Fixing prototype methods being discarded when using setData

src/utils.ts

@@ -80,20 +80,22 @@ export const mergeDeep = (
   if (!isObject(target) || !isObject(source)) {
     return source
   }
-  Object.keys(source).forEach((key) => {
-    const targetValue = target[key]
-    const sourceValue = source[key]
-
-    if (Array.isArray(targetValue) && Array.isArray(sourceValue)) {
-      target[key] = sourceValue
-    } else if (sourceValue instanceof Date) {
-      target[key] = sourceValue
-    } else if (isObject(targetValue) && isObject(sourceValue)) {
-      target[key] = mergeDeep(Object.assign({}, targetValue), sourceValue)
-    } else {
-      target[key] = sourceValue
-    }
-  })
+  Object.keys(source)
+    .concat(Object.getOwnPropertyNames(Object.getPrototypeOf(source) ?? {}))
+    .forEach((key) => {
+      const targetValue = target[key]
+      const sourceValue = source[key]
+
+      if (Array.isArray(targetValue) && Array.isArray(sourceValue)) {
+        target[key] = sourceValue
+      } else if (sourceValue instanceof Date) {
+        target[key] = sourceValue
+      } else if (isObject(targetValue) && isObject(sourceValue)) {
+        target[key] = mergeDeep(Object.assign({}, targetValue), sourceValue)
+      } else {
+        target[key] = sourceValue
+      }
+    })
 
   return target
 }

tests/setData.spec.ts

@@ -214,4 +214,36 @@ describe('setData', () => {
     expect(wrapper.vm.value).toBeInstanceOf(Date)
     expect(wrapper.vm.value!.toISOString()).toBe('2022-08-11T12:15:54.000Z')
   })
+
+  it('should retain prototype methods for constructed objects when calling setData', async () => {
+    const expectedResult = 'success!'
+    class TestClass {
+      constructor(readonly name: string) {}
+      getResult(): string {
+        return expectedResult
+      }
+    }
+
+    const wrapper = mount(
+      defineComponent({
+        template: '<div/>',
+        data() {
+          return { value: new TestClass('test1') }
+        },
+        methods: {
+          getResult() {
+            return `${this.value.name}: ${this.value.getResult()}`
+          }
+        }
+      })
+    )
+
+    expect(wrapper.vm.getResult()).toStrictEqual(`test1: ${expectedResult}`)
+
+    await wrapper.setData({
+      value: new TestClass('test2')
+    })
+
+    expect(wrapper.vm.getResult()).toStrictEqual(`test2: ${expectedResult}`)
+  })
 })