller.ts
index 5771e049533a..4539569f7922 100644
--- a/packages/devextreme/js/__internal/grids/grid_core/data_controller/data_controller.ts
+++ b/packages/devextreme/js/__internal/grids/grid_core/data_controller/data_controller.ts
@@ -33,7 +33,6 @@ import type {
   CallbackFlags,
   DataChange,
   DataFilter,
-  DataSourceAdapterLike,
   GeneratedItem,
   ItemChange,
   ItemProcessingOptions,
@@ -70,8 +69,7 @@ import {
 import { generateRowValues } from './utils/row_values';
 
 export class DataController extends modules.Controller {
-  // eslint-disable-next-line @typescript-eslint/no-explicit-any
-  public _dataSource?: any;
+  public _dataSource?: DataSourceAdapter | null;
 
   protected isSharedDataSource?: boolean;
 
@@ -192,7 +190,8 @@ export class DataController extends modules.Controller {
    * @extended: virtual_scrolling
    */
   protected _getPagingOptionValue(optionName: PagingOptionName): number {
-    return this._dataSource[optionName]() as number;
+    // eslint-disable-next-line @typescript-eslint/no-non-null-assertion
+    return this._dataSource![optionName]() as number;
   }
 
   protected callbackNames(): string[] {
@@ -336,9 +335,8 @@ export class DataController extends modules.Controller {
     return !this._isLoading;
   }
 
-  public getDataSource(): DataSource | null | undefined {
-    const adapter: DataSourceAdapterLike | null | undefined = this._dataSource;
-    return adapter ? adapter._dataSource : null;
+  public getDataSource(): DataSource | null {
+    return this._dataSource?._dataSource ?? null;
   }
 
   public getCombinedFilter(returnDataField?: boolean): DataFilter {
@@ -422,6 +420,9 @@ export class DataController extends modules.Controller {
   private readonly customizeStoreLoadOptionsHandler = (e: LoadOperation): void => {
     const columnsController = this._columnsController;
     const dataSource = this._dataSource;
+    if (!dataSource) {
+      return;
+    }
     const { storeLoadOptions } = e;
 
     if (e.isCustomLoading && !storeLoadOptions.isLoadingAll) {
@@ -730,7 +731,7 @@ export class DataController extends modules.Controller {
         dataSource.load().done((...args: unknown[]) => {
           this._isPaging = false;
           result.resolve(...args);
-        }).fail(result.reject);
+        }).fail((...args: unknown[]) => { result.reject(...args); });
       } else {
         result.resolve();
       }
@@ -1181,7 +1182,8 @@ export class DataController extends modules.Controller {
 
     // change.items at this stage is defined only if virtualScrolling
     // + legacyScrollingMode enabled
-    const dataItems = this._beforeProcessItems(change.items ?? this._dataSource.items());
+    // eslint-disable-next-line @typescript-eslint/no-non-null-assertion
+    const dataItems = this._beforeProcessItems(change.items ?? this._dataSource!.items());
     const processedItems = this._processItems(dataItems, change);
 
     this._cachedProcessedItems = processedItems;
@@ -1470,7 +1472,7 @@ export class DataController extends modules.Controller {
   }
 
   public pageCount(): number {
-    return this._dataSource ? this._dataSource.pageCount() as number : 1;
+    return this._dataSource ? this._dataSource.pageCount() : 1;
   }
 
   // eslint-disable-next-line @typescript-eslint/no-explicit-any
@@ -1509,10 +1511,12 @@ export class DataController extends modules.Controller {
         group: dataSource.group(),
         sort: dataSource.sort(),
       })
+        // @ts-expect-error badly typed CustomLoadResult
         .done(resolveWithProcessedItems)
         .fail(d.reject as (...args: unknown[]) => void);
     } else if (!dataSource.isLoading()) {
       dataSource.customLoadAll()
+        // @ts-expect-error badly typed CustomLoadResult
         .done(resolveWithProcessedItems)
         .fail(d.reject as (...args: unknown[]) => void);
     } else {
@@ -1606,7 +1610,8 @@ export class DataController extends modules.Controller {
       this._skipProcessingPagingChange = false;
     }
 
-    const pageIndex = dataSource.pageIndex();
+    // @ts-expect-error badly typed DataSourceAdapter
+    const pageIndex: number = dataSource.pageIndex();
     this._isPaging = optionName === 'pageIndex';
 
     const loadResult: DeferredObj<unknown> = dataSource[optionName === 'pageIndex' ? 'load' : 'reload']();
@@ -1757,19 +1762,20 @@ export class DataController extends modules.Controller {
   }
 
   public push(...args: unknown[]): unknown {
+    // @ts-expect-error badly typed DataSourceAdapter
     return this._dataSource?.push(...args);
   }
 
   private itemsCount(): number {
-    return (this._dataSource ? this._dataSource.itemsCount() : 0) as number;
+    return (this._dataSource ? this._dataSource.itemsCount() : 0);
   }
 
   public totalItemsCount(): number {
-    return (this._dataSource ? this._dataSource.totalItemsCount() : 0) as number;
+    return (this._dataSource ? this._dataSource.totalItemsCount() : 0);
   }
 
   public hasKnownLastPage(): boolean {
-    return (this._dataSource ? this._dataSource.hasKnownLastPage() : true) as boolean;
+    return (this._dataSource ? this._dataSource.hasKnownLastPage() : true);
   }
 
   /**
@@ -1780,7 +1786,7 @@ export class DataController extends modules.Controller {
   }
 
   public totalCount(): number {
-    return (this._dataSource ? this._dataSource.totalCount() : 0) as number;
+    return (this._dataSource ? this._dataSource.totalCount() : 0);
   }
 
   public hasLoadOperation(): boolean {
diff --git a/packages/devextreme/js/__internal/grids/grid_core/data_source_adapter/m_data_source_adapter.ts b/packages/devextreme/js/__internal/grids/grid_core/data_source_adapter/m_data_source_adapter.ts
index 23c4014ca398..e25b4bc85757 100644
--- a/packages/devextreme/js/__internal/grids/grid_core/data_source_adapter/m_data_source_adapter.ts
+++ b/packages/devextreme/js/__internal/grids/grid_core/data_source_adapter/m_data_source_adapter.ts
@@ -8,7 +8,7 @@ import type { DeferredObj } from '@js/core/utils/deferred';
 import { Deferred, when } from '@js/core/utils/deferred';
 import { extend } from '@js/core/utils/extend';
 import { each } from '@js/core/utils/iterator';
-import { isDefined, isFunction, isPlainObject } from '@js/core/utils/type';
+import { isDefined, isPlainObject } from '@js/core/utils/type';
 import type { StoreChange } from '@js/data/store';
 import type { ChangingEvent, DataSource, StoreLoadOptions } from '@ts/data/data_source/types';
 import type { BeforePushEvent } from '@ts/data/types';
@@ -30,7 +30,7 @@ import type {
 import { normalizeRemoteOperations } from './utils/remoteOperations';
 
 export default class DataSourceAdapter extends modules.Controller {
-  protected _dataSource!: DataSource;
+  public _dataSource!: DataSource;
 
   private _remoteOperations!: RemoteOperationsOptions;
 
