Also handle cloneProtoObject in mergeObject

index.js

@@ -109,11 +109,21 @@ function deepmergeConstructor (options) {
     }
 
     for (i = 0, il = sourceKeys.length; i < il; ++i) {
-      isNotPrototypeKey(key = sourceKeys[i]) &&
-      (
-        (key in target && (targetKeys.indexOf(key) !== -1 && (result[key] = _deepmerge(target[key], source[key])), true)) ||
-        (result[key] = clone(source[key]))
-      )
+      if (!isNotPrototypeKey(key = sourceKeys[i])) {
+        continue
+      }
+
+      if (key in target) {
+        if (targetKeys.indexOf(key) !== -1) {
+          if (cloneProtoObject && isMergeableObject(source[key]) && Object.getPrototypeOf(source[key]) !== JSON_PROTO) {
+            result[key] = cloneProtoObject(source[key])
+          } else {
+            result[key] = _deepmerge(target[key], source[key])
+          }
+        }
+      } else {
+        result[key] = clone(source[key])
+      }
     }
     return result
   }

test/merge-proto-objects.test.js

@@ -6,6 +6,12 @@ const { Readable } = require('node:stream')
 const deepmerge = require('../index')
 const { test } = require('tape')
 
+class Foo {
+  constructor (foo) {
+    this.foo = foo
+  }
+}
+
 test('merge nested objects should be immutable', function (t) {
   t.plan(3)
   const src = {
@@ -86,6 +92,23 @@ test('should not merge the buffers when cloned by reference', async t => {
   t.same(result.logger.buffer, Buffer.of(1, 2, 3))
 })
 
+test('should clone by reference with proto object in both source and target', async t => {
+  t.plan(4)
+  const foo2 = new Foo(2)
+  const result = deepmerge({
+    cloneProtoObject (x) {
+      t.ok(x instanceof Foo)
+      return x
+    }
+  })(
+    { foo: new Foo(1) },
+    { foo: foo2 }
+  )
+  t.equal(typeof result.foo, 'object')
+  t.ok(result.foo instanceof Foo)
+  t.same(result.foo, foo2)
+})
+
 test('doc example', async t => {
   const stream = process.stdout