fix: avoid array length truncation in mergeWith and add sparse merge test

Author: QDyanbing

src/test/mergeWith.test.ts

@@ -0,0 +1,16 @@
+import { mergeWith } from '../utils/set';
+
+describe('mergeWith array merge', () => {
+  it('should keep existing array length when merging sparse updates', () => {
+    const allValues = { list: ['A', 'B', 'C', 'D'] };
+    const changedValues = { list: new Array(2) };
+    changedValues.list[1] = 'BB'; // 仅更新第 2 项，长度为 2
+
+    const merged = mergeWith([allValues, changedValues], {
+      prepareArray: current => [...(current || [])],
+    });
+
+    expect(merged.list).toEqual(['A', 'BB', 'C', 'D']);
+    expect(merged.list).toHaveLength(4);
+  });
+});

src/utils/set.ts

@@ -110,7 +110,10 @@ export function mergeWith<T extends object>(
             clone = set(clone, path, createEmpty(value));
           }
 
-          keys(value).forEach(key => {
+          const valueKeys = isArr
+            ? keys(value).filter(key => key !== 'length')
+            : keys(value);
+          valueKeys.forEach(key => {
             internalMerge([...path, key], loopSet);
           });
         }