@@ -96,10 +96,6 @@ export default class DataSourceAdapter extends modules.Controller {
 
   private changingHandlerProxy!: (e: ChangingEvent) => void;
 
-  protected store!: () => any;
-
-  private readonly group!: (args?: any) => any;
-
   private customLoader!: CustomLoader;
 
   public init(dataSource?: DataSource): void {
@@ -107,59 +103,48 @@ export default class DataSourceAdapter extends modules.Controller {
       return;
     }
 
-    const that = this;
-
-    that._dataSource = dataSource;
-    that._remoteOperations = normalizeRemoteOperations(
+    this._dataSource = dataSource;
+    this._remoteOperations = normalizeRemoteOperations(
       this.option('remoteOperations'),
       dataSource.store(),
     );
 
-    that._isLastPage = !dataSource.isLastPage();
-    that._hasLastPage = false;
-    that._currentTotalCount = 0;
-    that._cachedData = createEmptyCachedData();
-    that._lastOperationTypes = {};
-    that._eventsStrategy = dataSource._eventsStrategy;
-    that._totalCountCorrection = 0;
-    that.customLoader = new CustomLoader(
+    this._isLastPage = !dataSource.isLastPage();
+    this._hasLastPage = false;
+    this._currentTotalCount = 0;
+    this._cachedData = createEmptyCachedData();
+    this._lastOperationTypes = {};
+    this._eventsStrategy = dataSource._eventsStrategy;
+    this._totalCountCorrection = 0;
+    this.customLoader = new CustomLoader(
       dataSource,
       () => this.option('loadingTimeout'),
       (operation) => this.customizeStoreLoadOptionsHandler(operation),
       (operation) => this.customizeLoadResultHandler(operation),
     );
 
-    that.changed = Callbacks();
-    that.loadingChanged = Callbacks();
-    that.loadError = Callbacks();
-    that.customizeStoreLoadOptions = Callbacks();
-    that.changing = Callbacks();
-    that.pushed = Callbacks();
-
-    that.dataChangedHandlerProxy = that.dataChangedHandler.bind(that);
-    that.customizeStoreLoadOptionsHandlerProxy = that.customizeStoreLoadOptionsHandler.bind(that);
-    that.customizeLoadResultHandlerProxy = that.customizeLoadResultHandler.bind(that);
-    that.loadingChangedHandlerProxy = that.loadingChangedHandler.bind(that);
-    that.loadErrorHandlerProxy = that.loadErrorHandler.bind(that);
-    that.pushHandlerProxy = that.pushHandler.bind(that);
-    that.changingHandlerProxy = that.changingHandler.bind(that);
-
-    dataSource.on('changed', that.dataChangedHandlerProxy);
-    dataSource.on('customizeStoreLoadOptions', that.customizeStoreLoadOptionsHandlerProxy);
-    dataSource.on('customizeLoadResult', that.customizeLoadResultHandlerProxy);
-    dataSource.on('loadingChanged', that.loadingChangedHandlerProxy);
-    dataSource.on('loadError', that.loadErrorHandlerProxy);
-    dataSource.on('changing', that.changingHandlerProxy);
-    dataSource.store().on('beforePush', that.pushHandlerProxy);
-
-    // TODO: remove copying dataSource's members
-    each(dataSource, (memberName, member) => {
-      if (!that[memberName] && isFunction(member)) {
-        that[memberName] = function () {
-          return this._dataSource[memberName].apply(this._dataSource, arguments);
-        };
-      }
-    });
+    this.changed = Callbacks();
+    this.loadingChanged = Callbacks();
+    this.loadError = Callbacks();
+    this.customizeStoreLoadOptions = Callbacks();
+    this.changing = Callbacks();
+    this.pushed = Callbacks();
+
+    this.dataChangedHandlerProxy = this.dataChangedHandler.bind(this);
+    this.customizeStoreLoadOptionsHandlerProxy = this.customizeStoreLoadOptionsHandler.bind(this);
+    this.customizeLoadResultHandlerProxy = this.customizeLoadResultHandler.bind(this);
+    this.loadingChangedHandlerProxy = this.loadingChangedHandler.bind(this);
+    this.loadErrorHandlerProxy = this.loadErrorHandler.bind(this);
+    this.pushHandlerProxy = this.pushHandler.bind(this);
+    this.changingHandlerProxy = this.changingHandler.bind(this);
+
+    dataSource.on('changed', this.dataChangedHandlerProxy);
+    dataSource.on('customizeStoreLoadOptions', this.customizeStoreLoadOptionsHandlerProxy);
+    dataSource.on('customizeLoadResult', this.customizeLoadResultHandlerProxy);
+    dataSource.on('loadingChanged', this.loadingChangedHandlerProxy);
+    dataSource.on('loadError', this.loadErrorHandlerProxy);
+    dataSource.on('changing', this.changingHandlerProxy);
+    dataSource.store().on('beforePush', this.pushHandlerProxy);
   }
 
   public dispose(isSharedDataSource?: boolean): void {
@@ -182,6 +167,76 @@ export default class DataSourceAdapter extends modules.Controller {
   /**
    * @extended: TreeLists's data_source_adapter
    */
+  public filter(): StoreLoadOptions['filter'];
+  public filter(filterExpr: StoreLoadOptions['filter']): void;
+  public filter(...args: unknown[]): unknown {
+    return (this._dataSource.filter as (...a: unknown[]) => unknown)(...args);
+  }
+
+  public sort(): StoreLoadOptions['sort'];
+  public sort(sortExpr: StoreLoadOptions['sort']): void;
+  public sort(...args: unknown[]): unknown {
+    return (this._dataSource.sort as (...a: unknown[]) => unknown)(...args);
+  }
+
+  public group(): StoreLoadOptions['group'];
+  public group(groupExpr: StoreLoadOptions['group']): void;
+  public group(...args: unknown[]): unknown {
+    return (this._dataSource.group as (...a: unknown[]) => unknown)(...args);
+  }
+
+  public select(): StoreLoadOptions['select'];
+  public select(selectExpr: StoreLoadOptions['select']): void;
+  public select(...args: unknown[]): unknown {
+    return (this._dataSource.select as (...a: unknown[]) => unknown)(...args);
+  }
+
+  public paginate(): boolean | undefined;
+  public paginate(value: boolean): void;
+  public paginate(value?: boolean): boolean | undefined {
+    return (this._dataSource.paginate as (...a: unknown[]) => boolean | undefined)(value);
+  }
+
+  public requireTotalCount(): StoreLoadOptions['requireTotalCount'];
+  public requireTotalCount(value: boolean): void;
+  public requireTotalCount(value?: boolean): unknown {
+    return (this._dataSource.requireTotalCount as (...a: unknown[]) => unknown)(value);
+  }
+
+  // eslint-disable-next-line @typescript-eslint/no-explicit-any
+  public store(): any {
+    return this._dataSource.store();
+  }
+
+  // eslint-disable-next-line @typescript-eslint/no-explicit-any
+  public key(): any {
+    return this._dataSource.key();
+  }
+
+  public isLoading(): boolean {
+    return this._dataSource.isLoading();
+  }
+
+  public beginLoading(): void {
+    this._dataSource.beginLoading();
+  }
+
+  public endLoading(): void {
+    this._dataSource.endLoading();
+  }
+
+  public loadOptions(): StoreLoadOptions {
+    return this._dataSource.loadOptions();
+  }
+
+  public cancel(operationId: number): boolean {
+    return this._dataSource.cancel(operationId);
+  }
+
+  public cancelAll(): void {
+    (this._dataSource as unknown as { cancelAll: () => void }).cancelAll();
+  }
+
   public remoteOperations(): RemoteOperationsOptions {
     return this._remoteOperations;
   }
@@ -190,17 +245,16 @@ export default class DataSourceAdapter extends modules.Controller {
    * @extended: virtual_scrolling
    */
   public refresh(options, operationTypes) {
-    const that = this;
-    const dataSource = that._dataSource;
+    const dataSource = this._dataSource;
 
