From 7ace2db4dc2cf0f5b99e14089f1b76857295ccd5 Mon Sep 17 00:00:00 2001
From: Matthew Pietz <sirlancelot@users.noreply.github.com>
Date: Fri, 24 Oct 2025 15:22:03 -0700
Subject: [PATCH 1/3] Use `function` signature to support Vitest v4

---
 src/stubs.ts | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/src/stubs.ts b/src/stubs.ts
index 17c1a59..b913893 100644
--- a/src/stubs.ts
+++ b/src/stubs.ts
@@ -33,7 +33,7 @@ export const configureMock = <TMock extends Mock>(
   const behaviorStack = createBehaviorStack<TMock>()
   const fallbackImplementation = getFallbackImplementation(mock)
 
-  const implementation = (...args: ParametersOf<TMock>) => {
+  function implementation(...args: ParametersOf<TMock>) {
     const behavior = behaviorStack.use(args)?.behavior ?? {
       type: BehaviorType.DO,
       callback: fallbackImplementation,
@@ -59,7 +59,7 @@ export const configureMock = <TMock extends Mock>(
 
       case BehaviorType.DO: {
         // eslint-disable-next-line @typescript-eslint/no-unsafe-return
-        return behavior.callback?.(...args)
+        return behavior.callback?.call(this, ...args)
       }
     }
   }

From f92e4c265ba93704c2ead6c608b4e2046b8326ff Mon Sep 17 00:00:00 2001
From: Matthew Pietz <sirlancelot@users.noreply.github.com>
Date: Sat, 25 Oct 2025 10:44:06 -0700
Subject: [PATCH 2/3] Add test for `.thenDo()`

---
 src/stubs.ts             |  2 +-
 test/vitest-when.test.ts | 13 +++++++++++++
 2 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/src/stubs.ts b/src/stubs.ts
index b913893..da092a9 100644
--- a/src/stubs.ts
+++ b/src/stubs.ts
@@ -33,7 +33,7 @@ export const configureMock = <TMock extends Mock>(
   const behaviorStack = createBehaviorStack<TMock>()
   const fallbackImplementation = getFallbackImplementation(mock)
 
-  function implementation(...args: ParametersOf<TMock>) {
+  function implementation(this: ThisType<TMock>, ...args: ParametersOf<TMock>) {
     const behavior = behaviorStack.use(args)?.behavior ?? {
       type: BehaviorType.DO,
       callback: fallbackImplementation,
diff --git a/test/vitest-when.test.ts b/test/vitest-when.test.ts
index 063df06..786378c 100644
--- a/test/vitest-when.test.ts
+++ b/test/vitest-when.test.ts
@@ -129,6 +129,19 @@ describe('vitest-when', () => {
     expect(callback).toHaveBeenCalledTimes(1)
   })
 
+  it('should pass this to the callback', () => {
+    const ok = Symbol('ok')
+    const Spy = subject
+      .when(vi.fn())
+      .calledWith()
+      .thenDo(function (this: unknown) {
+        expect(this).toBeInstanceOf(Spy)
+        return { ok }
+      })
+
+    expect(new Spy()).toEqual({ ok })
+  })
+
   it('should return multiple values', () => {
     const spy = subject.when(vi.fn()).calledWith(1, 2, 3).thenReturn(4, 5, 6)
 

From 639c131e1a82fa947ba598e84ea567a41718ddb3 Mon Sep 17 00:00:00 2001
From: Michael Cousins <michael@cousins.io>
Date: Sun, 26 Oct 2025 23:25:56 -0400
Subject: [PATCH 3/3] fixup: make tests more focused on the end result

---
 test/vitest-when.test.ts | 24 ++++++++++++++++--------
 1 file changed, 16 insertions(+), 8 deletions(-)

diff --git a/test/vitest-when.test.ts b/test/vitest-when.test.ts
index 786378c..c04758d 100644
--- a/test/vitest-when.test.ts
+++ b/test/vitest-when.test.ts
@@ -1,6 +1,7 @@
 import { describe, expect, it, vi } from 'vitest'
 
 import * as subject from '../src/vitest-when.ts'
+import { SimpleClass } from './fixtures.ts'
 
 declare module 'vitest' {
   interface AsymmetricMatchersContaining {
@@ -129,17 +130,24 @@ describe('vitest-when', () => {
     expect(callback).toHaveBeenCalledTimes(1)
   })
 
-  it('should pass this to the callback', () => {
-    const ok = Symbol('ok')
+  it('should mock a constructor via thenDo', () => {
     const Spy = subject
-      .when(vi.fn())
-      .calledWith()
-      .thenDo(function (this: unknown) {
-        expect(this).toBeInstanceOf(Spy)
-        return { ok }
+      .when(vi.fn<typeof SimpleClass>())
+      .calledWith(42)
+      .thenDo(function (this: SimpleClass) {
+        this.simpleMethod = () => 'hello'
       })
 
-    expect(new Spy()).toEqual({ ok })
+    expect(new Spy(42).simpleMethod()).toBe('hello')
+  })
+
+  it('should mock a constructor via thenReturn', () => {
+    const Spy = subject
+      .when(vi.fn<typeof SimpleClass>())
+      .calledWith(42)
+      .thenReturn({ simpleMethod: () => 'hello' })
+
+    expect(new Spy(42).simpleMethod()).toBe('hello')
   })
 
   it('should return multiple values', () => {