From eb9be5aca24951246842a40d45fd1a77e539ca4f Mon Sep 17 00:00:00 2001
From: Aaron Abramov <dmitrii@abramov.io>
Date: Thu, 29 Jun 2017 00:58:00 -0700
Subject: [PATCH] fix mock return value (#3933)

---
 packages/jest-mock/src/__tests__/jest_mock.test.js | 8 ++++++++
 packages/jest-mock/src/index.js                    | 1 +
 2 files changed, 9 insertions(+)

diff --git a/packages/jest-mock/src/__tests__/jest_mock.test.js b/packages/jest-mock/src/__tests__/jest_mock.test.js
index 5b07888980ca..4aa0645df5cd 100644
--- a/packages/jest-mock/src/__tests__/jest_mock.test.js
+++ b/packages/jest-mock/src/__tests__/jest_mock.test.js
@@ -422,6 +422,14 @@ describe('moduleMocker', () => {
     });
   });
 
+  test('mockImplementation resets the mock', () => {
+    const fn = jest.fn();
+    expect(fn()).toBeUndefined();
+    fn.mockReturnValue('returnValue');
+    fn.mockImplementation(() => 'foo');
+    expect(fn()).toBe('foo');
+  });
+
   it('should recognize a mocked function', () => {
     const mockFn = moduleMocker.fn();
 
diff --git a/packages/jest-mock/src/index.js b/packages/jest-mock/src/index.js
index e41b5c221ff5..ea49d9452fff 100644
--- a/packages/jest-mock/src/index.js
+++ b/packages/jest-mock/src/index.js
@@ -348,6 +348,7 @@ class ModuleMockerClass {
         // next function call will use mock implementation return value
         const mockConfig = this._ensureMockConfig(f);
         mockConfig.isReturnValueLastSet = false;
+        mockConfig.defaultReturnValue = undefined;
         mockConfig.mockImpl = fn;
         return f;
       };