     if (operationTypes.reload) {
-      that.resetCurrentTotalCount();
-      that._isLastPage = !dataSource.paginate();
-      that._hasLastPage = that._isLastPage;
+      this.resetCurrentTotalCount();
+      this._isLastPage = !dataSource.paginate();
+      this._hasLastPage = this._isLastPage;
     }
   }
 
-  private resetCurrentTotalCount() {
+  public resetCurrentTotalCount() {
     this._currentTotalCount = 0;
     this._totalCountCorrection = 0;
   }
@@ -231,7 +285,7 @@ export default class DataSourceAdapter extends modules.Controller {
     return !isLocalOperations;
   }
 
-  private push(changes: StoreChange[], fromStore: boolean): void {
+  public push(changes: StoreChange[], fromStore: boolean): void {
     const store = this.store();
 
     if (this._needClearStoreDataCache()) {
@@ -260,7 +314,7 @@ export default class DataSourceAdapter extends modules.Controller {
     this.pushed.fire(changes);
   }
 
-  private getDataIndexGetter() {
+  public getDataIndexGetter() {
     if (!this._dataIndexGetter) {
       const store = this.store();
 
@@ -520,7 +574,7 @@ export default class DataSourceAdapter extends modules.Controller {
   /**
    * @extended: TreeLists's data_source_adapter
    */
-  protected customizeLoadResultHandler(options) {
+  public customizeLoadResultHandler(options) {
     const { loadOptions } = options;
     const localPaging = options.remoteOperations && !options.remoteOperations.paging;
     const { cachedData } = options;
@@ -716,11 +770,11 @@ export default class DataSourceAdapter extends modules.Controller {
     return this._loadingOperationTypes;
   }
 
-  public operationTypes(): OperationTypes | undefined {
-    return this._operationTypes;
+  public operationTypes(): OperationTypes | null {
+    return this._operationTypes ?? null;
   }
 
-  private lastLoadOptions() {
+  public lastLoadOptions() {
     return this._lastLoadOptions || {};
   }
 
@@ -745,14 +799,14 @@ export default class DataSourceAdapter extends modules.Controller {
    * @extended: TreeLists's data_source_adapter
    */
   // eslint-disable-next-line @typescript-eslint/no-unused-vars
-  protected changeRowExpand(path?: any): any {}
+  public changeRowExpand(path?: any): any {}
 
-  private totalCount() {
+  public totalCount() {
     // eslint-disable-next-line radix
     return parseInt((this._currentTotalCount || this._dataSourceTotalCount()) + this._totalCountCorrection);
   }
 
-  private totalCountCorrection() {
+  public totalCountCorrection() {
     return this._totalCountCorrection;
   }
 
@@ -760,19 +814,19 @@ export default class DataSourceAdapter extends modules.Controller {
    * @extended: virtual_scrolling
    * @protected
    */
-  protected items(): any {}
+  public items(): any {}
 
   /**
    * @extended: virtual_scrolling
    */
-  protected itemsCount() {
+  public itemsCount() {
     return this._dataSource.items().length;
   }
 
   /**
    * @extended: TreeLists's data_source_adapter
    */
-  protected totalItemsCount() {
+  public totalItemsCount() {
     return this.totalCount();
   }
 
@@ -787,10 +841,9 @@ export default class DataSourceAdapter extends modules.Controller {
     return this._dataSource.pageSize(value);
   }
 
-  protected pageCount() {
-    const that = this;
-    const count = that.totalItemsCount() - that._totalCountCorrection;
-    const pageSize = that.pageSize();
+  public pageCount() {
+    const count = this.totalItemsCount() - this._totalCountCorrection;
+    const pageSize = this.pageSize();
 
     if (pageSize && count > 0) {
       return Math.max(1, Math.ceil(count / pageSize));
@@ -798,7 +851,7 @@ export default class DataSourceAdapter extends modules.Controller {
     return 1;
   }
 
-  protected hasKnownLastPage() {
+  public hasKnownLastPage() {
     return this._hasLastPage || this._dataSource.totalCount() >= 0;
   }
 
@@ -806,7 +859,7 @@ export default class DataSourceAdapter extends modules.Controller {
     return this.customLoader.loadFromStore(loadOptions);
   }
 
-  protected isCustomLoading(): boolean {
+  public isCustomLoading(): boolean {
     return this.customLoader.isLoading();
   }
 
@@ -817,7 +870,7 @@ export default class DataSourceAdapter extends modules.Controller {
   /**
    * @extended: virtual_scrolling
    */
-  protected load(options?: CustomLoadOptions): DeferredObj<unknown> {
+  public load(options?: CustomLoadOptions): DeferredObj<unknown> {
     if (options) {
       return this.customLoader.load(options);
     }
@@ -839,12 +892,13 @@ export default class DataSourceAdapter extends modules.Controller {
   /**
    * @extended: virtual_scrolling
    */
-  protected reload(full: boolean): DeferredObj<unknown> {
+  // eslint-disable-next-line @typescript-eslint/no-unused-vars
+  public reload(full?: boolean, changesOnly?: boolean): DeferredObj<unknown> {
     const result = full ? this._dataSource.reload() : this._dataSource.load();
     return result as unknown as DeferredObj<unknown>;
   }
 
-  private getCachedStoreData() {
+  public getCachedStoreData() {
     return this._cachedStoreData;
   }
 
