diff --git a/test/mergeAndCompare.test.ts b/test/mergeAndCompare.test.ts
index 5e5e6f5..154bdd3 100644
--- a/test/mergeAndCompare.test.ts
+++ b/test/mergeAndCompare.test.ts
@@ -1,5 +1,5 @@
 import { test, expect } from 'vitest'
-import { isDate, isString, isArray, isObject } from 'is-what'
+import { isDate, isString, isArray } from 'is-what'
 import { mergeAndCompare } from '../src/index'
 
 test('conversion based on original val', () => {
diff --git a/test/mergeArrays.test.ts b/test/mergeArrays.test.ts
new file mode 100644
index 0000000..2ed3cc9
--- /dev/null
+++ b/test/mergeArrays.test.ts
@@ -0,0 +1,70 @@
+import { test, expect } from 'vitest'
+import { isArray } from 'is-what'
+import { mergeAndCompare, merge } from '../src/index'
+
+function mergeArrays(originVal: any, newVal: any): any | any[] {
+  if (isArray(originVal) && isArray(newVal)) {
+    // concat & merge logic
+    const overlappingPart = originVal.slice(0, newVal.length)
+
+    return overlappingPart
+      .map((p, i) => (newVal[i] ? merge(p, newVal[i]) : p))
+      .concat(
+        newVal.length > originVal.length
+          ? originVal.slice(newVal.length)
+          : newVal.slice(originVal.length)
+      )
+  }
+  return newVal // always return newVal as fallback!!
+}
+
+test('undefined object', () => {
+  function merge(originVal: any, targetVal: any, key: any) {
+    if (originVal !== undefined) return targetVal
+  }
+
+  const origin = {
+    pages: {
+      aa: 'ttt',
+    },
+  }
+
+  const newData = {
+    pages: {
+      aa: '1111',
+      bb: '2222',
+    },
+  }
+
+  const res = mergeAndCompare(mergeArrays, origin, newData)
+  expect(res as any).toEqual({ pages: { aa: '1111', bb: '2222' } })
+})
+
+test('undefined array', () => {
+  const origin = {
+    date: [
+      {
+        new: 'aa',
+        something: 'yy',
+      },
+    ],
+  }
+  const target = {
+    date: [
+      {
+        new: 'bb',
+        old: 'zz',
+      },
+    ],
+  }
+  const res = mergeAndCompare(mergeArrays, origin, target)
+  expect(res as any).toEqual({
+    date: [
+      {
+        new: 'bb',
+        something: 'yy',
+        old: 'zz',
+      },
+    ],
+  })
+})