From c759a9aacfc5f46c508f6c44a604ef0d01bf50af Mon Sep 17 00:00:00 2001
From: Alex Miller <codex.nz@gmail.com>
Date: Fri, 28 Apr 2023 00:29:55 +1200
Subject: [PATCH] fix(spy): update to set initial implementation through normal
 logic (fixes #3260) (#3263)

---
 packages/spy/src/index.ts        |  6 +++++-
 test/core/test/jest-mock.test.ts | 13 ++++++++++++-
 2 files changed, 17 insertions(+), 2 deletions(-)

diff --git a/packages/spy/src/index.ts b/packages/spy/src/index.ts
index 812459518a75..02c9a7e60362 100644
--- a/packages/spy/src/index.ts
+++ b/packages/spy/src/index.ts
@@ -324,5 +324,9 @@ export function fn<TArgs extends any[] = any[], R = any>(
 export function fn<TArgs extends any[] = any[], R = any>(
   implementation?: (...args: TArgs) => R,
 ): Mock<TArgs, R> {
-  return enhanceSpy(tinyspy.internalSpyOn({ fn: implementation || (() => {}) }, 'fn')) as unknown as Mock
+  const enhancedSpy = enhanceSpy(tinyspy.internalSpyOn({ spy: () => {} }, 'spy'))
+  if (implementation)
+    enhancedSpy.mockImplementation(implementation)
+
+  return enhancedSpy as Mock
 }
diff --git a/test/core/test/jest-mock.test.ts b/test/core/test/jest-mock.test.ts
index 92ef87786881..12bb6fa189ae 100644
--- a/test/core/test/jest-mock.test.ts
+++ b/test/core/test/jest-mock.test.ts
@@ -41,6 +41,17 @@ describe('jest mock compat layer', () => {
     expect(Spy.mock.instances).toHaveLength(0)
   })
 
+  it('implementation is set correctly on init', () => {
+    const impl = () => 1
+    const mock1 = vi.fn(impl)
+
+    expect(mock1.getMockImplementation()).toEqual(impl)
+
+    const mock2 = vi.fn()
+
+    expect(mock2.getMockImplementation()).toBeUndefined()
+  })
+
   it('implementation sync fn', () => {
     const originalFn = function () {
       return 'original'
@@ -49,7 +60,7 @@ describe('jest mock compat layer', () => {
 
     spy() // returns 'original'
 
-    expect(spy.getMockImplementation()).toBe(undefined)
+    expect(spy.getMockImplementation()).toBe(originalFn)
 
     spy.mockReturnValueOnce('2-once').mockReturnValueOnce('3-once')