@@ -857,5 +911,5 @@ export default class DataSourceAdapter extends modules.Controller {
    * @extended: virtual_scrolling
    */
   // eslint-disable-next-line @typescript-eslint/no-unused-vars
-  protected pageIndex(pageIndex?) {}
+  public pageIndex(pageIndex?) {}
 }
diff --git a/packages/devextreme/js/__internal/grids/grid_core/focus/m_focus.ts b/packages/devextreme/js/__internal/grids/grid_core/focus/m_focus.ts
index 39fb37dd160b..aa3d5fe659dc 100644
--- a/packages/devextreme/js/__internal/grids/grid_core/focus/m_focus.ts
+++ b/packages/devextreme/js/__internal/grids/grid_core/focus/m_focus.ts
@@ -546,7 +546,7 @@ const columns = (Base: ModuleType<ColumnsController>) => class FocusColumnsExten
       if (notSortedKeys.length) {
         result = result || [];
         if (isLocalOperations) {
-          result.push({ selector: dataSource.getDataIndexGetter(), desc: false });
+          result.push({ selector: dataSource?.getDataIndexGetter(), desc: false });
         } else {
           notSortedKeys.forEach((notSortedKey) => result.push({ selector: notSortedKey, desc: false }));
         }
@@ -708,7 +708,7 @@ const focusDataControllerExtender = (
   }
 
   private getGlobalRowIndexByKey(key) {
-    if (this._dataSource.group()) {
+    if (this._dataSource!.group()) {
       // @ts-expect-error
       return this._calculateGlobalRowIndexByGroupedData(key);
     }
@@ -719,7 +719,7 @@ const focusDataControllerExtender = (
   protected _calculateGlobalRowIndexByFlatData(key, groupFilter, useGroup) {
     // @ts-expect-error
     const deferred = new Deferred();
-    const dataSource = this._dataSource;
+    const dataSource = this._dataSource!;
 
     if (Array.isArray(key) || isNewRowTempKey(key)) {
       return deferred.resolve(-1).promise();
@@ -736,8 +736,8 @@ const focusDataControllerExtender = (
         deferred.resolve(-1);
         return;
       }
-      if (data.length > 0) {
-        filter = this._generateOperationFilterByKey(key, data[0], useGroup);
+      if ((data as unknown[]).length > 0) {
+        filter = this._generateOperationFilterByKey(key, (data as unknown[])[0], useGroup);
         dataSource.load({
           filter: this._concatWithCombinedFilter(filter, groupFilter),
           skip: 0,
@@ -748,7 +748,7 @@ const focusDataControllerExtender = (
             deferred.resolve(-1);
             return;
           }
-          deferred.resolve(extra.totalCount);
+          deferred.resolve((extra as { totalCount: number }).totalCount);
         });
       } else {
         deferred.resolve(-1);
@@ -784,8 +784,8 @@ const focusDataControllerExtender = (
   private _generateOperationFilterByKey(key, rowData, useGroup) {
     const that = this;
     const dateSerializationFormat = that.option('dateSerializationFormat');
-    const isRemoteFiltering = that._dataSource.remoteOperations().filtering;
-    const isRemoteSorting = that._dataSource.remoteOperations().sorting;
+    const isRemoteFiltering = that._dataSource!.remoteOperations().filtering;
+    const isRemoteSorting = that._dataSource!.remoteOperations().sorting;
 
     let filter = that._generateFilterByKey(key, '<');
     // @ts-expect-error
@@ -846,7 +846,7 @@ const focusDataControllerExtender = (
   }
 
   protected _generateFilterByKey(key, operation?) {
-    const dataSourceKey = this._dataSource.key();
+    const dataSourceKey = this._dataSource!.key();
     let filter: any = [];
 
     if (!operation) {
diff --git a/packages/devextreme/js/__internal/grids/grid_core/virtual_scrolling/extenders/virtual_scrolling_data_controller.ts b/packages/devextreme/js/__internal/grids/grid_core/virtual_scrolling/extenders/virtual_scrolling_data_controller.ts
index 8903563cd5e9..ed08c807aeda 100644
--- a/packages/devextreme/js/__internal/grids/grid_core/virtual_scrolling/extenders/virtual_scrolling_data_controller.ts
+++ b/packages/devextreme/js/__internal/grids/grid_core/virtual_scrolling/extenders/virtual_scrolling_data_controller.ts
@@ -34,8 +34,10 @@ import {
   LOAD_TIMEOUT,
   VISIBLE_PAGE_INDEX,
 } from '../const';
+import type { dataSourceAdapterExtender } from '../m_virtual_scrolling';
 import { VirtualScrollController } from '../m_virtual_scrolling_core';
 import type { ChangedLoadParams } from '../types';
+import type { GroupCountableDataSource } from '../utils/items';
 import {
   correctCount,
   isItemCountableByDataSource,
@@ -51,11 +53,15 @@ export interface VirtualScrollingDataControllerExtension {
   virtualItemsCount: () => VirtualItemsCount | undefined;
 }
 
+type VirtualScrollingDataSourceAdapter = InstanceType<ReturnType<typeof dataSourceAdapterExtender>>;
+
 export const virtualScrollingDataControllerExtender = (
   Base: ModuleType<DataController>,
 ): ModuleType<
   DataController & VirtualScrollingDataControllerExtension
 > => class VirtualScrollingDataControllerExtender extends Base {
+  public declare _dataSource?: VirtualScrollingDataSourceAdapter | null;
+
   private _loadViewportParams: any;
 
   private _allItems: any;
@@ -106,6 +112,7 @@ export const virtualScrollingDataControllerExtender = (
   // eslint-disable-next-line @typescript-eslint/no-unused-vars
   public reload(reload?: boolean, changesOnly?: boolean): DeferredObj<unknown> {
     const rowsScrollController = this._rowsScrollController || this._dataSource;
+    // @ts-expect-error badly typed DataSourceAdapter
     const itemIndex = rowsScrollController?.getItemIndexByPosition();
     const result = super.reload.apply(this, arguments as any);
     return result?.done(() => {
@@ -175,7 +182,7 @@ export const virtualScrollingDataControllerExtender = (
   private _getRowsScrollDataOptions() {
     const that = this;
     const isItemCountable = function (item) {
-      return isItemCountableByDataSource(item, that._dataSource);
+      return isItemCountableByDataSource(item, that._dataSource as unknown as GroupCountableDataSource);
     };
 
     return {
@@ -369,7 +376,7 @@ export const virtualScrollingDataControllerExtender = (
 
       processedItems.forEach((item) => {
         const { rowType } = item;
-        const itemCountable = isItemCountableByDataSource(item, dataSource);
+        const itemCountable = isItemCountableByDataSource(item, dataSource as unknown as GroupCountableDataSource);
 
         const isNextGroupItem = rowType === 'group' && (prevCountable || (prevRowType !== 'group' && currentIndex > 0));
         const isNextDataItem = rowType === 'data' && itemCountable && (prevCountable || prevRowType !== 'group');
@@ -403,7 +410,7 @@ export const virtualScrollingDataControllerExtender = (
   }
 
