Fix flatten hit modify original array

changelogs/fragments/9347.yml

@@ -0,0 +1,2 @@
+fix:
+- Flatten hit modify original array ([#9347](https://github.com/opensearch-project/OpenSearch-Dashboards/pull/9347))

src/plugins/data/common/index_patterns/index_patterns/flatten_hit.ts

@@ -62,7 +62,8 @@ function flattenHit(indexPattern: IndexPattern, hit: Record<string, any>, deep:
 
       if (hasValidMapping || isValue) {
         if (!flat[key]) {
-          flat[key] = val;
+          // Create a new array to avoid modifying the original array when pushing elements
+          flat[key] = Array.isArray(val) ? [...val] : val;
         } else if (Array.isArray(flat[key])) {
           flat[key].push(val);
         } else {

src/plugins/data/common/index_patterns/index_patterns/index_pattern.test.ts

@@ -28,7 +28,7 @@
  * under the License.
  */
 
-import { map, last } from 'lodash';
+import { map, last, cloneDeep } from 'lodash';
 
 import { IndexPattern } from './index_pattern';
 
@@ -319,4 +319,77 @@ describe('IndexPatternWithDataSource', () => {
       expect(indexPattern.getSaveObjectReference()[0]?.name).toEqual('dataSource');
     });
   });
+
+  describe('flattenHit', () => {
+    test('should not modify original hit', () => {
+      const nestedArrayIndexPattern = new IndexPattern({
+        spec: {
+          id: 'test-nested-array',
+          type: 'index-pattern',
+          fields: {
+            'nested_test1.d_values': {
+              count: 0,
+              name: 'nested_test1.d_values',
+              type: 'number',
+              esTypes: ['double'],
+              scripted: false,
+              searchable: true,
+              aggregatable: true,
+              readFromDocValues: true,
+              subType: {
+                nested: {
+                  path: 'nested_test1',
+                },
+              },
+            },
+            'nested_test1.s_entry': {
+              count: 0,
+              name: 'nested_test1.s_entry',
+              type: 'string',
+              esTypes: ['keyword'],
+              scripted: false,
+              searchable: true,
+              aggregatable: true,
+              readFromDocValues: true,
+              subType: {
+                nested: {
+                  path: 'nested_test1',
+                },
+              },
+            },
+          },
+        },
+        savedObjectsClient: {} as any,
+        fieldFormats: fieldFormatsMock,
+        shortDotsEnable: false,
+        metaFields: [],
+      });
+
+      const hit = {
+        _index: 'test-nested-array',
+        _id: 'JPas2pQBluzwIEYCwD0y',
+        _score: 1,
+        _source: {
+          nested_test1: [
+            {
+              d_values: [0.1, 0.2],
+              s_entry: '4',
+            },
+            {
+              d_values: [0.3, 0.4],
+              s_entry: '5',
+            },
+            {
+              d_values: [0.5, 0.6],
+              s_entry: '6',
+            },
+          ],
+        },
+      };
+      const hitClone = cloneDeep(hit);
+      nestedArrayIndexPattern.flattenHit(hit);
+
+      expect(hit).toEqual(hitClone);
+    });
+  });
 });