List: custom properties should be accessible in groupTemplate arguments(T1319741)

packages/devextreme/js/__internal/data/data_converter/grouped.ts

@@ -8,7 +8,10 @@ const groupKey = 'key';
 export function getDataSourceOptions<TItem>(
   dataSource: DataSourceLike<TItem>,
 ): DataSourceLike<TItem> | DataSourceOptions<GroupItem<TItem>> {
-  if (!isGroupItemsArray<TItem>(dataSource)) {
+  if (
+    !isGroupItemsArray<TItem>(dataSource)
+    || (dataSource as GroupItem<TItem>[]).some((item) => Object.keys(item).length !== 2)
+  ) {
     return dataSource;
   }
 

packages/devextreme/testing/tests/DevExpress.ui.widgets/listParts/commonTests.js

@@ -3791,6 +3791,68 @@ QUnit.module('regressions', moduleSetup, () => {
             done();
         });
     });
+
+    QUnit.test('custom properties should be preserved in grouped items (T1319741)', function(assert) {
+        const data = [
+            { key: 'Group 1', items: [{ text: 'Item 1' }], customProp: 'customValue' }
+        ];
+
+        const list = this.element.dxList({
+            dataSource: data,
+            grouped: true
+        }).dxList('instance');
+
+        const dataSourceItems = list.getDataSource().items();
+
+        assert.strictEqual(dataSourceItems.length, 1, 'One group loaded');
+        assert.strictEqual(dataSourceItems[0].key, 'Group 1', 'Key preserved');
+        assert.strictEqual(dataSourceItems[0].customProp, 'customValue', 'Custom property preserved');
+    });
+
+    QUnit.test('custom properties should be preserved in mixed grouped items (T1319741)', function(assert) {
+        const data = [
+            { key: 'Group 1', items: [{ text: 'Item 1' }] },
+            { key: 'Group 2', items: [{ text: 'Item 2' }], customProp: 'customValue' }
+        ];
+
+        const list = this.element.dxList({
+            dataSource: data,
+            grouped: true
+        }).dxList('instance');
+
+        const dataSourceItems = list.getDataSource().items();
+
+        assert.strictEqual(dataSourceItems.length, 2, 'Two groups loaded');
+
+        assert.strictEqual(dataSourceItems[0].key, 'Group 1');
+        assert.strictEqual(dataSourceItems[1].key, 'Group 2');
+        assert.strictEqual(dataSourceItems[1].customProp, 'customValue', 'Custom property preserved in second group');
+
+        const $groups = $(list.element()).find(`.${LIST_GROUP_CLASS}`);
+        assert.strictEqual($groups.length, 2, 'Two groups rendered');
+        assert.strictEqual($groups.eq(1).find(`.${LIST_ITEM_CLASS}`).text(), 'Item 2', 'Item in second group rendered');
+    });
+
+    QUnit.test('custom properties should be available in groupTemplate (T1319741)', function(assert) {
+        const data = [
+            { key: 'Group 1', customProp: 'customValue', items: [{ text: 'Item 1' }] }
+        ];
+
+        const groupTemplateStub = sinon.stub().returns($('<div>'));
+
+        this.element.dxList({
+            dataSource: data,
+            grouped: true,
+            groupTemplate: groupTemplateStub
+        });
+
+        assert.strictEqual(groupTemplateStub.callCount, 1, 'Template was called once');
+        const groupItem = groupTemplateStub.getCall(0).args[0];
+
+        assert.strictEqual(groupItem.key, 'Group 1', 'Key is correct in template');
+        assert.ok(Object.prototype.hasOwnProperty.call(groupItem, 'customProp'), 'Custom property is present in group item');
+        assert.strictEqual(groupItem.customProp, 'customValue', 'Custom property value is correct');
+    });
 });
 
 QUnit.module('widget sizing render', {}, () => {