   protected _afterProcessItems(processedItems: ProcessedItem[]): ProcessedItem[] {
-    this._itemCount = processedItems.filter((item) => isItemCountableByDataSource(item, this._dataSource)).length;
+    this._itemCount = processedItems.filter((item) => isItemCountableByDataSource(item, this._dataSource as unknown as GroupCountableDataSource)).length;
 
     if (isDefined(this._loadViewportParams)) {
       this._updateLoadViewportParams();
@@ -438,7 +445,7 @@ export const virtualScrollingDataControllerExtender = (
 
     if (removeCount) {
       const fromEnd = changeType === 'prepend';
-      removeCount = correctCount(that._items, removeCount, fromEnd, (item, isNextAfterLast) => isItemCountableByDataSource(item, that._dataSource) || (item.rowType === 'group' && isNextAfterLast));
+      removeCount = correctCount(that._items, removeCount, fromEnd, (item, isNextAfterLast) => isItemCountableByDataSource(item, that._dataSource as unknown as GroupCountableDataSource) || (item.rowType === 'group' && isNextAfterLast));
 
       change.removeCount = removeCount;
     }
@@ -527,46 +534,46 @@ export const virtualScrollingDataControllerExtender = (
     return isDefined(lastVisibleItem?.dataIndex) ? lastVisibleItem!.dataIndex + 1 : 0;
   }
 
-  private viewportSize() {
+  private viewportSize(size?) {
     const rowsScrollController = this._rowsScrollController;
     const dataSource = this._dataSource;
     // @ts-expect-error
-    const result = rowsScrollController?.viewportSize.apply(rowsScrollController, arguments);
+    const result = rowsScrollController?.viewportSize(size);
 
     if (this.option(LEGACY_SCROLLING_MODE) === false) {
       return result;
     }
 
-    return dataSource?.viewportSize.apply(dataSource, arguments);
+    return dataSource?.viewportSize(size);
   }
 
   private viewportHeight(height, scrollTop) {
     this._rowsScrollController?.viewportHeight(height, scrollTop);
   }
 
-  private viewportItemSize() {
+  private viewportItemSize(size?) {
     const rowsScrollController = this._rowsScrollController;
     const dataSource = this._dataSource;
     // @ts-expect-error
-    const result = rowsScrollController?.viewportItemSize.apply(rowsScrollController, arguments);
+    const result = rowsScrollController?.viewportItemSize(size);
 
     if (this.option(LEGACY_SCROLLING_MODE) === false) {
       return result;
     }
 
-    return dataSource?.viewportItemSize.apply(dataSource, arguments);
+    return dataSource?.viewportItemSize(size);
   }
 
-  private setViewportPosition() {
+  private setViewportPosition(position?) {
     const rowsScrollController = this._rowsScrollController;
     const dataSource = this._dataSource;
     this._isPaging = false;
 
     if (rowsScrollController) {
       // @ts-expect-error
-      rowsScrollController.setViewportPosition.apply(rowsScrollController, arguments);
+      rowsScrollController.setViewportPosition(position);
     } else {
-      dataSource?.setViewportPosition.apply(dataSource, arguments);
+      dataSource?.setViewportPosition(position);
     }
   }
 
@@ -738,8 +745,8 @@ export const virtualScrollingDataControllerExtender = (
   }
 
   private loadPages(changedParams: ChangedLoadParams): void {
-    this._dataSource.pageIndex(changedParams.pageIndex);
-    this._dataSource.loadPageCount(changedParams.loadPageCount);
+    this._dataSource!.pageIndex(changedParams.pageIndex);
+    this._dataSource!.loadPageCount(changedParams.loadPageCount);
     this._repaintChangesOnly = true;
     this._needUpdateDimensions = true;
 
@@ -827,11 +834,11 @@ export const virtualScrollingDataControllerExtender = (
 
     if (rowsScrollController) {
       // @ts-expect-error
-      return rowsScrollController.getItemSize.apply(rowsScrollController, arguments);
+      return rowsScrollController.getItemSize();
     }
 
     const dataSource = this._dataSource;
-    return dataSource?.getItemSize.apply(dataSource, arguments);
+    return dataSource?.getItemSize();
   }
 
   private getItemSizes() {
@@ -839,23 +846,23 @@ export const virtualScrollingDataControllerExtender = (
 
     if (rowsScrollController) {
       // @ts-expect-error
-      return rowsScrollController.getItemSizes.apply(rowsScrollController, arguments);
+      return rowsScrollController.getItemSizes();
     }
 
     const dataSource = this._dataSource;
-    return dataSource?.getItemSizes.apply(dataSource, arguments);
+    return dataSource?.getItemSizes();
   }
 
-  private getContentOffset() {
+  private getContentOffset(type?) {
     const rowsScrollController = this._rowsScrollController;
 
     if (rowsScrollController) {
       // @ts-expect-error
-      return rowsScrollController.getContentOffset.apply(rowsScrollController, arguments);
+      return rowsScrollController.getContentOffset(type);
     }
 
     const dataSource = this._dataSource;
-    return dataSource?.getContentOffset.apply(dataSource, arguments);
+    return dataSource?.getContentOffset(type);
   }
 
   public refresh(options?: boolean | RefreshOptions): DeferredObj<unknown> {
@@ -908,7 +915,7 @@ export const virtualScrollingDataControllerExtender = (
       const { fullReload, pageIndex } = operationTypes;
 
       if (e.isDataChanged && !fullReload && pageIndex) {
-        this._updateVisiblePageIndex(this._dataSource.pageIndex());
+        this._updateVisiblePageIndex(this._dataSource!.pageIndex());
       }
     }
   }
diff --git a/packages/devextreme/js/__internal/grids/grid_core/virtual_scrolling/m_virtual_scrolling.ts b/packages/devextreme/js/__internal/grids/grid_core/virtual_scrolling/m_virtual_scrolling.ts
index 5315499f0d30..094de621b840 100644
--- a/packages/devextreme/js/__internal/grids/grid_core/virtual_scrolling/m_virtual_scrolling.ts
+++ b/packages/devextreme/js/__internal/grids/grid_core/virtual_scrolling/m_virtual_scrolling.ts
@@ -33,6 +33,7 @@ import {
   VIRTUAL_ROW_CLASS,
 } from './const';
 import { subscribeToExternalScrollers, VirtualScrollController } from './m_virtual_scrolling_core';
+import type { GroupCountableDataSource } from './utils/items';
 import { isItemCountableByDataSource } from './utils/items';
 import { isInfiniteMode, isVirtualMode, isVirtualPaging } from './utils/scrolling_mode';
 
@@ -216,7 +217,7 @@ export const dataSourceAdapterExtender = (Base: ModuleType<DataSourceAdapter>) =
     super._customizeRemoteOperations.apply(this, arguments as any);
   }
 
