From 32e194e842d682c0dfc86165f795e2aaa3e06e25 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?=E9=AB=98=E8=89=B3=E5=85=B5?= <yanbing.gao@aliyun.com>
Date: Mon, 22 Dec 2025 16:42:52 +0800
Subject: [PATCH 1/3] fix: avoid array length truncation in mergeWith and add
 sparse merge test

---
 src/test/mergeWith.test.ts | 16 ++++++++++++++++
 src/utils/set.ts           |  5 ++++-
 2 files changed, 20 insertions(+), 1 deletion(-)
 create mode 100644 src/test/mergeWith.test.ts

diff --git a/src/test/mergeWith.test.ts b/src/test/mergeWith.test.ts
new file mode 100644
index 00000000..c01b5654
--- /dev/null
+++ b/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);
+  });
+});
diff --git a/src/utils/set.ts b/src/utils/set.ts
index c0db65d5..061ef0c9 100644
--- a/src/utils/set.ts
+++ b/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);
           });
         }

From 4550dd2bf59585715f8ffac3338bfcfe60a35afd Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?=E9=AB=98=E8=89=B3=E5=85=B5?= <yanbing.gao@aliyun.com>
Date: Mon, 22 Dec 2025 16:51:44 +0800
Subject: [PATCH 2/3] fix: skip array length key inline in mergeWith traversal

---
 src/utils/set.ts | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/src/utils/set.ts b/src/utils/set.ts
index 061ef0c9..0968e108 100644
--- a/src/utils/set.ts
+++ b/src/utils/set.ts
@@ -110,10 +110,10 @@ export function mergeWith<T extends object>(
             clone = set(clone, path, createEmpty(value));
           }
 
-          const valueKeys = isArr
-            ? keys(value).filter(key => key !== 'length')
-            : keys(value);
-          valueKeys.forEach(key => {
+          keys(value).forEach(key => {
+            if (isArr && key === 'length') {
+              return;
+            }
             internalMerge([...path, key], loopSet);
           });
         }

From a2f831cae6b5a8785dc7f6ebfc625a64c8fb5770 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?=E9=AB=98=E8=89=B3=E5=85=B5?= <yanbing.gao@aliyun.com>
Date: Mon, 22 Dec 2025 18:04:08 +0800
Subject: [PATCH 3/3] refactor: avoid hardcoded length handling in merge
 traversal

---
 src/utils/set.ts | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/src/utils/set.ts b/src/utils/set.ts
index 0968e108..7d465a56 100644
--- a/src/utils/set.ts
+++ b/src/utils/set.ts
@@ -111,10 +111,9 @@ export function mergeWith<T extends object>(
           }
 
           keys(value).forEach(key => {
-            if (isArr && key === 'length') {
-              return;
+            if (Object.getOwnPropertyDescriptor(value, key).enumerable) {
+              internalMerge([...path, key], loopSet);
             }
-            internalMerge([...path, key], loopSet);
           });
         }
       } else {