fix: stop changing the behaviour of spies on mockFn.mockReset

CHANGELOG.md

@@ -10,6 +10,7 @@
 
 ### Fixes
 
+- `[jest-mock]` fix mockReset and resetAllMocks undefined return ([#13692](https://github.com/facebook/jest/pull/13692))
 - `[jest-environment-node]` fix non-configurable globals ([#13687](https://github.com/facebook/jest/pull/13687))
 - `[@jest/expect-utils]` `toMatchObject` should handle `Symbol` properties ([#13639](https://github.com/facebook/jest/pull/13639))
 - `[jest-resolve]` Add global paths to `require.resolve.paths` ([#13633](https://github.com/facebook/jest/pull/13633))

docs/MockFunctionAPI.md

@@ -126,7 +126,7 @@ The [`clearMocks`](configuration#clearmocks-boolean) configuration option is ava
 
 Does everything that [`mockFn.mockClear()`](#mockfnmockclear) does, and also removes any mocked return values or implementations.
 
-This is useful when you want to completely reset a _mock_ back to its initial state. (Note that resetting a _spy_ will result in a function with no return value).
+This is useful when you want to completely reset a _mock_ back to its initial state.
 
 The [`resetMocks`](configuration#resetmocks-boolean) configuration option is available to reset mocks automatically before each test.
 

e2e/mock-reset/__tests__/index.test.js

@@ -0,0 +1,46 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates. All Rights Reserved.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+'use strict';
+
+describe('with mock reset', () => {
+  const myObject = {bar: () => 'bar'};
+
+  let barStub;
+
+  beforeAll(() => {
+    barStub = jest.spyOn(myObject, 'bar');
+  });
+
+  test('after mock reset, bar should return to its original value', () => {
+    barStub.mockReturnValue('POTATO!');
+    expect(myObject.bar()).toBe('POTATO!');
+    barStub.mockReset();
+
+    // This expect should be successful
+    expect(myObject.bar()).toBe('bar');
+  });
+});
+
+describe('with reset all mocks', () => {
+  const myObject = {bar: () => 'bar', foo: () => 'foo'};
+
+  let barStub;
+
+  beforeAll(() => {
+    barStub = jest.spyOn(myObject, 'bar');
+  });
+
+  test('after reset all, bar should return to its original value', () => {
+    barStub.mockReturnValue('POTATO!');
+    expect(myObject.bar()).toBe('POTATO!');
+    jest.resetAllMocks();
+
+    // This expect should be successful
+    expect(myObject.bar()).toBe('bar');
+  });
+});

e2e/mock-reset/package.json

@@ -0,0 +1,5 @@
+{
+  "jest": {
+    "testEnvironment": "node"
+  }
+}

packages/jest-mock/src/index.ts

@@ -487,6 +487,7 @@ export class ModuleMocker {
   private _mockConfigRegistry: WeakMap<Function, MockFunctionConfig>;
   private _spyState: Set<() => void>;
   private _invocationCallCounter: number;
+  private _originalFn: WeakMap<Mock, Function>;
 
   /**
    * @see README.md
@@ -499,6 +500,7 @@ export class ModuleMocker {
     this._mockConfigRegistry = new WeakMap();
     this._spyState = new Set();
     this._invocationCallCounter = 1;
+    this._originalFn = new WeakMap();
   }
 
   private _getSlots(object?: Record<string, any>): Array<string> {
@@ -753,7 +755,12 @@ export class ModuleMocker {
 
       f.mockReset = () => {
         f.mockClear();
-        this._mockConfigRegistry.delete(f);
+        const originalFn = this._originalFn.get(f);
+        const originalMockImpl = {
+          ...this._defaultMockConfig(),
+          mockImpl: originalFn,
+        };
+        this._mockConfigRegistry.set(f, originalMockImpl);
         return f;
       };
 
@@ -1166,7 +1173,7 @@ export class ModuleMocker {
         return original.apply(this, arguments);
       });
     }
-
+    this._originalFn.set(object[methodKey], original);
     return object[methodKey] as Mock;
   }
 
@@ -1235,6 +1242,7 @@ export class ModuleMocker {
   }
 
   resetAllMocks(): void {
+    this._spyState.forEach(reset => reset());
     this._mockConfigRegistry = new WeakMap();
     this._mockState = new WeakMap();
   }