-  protected items() {
+  public items() {
     return this._items;
   }
 
@@ -224,21 +225,21 @@ export const dataSourceAdapterExtender = (Base: ModuleType<DataSourceAdapter>) =
     return this.option(LEGACY_SCROLLING_MODE) === false && isVirtualMode(this) && !isBase ? this._totalCount : super._dataSourceTotalCount();
   }
 
-  protected itemsCount(isBase?) {
+  public itemsCount(isBase?) {
     if (isBase || this.option(LEGACY_SCROLLING_MODE) === false) {
       return super.itemsCount();
     }
     return this._virtualScrollController.itemsCount();
   }
 
-  protected load(loadOptions) {
+  public load(loadOptions) {
     if (this.option(LEGACY_SCROLLING_MODE) === false || loadOptions) {
       return super.load(loadOptions);
     }
     return this._virtualScrollController.load();
   }
 
-  private isLoading() {
+  public isLoading() {
     return this.option(LEGACY_SCROLLING_MODE) === false ? this._dataSource.isLoading() : this._isLoading;
   }
 
@@ -266,7 +267,7 @@ export const dataSourceAdapterExtender = (Base: ModuleType<DataSourceAdapter>) =
     return result;
   }
 
-  protected reload() {
+  public reload() {
     this._dataSource.pageIndex(this.pageIndex());
     const virtualScrollController = this._virtualScrollController;
 
@@ -320,7 +321,7 @@ export const dataSourceAdapterExtender = (Base: ModuleType<DataSourceAdapter>) =
     return super.refresh.apply(this, arguments as any);
   }
 
-  private loadPageCount(count?) {
+  public loadPageCount(count?) {
     if (!isDefined(count)) {
       return this._loadPageCount;
     }
@@ -354,27 +355,30 @@ export const dataSourceAdapterExtender = (Base: ModuleType<DataSourceAdapter>) =
   }
 
   // eslint-disable-next-line @typescript-eslint/no-unused-vars
-  protected pageIndex(pageIndex?): any {
+  public pageIndex(pageIndex?): any {
     return proxyDataSourceAdapterMethod(this, 'pageIndex', [...arguments]);
   }
 
-  private virtualItemsCount(): any {
+  public virtualItemsCount(): any {
     return proxyDataSourceAdapterMethod(this, 'virtualItemsCount', [...arguments]);
   }
 
-  private getContentOffset(): any {
+  // eslint-disable-next-line @typescript-eslint/no-unused-vars
+  public getContentOffset(type?): any {
     return proxyDataSourceAdapterMethod(this, 'getContentOffset', [...arguments]);
   }
 
-  private getVirtualContentSize(): any {
+  public getVirtualContentSize(): any {
     return proxyDataSourceAdapterMethod(this, 'getVirtualContentSize', [...arguments]);
   }
 
-  private setContentItemSizes(): any {
+  // eslint-disable-next-line @typescript-eslint/no-unused-vars
+  public setContentItemSizes(sizes?): any {
     return proxyDataSourceAdapterMethod(this, 'setContentItemSizes', [...arguments]);
   }
 
-  private setViewportPosition(): any {
+  // eslint-disable-next-line @typescript-eslint/no-unused-vars
+  public setViewportPosition(position?): any {
     return proxyDataSourceAdapterMethod(this, 'setViewportPosition', [...arguments]);
   }
 
@@ -387,27 +391,29 @@ export const dataSourceAdapterExtender = (Base: ModuleType<DataSourceAdapter>) =
     return proxyDataSourceAdapterMethod(this, 'setViewportItemIndex', [...arguments]);
   }
 
-  private getItemIndexByPosition(): any {
+  public getItemIndexByPosition(): any {
     return proxyDataSourceAdapterMethod(this, 'getItemIndexByPosition', [...arguments]);
   }
 
-  private viewportSize(): any {
+  // eslint-disable-next-line @typescript-eslint/no-unused-vars
+  public viewportSize(size?): any {
     return proxyDataSourceAdapterMethod(this, 'viewportSize', [...arguments]);
   }
 
-  private viewportItemSize(): any {
+  // eslint-disable-next-line @typescript-eslint/no-unused-vars
+  public viewportItemSize(size?): any {
     return proxyDataSourceAdapterMethod(this, 'viewportItemSize', [...arguments]);
   }
 
-  private getItemSize(): any {
+  public getItemSize(): any {
     return proxyDataSourceAdapterMethod(this, 'getItemSize', [...arguments]);
   }
 
-  private getItemSizes(): any {
+  public getItemSizes(): any {
     return proxyDataSourceAdapterMethod(this, 'getItemSizes', [...arguments]);
   }
 
-  private loadIfNeed(): any {
+  public loadIfNeed(): any {
     return proxyDataSourceAdapterMethod(this, 'loadIfNeed', [...arguments]);
   }
 };
@@ -569,7 +575,7 @@ export const rowsView = (Base: ModuleType<RowsView>) => class VirtualScrollingRo
   }
 
   protected _renderCore(e) {
-    const startRenderTime: any = new Date();
+    const startRenderTime = Date.now();
 
     const deferred = super._renderCore.apply(this, arguments as any);
 
@@ -582,9 +588,11 @@ export const rowsView = (Base: ModuleType<RowsView>) => class VirtualScrollingRo
         .viewportSize() || 20;
 
       if (gridCoreUtils.isVirtualRowRendering(this) && itemCount > 0 && this.option(LEGACY_SCROLLING_MODE) !== false) {
-        dataSource._renderTime = ((new Date()) as any - startRenderTime) * viewportSize / itemCount;
+        // @ts-expect-error badly typed DataSourceAdapter
+        dataSource._renderTime = (Date.now() - startRenderTime) * viewportSize / itemCount;
       } else {
-        dataSource._renderTime = ((new Date()) as any - startRenderTime);
+        // @ts-expect-error badly typed DataSourceAdapter
+        dataSource._renderTime = Date.now() - startRenderTime;
       }
     }
     return deferred;
@@ -718,7 +726,7 @@ export const rowsView = (Base: ModuleType<RowsView>) => class VirtualScrollingRo
           itemSize = 0;
         }
         lastLoadIndex = currentItem.loadIndex;
-      } else if (isItemCountableByDataSource(currentItem, dataSource)) {
+      } else if (isItemCountableByDataSource(currentItem, dataSource as unknown as GroupCountableDataSource)) {
         if (firstCountableItem) {
           firstCountableItem = false;
         } else {
@@ -964,7 +972,7 @@ export const rowsView = (Base: ModuleType<RowsView>) => class VirtualScrollingRo
     }
   }
 
