Fix mock reassigning to previously unexisting method

CHANGELOG.md

@@ -22,6 +22,7 @@
 - `[jest-haste-map]` Don't throw on missing mapper in Node crawler ([#8558](https://github.com/facebook/jest/pull/8558))
 - `[jest-core]` Fix incorrect `passWithNoTests` warning ([#8595](https://github.com/facebook/jest/pull/8595))
 - `[jest-snapshots]` Fix test retries that contain snapshots ([#8629](https://github.com/facebook/jest/pull/8629))
+- `[jest-mock]` Fix incorrect assignments when restoring mocks in instances where they originally didn't exist ([#8631](https://github.com/facebook/jest/pull/8631))
 
 ### Chore & Maintenance
 

packages/jest-mock/src/__tests__/index.test.ts

@@ -542,6 +542,32 @@ describe('moduleMocker', () => {
       });
     });
 
+    it('mocks the method in the passed object itself', () => {
+      const parent = {func: () => 'abcd'};
+      const child = Object.create(parent);
+
+      moduleMocker.spyOn(child, 'func').mockReturnValue('efgh');
+
+      expect(child.hasOwnProperty('func')).toBe(true);
+      expect(child.func()).toEqual('efgh');
+      expect(parent.func()).toEqual('abcd');
+    });
+
+    it('should delete previously inexistent methods when restoring', () => {
+      const parent = {func: () => 'abcd'};
+      const child = Object.create(parent);
+
+      moduleMocker.spyOn(child, 'func').mockReturnValue('efgh');
+
+      moduleMocker.restoreAllMocks();
+      expect(child.func()).toEqual('abcd');
+
+      moduleMocker.spyOn(parent, 'func').mockReturnValue('jklm');
+
+      expect(child.hasOwnProperty('func')).toBe(false);
+      expect(child.func()).toEqual('jklm');
+    });
+
     it('supports mock value returning undefined', () => {
       const obj = {
         func: () => 'some text',

packages/jest-mock/src/index.ts

@@ -1009,9 +1009,15 @@ class ModuleMockerClass {
         );
       }
 
+      const isMethodOwner = object.hasOwnProperty(methodName);
+
       // @ts-ignore overriding original method with a Mock
       object[methodName] = this._makeComponent({type: 'function'}, () => {
-        object[methodName] = original;
+        if (isMethodOwner) {
+          object[methodName] = original;
+        } else {
+          delete object[methodName];
+        }
       });
 
       // @ts-ignore original method is now a Mock