-  private loadIfNeed() {
+  public loadIfNeed() {
     this._dataController
       // @ts-expect-error
       ?.loadIfNeed?.();
diff --git a/packages/devextreme/js/__internal/grids/grid_core/virtual_scrolling/utils/items.ts b/packages/devextreme/js/__internal/grids/grid_core/virtual_scrolling/utils/items.ts
index fc8b52d079dc..28412664c581 100644
--- a/packages/devextreme/js/__internal/grids/grid_core/virtual_scrolling/utils/items.ts
+++ b/packages/devextreme/js/__internal/grids/grid_core/virtual_scrolling/utils/items.ts
@@ -1,6 +1,6 @@
 import type { ProcessedItem } from '@ts/grids/grid_core/data_controller/types';
 
-interface GroupCountableDataSource {
+export interface GroupCountableDataSource {
   isGroupItemCountable: (data: unknown) => boolean;
 }
 
diff --git a/packages/devextreme/js/__internal/grids/tree_list/data_controller/m_data_controller.ts b/packages/devextreme/js/__internal/grids/tree_list/data_controller/m_data_controller.ts
index f0fa6c4d443e..55c9e35e78fa 100644
--- a/packages/devextreme/js/__internal/grids/tree_list/data_controller/m_data_controller.ts
+++ b/packages/devextreme/js/__internal/grids/tree_list/data_controller/m_data_controller.ts
@@ -5,10 +5,13 @@ import { DataController, dataControllerModule } from '@ts/grids/grid_core/data_c
 import type { DataSourceAdapterProvider } from '@ts/grids/grid_core/data_source_adapter/types';
 import type { RowKey } from '@ts/grids/grid_core/m_types';
 
+import type { DataSourceAdapterTreeList } from '../data_source_adapter/m_data_source_adapter';
 import dataSourceAdapterProvider from '../data_source_adapter/m_data_source_adapter';
 import treeListCore from '../m_core';
 
 export class TreeListDataController extends DataController {
+  public declare _dataSource?: DataSourceAdapterTreeList | null;
+
   protected _getDataSourceAdapterProvider(): DataSourceAdapterProvider {
     return dataSourceAdapterProvider;
   }
@@ -36,7 +39,7 @@ export class TreeListDataController extends DataController {
   }
 
   private _loadOnOptionChange() {
-    this._dataSource.load();
+    this._dataSource!.load();
   }
 
   protected isSameRowState(item1, item2): boolean {
@@ -163,6 +166,7 @@ export class TreeListDataController extends DataController {
         break;
       case 'expandedRowKeys':
       case 'onNodesInitialized':
+        // @ts-expect-error badly typed DataSourceAdapter
         if (this._dataSource && !this._dataSource._isNodesInitializing && !equalByValue(args.value, args.previousValue)) {
           this._loadOnOptionChange();
         }
@@ -201,7 +205,7 @@ export class TreeListDataController extends DataController {
   }
 
   private forEachNode() {
-    this._dataSource.forEachNode.apply(this, arguments);
+    this._dataSource!.forEachNode.apply(this, arguments as any);
   }
 
   // Collect keys by walking the loaded node tree (depth-first, parent before
diff --git a/packages/devextreme/js/__internal/grids/tree_list/data_source_adapter/m_data_source_adapter.ts b/packages/devextreme/js/__internal/grids/tree_list/data_source_adapter/m_data_source_adapter.ts
index 722fdef004b4..794e20ebb1bd 100644
--- a/packages/devextreme/js/__internal/grids/tree_list/data_source_adapter/m_data_source_adapter.ts
+++ b/packages/devextreme/js/__internal/grids/tree_list/data_source_adapter/m_data_source_adapter.ts
@@ -617,7 +617,7 @@ export class DataSourceAdapterTreeList extends DataSourceAdapter {
     return baseChanges;
   }
 
-  protected customizeLoadResultHandler(options) {
+  public customizeLoadResultHandler(options) {
     const data = options.data = this._convertDataToPlainStructure(options.data);
     if (!options.remoteOperations.filtering && options.loadOptions.filter) {
       // @ts-expect-error
@@ -773,7 +773,7 @@ export class DataSourceAdapterTreeList extends DataSourceAdapter {
     this.createAction('onNodesInitialized');
   }
 
-  private getKeyExpr() {
+  public getKeyExpr() {
     const store = this.store();
     const key = store && store.key();
     const keyExpr = this.option('keyExpr');
@@ -787,23 +787,23 @@ export class DataSourceAdapterTreeList extends DataSourceAdapter {
     return key || keyExpr || DEFAULT_KEY_EXPRESSION;
   }
 
-  private keyOf(data) {
+  public keyOf(data) {
     return this._keyGetter && this._keyGetter(data);
   }
 
-  private parentKeyOf(data) {
+  public parentKeyOf(data) {
     return this._parentIdGetter && this._parentIdGetter(data);
   }
 
-  private getRootNode() {
+  public getRootNode() {
     return this._rootNode;
   }
 
-  protected totalItemsCount() {
+  public totalItemsCount() {
     return this._totalItemsCount + this._totalCountCorrection;
   }
 
-  private isRowExpanded(key, cache?) {
+  public isRowExpanded(key, cache?) {
     if (cache) {
       let { isExpandedByKey } = cache;
       if (!isExpandedByKey) {
@@ -836,13 +836,13 @@ export class DataSourceAdapterTreeList extends DataSourceAdapter {
     this.option('expandedRowKeys', expandedRowKeys);
   }
 
-  protected changeRowExpand(key) {
+  public changeRowExpand(key) {
     this._changeRowExpandCore(key);
     // @ts-expect-error
     return this._isNodesInitializing ? new Deferred().resolve() : this.load();
   }
 
-  private getNodeByKey(key) {
+  public getNodeByKey(key) {
     if (this._nodeByKey) {
       return this._nodeByKey[key];
     }
@@ -864,7 +864,7 @@ export class DataSourceAdapterTreeList extends DataSourceAdapter {
     return result;
   }
 
-  private getChildNodeKeys(parentKey) {
+  public getChildNodeKeys(parentKey) {
     const node = this.getNodeByKey(parentKey);
     const childrenKeys: any[] = [];
 
@@ -875,7 +875,7 @@ export class DataSourceAdapterTreeList extends DataSourceAdapter {
     return childrenKeys;
   }
 
-  private loadDescendants(keys, childrenOnly) {
+  public loadDescendants(keys, childrenOnly) {
     const that = this;
     // @ts-expect-error
     const d = new Deferred();
@@ -911,7 +911,8 @@ export class DataSourceAdapterTreeList extends DataSourceAdapter {
     return d.promise();
   }
 
-  private forEachNode() {
+  // eslint-disable-next-line @typescript-eslint/no-unused-vars, @typescript-eslint/no-explicit-any
+  public forEachNode(nodeCallback?: (node: any) => void) {
     let nodes = [];
     let callback;
 
diff --git a/packages/devextreme/js/__internal/grids/tree_list/m_focus.ts b/packages/devextreme/js/__internal/grids/tree_list/m_focus.ts
index f65678a32c3c..976b33b0d7d0 100644
--- a/packages/devextreme/js/__internal/grids/tree_list/m_focus.ts
+++ b/packages/devextreme/js/__internal/grids/tree_list/m_focus.ts
@@ -3,6 +3,7 @@ import { focusModule } from '@ts/grids/grid_core/focus/m_focus';
 
 import type { DataController } from '../grid_core/data_controller/data_controller';
 import type { ModuleType } from '../grid_core/m_types';
+import type { DataSourceAdapterTreeList } from './data_source_adapter/m_data_source_adapter';
 import core from './m_core';
 
 function findIndex(items, callback) {
@@ -51,7 +52,7 @@ const data = (
 
   private getParentKey(key) {
     const that = this;
-    const dataSource = that._dataSource;
+    const dataSource = that._dataSource as unknown as DataSourceAdapterTreeList;
     // @ts-expect-error
     const node = that.getNodeByKey(key);
     // @ts-expect-error
@@ -63,7 +64,7 @@ const data = (
       dataSource.load({
         filter: [dataSource.getKeyExpr(), '=', key],
       }).done((items) => {
-        const parentData = items[0];
+        const parentData = (items as unknown[])[0];
 
         if (parentData) {
           d.resolve(dataSource.parentKeyOf(parentData));
@@ -78,15 +79,17 @@ const data = (
 
   private expandAscendants(key) {
     const that = this;
-    const dataSource = that._dataSource;
+    const dataSource = that._dataSource as unknown as DataSourceAdapterTreeList;
     // @ts-expect-error
     const d = new Deferred();
 
     that.getParentKey(key).done((parentKey) => {
       if (dataSource && parentKey !== undefined && parentKey !== that.option('rootValue')) {
+        // @ts-expect-error badly typed DataSourceAdapter
         dataSource._isNodesInitializing = true;
         // @ts-expect-error
         that.expandRow(parentKey);
+        // @ts-expect-error badly typed DataSourceAdapter
         dataSource._isNodesInitializing = false;
         that.expandAscendants(parentKey).done(d.resolve).fail(d.reject);
       } else {
@@ -98,7 +101,7 @@ const data = (
   }
 
   protected getPageIndexByKey(key) {
-    const dataSource = this._dataSource;
+    const dataSource = this._dataSource as unknown as DataSourceAdapterTreeList;
     // @ts-expect-error
     const d = new Deferred();
 
@@ -106,7 +109,7 @@ const data = (
       dataSource.load({
         parentIds: [],
       }).done((nodes) => {
-        if (this._dataSource !== dataSource) {
+        if ((this._dataSource as unknown) !== (dataSource as unknown)) {
           d.resolve(-1);
           return;
         }
diff --git a/packages/devextreme/js/__internal/grids/tree_list/m_virtual_scrolling.ts b/packages/devextreme/js/__internal/grids/tree_list/m_virtual_scrolling.ts
index 087fcd9315ca..169ec0172d0a 100644
--- a/packages/devextreme/js/__internal/grids/tree_list/m_virtual_scrolling.ts
+++ b/packages/devextreme/js/__internal/grids/tree_list/m_virtual_scrolling.ts
@@ -35,6 +35,7 @@ virtualScrollingModule.extenders.views.rowsView = (Base: ModuleType<RowsView>) =
 
 virtualScrollingModule.extenders.controllers.data = (Base: ModuleType<DataController>) => class TreeListVirtualScrollingDataControllerExtender extends virtualScrollingDataControllerExtender(Base) {
   protected _loadOnOptionChange() {
+    // @ts-expect-error badly typed DataSourceAdapter
     const virtualScrollController = this._dataSource?._virtualScrollController;
 
     virtualScrollController?.reset();
@@ -44,7 +45,7 @@ virtualScrollingModule.extenders.controllers.data = (Base: ModuleType<DataContro
 };
 
 const dataSourceAdapterExtender = (Base: ModuleType<DataSourceAdapter>) => class VirtualScrollingDataSourceAdapterExtender extends virtualScrollingDataSourceAdapterExtender(Base) {
-  protected changeRowExpand() {
+  public changeRowExpand() {
     return super.changeRowExpand.apply(this, arguments as any).done(() => {
       const viewportItemIndex = this.getViewportItemIndex();
 
diff --git a/packages/devextreme/testing/helpers/gridBaseMocks.js b/packages/devextreme/testing/helpers/gridBaseMocks.js
index 029e59537099..9f4eb8f29e91 100644
--- a/packages/devextreme/testing/helpers/gridBaseMocks.js
+++ b/packages/devextreme/testing/helpers/gridBaseMocks.js
@@ -808,6 +808,21 @@ module.exports = function($, gridCore, columnResizingReordering, domUtils, commo
 
             cancelAll: commonUtils.noop,
 
+            loadOptions: function() {
+                return {};
+            },
+            beginLoading: commonUtils.noop,
+            endLoading: commonUtils.noop,
+            key: function() {
+                return options.key;
+            },
+            select: function() {
+                return options.select;
+            },
+            cancel: function() {
+                return false;
+            },
+
             on(eventName, eventHandler) {
                 this[eventName].add(eventHandler);
             },
diff --git a/packages/devextreme/testing/helpers/vizMocks.js b/packages/devextreme/testing/helpers/vizMocks.js
index 8a6838ea73a5..ae1a8310d41b 100644
--- a/packages/devextreme/testing/helpers/vizMocks.js
+++ b/packages/devextreme/testing/helpers/vizMocks.js
@@ -259,14 +259,24 @@ function stubClass(target, members, settings) {
     settings = settings || {};
     proto.prototype = typeof target === 'function' ? target.prototype : target;
     const stubPrototype = stub.prototype = new proto();
-    $.each(stubPrototype, function(name, member) {
-        if(typeof member === 'function' && name !== 'constructor') {
-            stubPrototype[name] = function() {
-                createStub(this, name);
-                return this[name].apply(this, arguments);
-            };
-        }
-    });
+    // Walk the prototype chain so non-enumerable methods (ES6 class methods) are
+    // stubbed too; a plain `for...in`/`$.each` only sees enumerable members.
+    const stubbedNames = {};
+    for(let currentProto = stubPrototype; currentProto && currentProto !== Object.prototype; currentProto = Object.getPrototypeOf(currentProto)) {
+        Object.getOwnPropertyNames(currentProto).forEach(function(name) {
+            if(name === 'constructor' || stubbedNames[name]) {
+                return;
+            }
+            const descriptor = Object.getOwnPropertyDescriptor(currentProto, name);
+            if(descriptor && typeof descriptor.value === 'function') {
+                stubbedNames[name] = true;
+                stubPrototype[name] = function() {
+                    createStub(this, name);
+                    return this[name].apply(this, arguments);
+                };
+            }
+        });
+    }
     settings.$extraFunctions && $.each(settings.$extraFunctions, function(_, name) {
         _members[name] = 'name' in _members ? _members[name] : function() { };
     });