fix: support mocked constructors in Vitest v4 (#33)

Co-authored-by: Michael Cousins <michael@cousins.io>

Author: sirlancelot

src/stubs.ts

@@ -33,7 +33,7 @@ export const configureMock = <TFunc extends AnyMockable>(
   const behaviorStack = createBehaviorStack<TFunc>()
   const fallbackImplementation = getFallbackImplementation(mock)
 
-  const implementation = (...args: ParametersOf<TFunc>) => {
+  function implementation(this: ThisType<TFunc>, ...args: ParametersOf<TFunc>) {
     const behavior = behaviorStack.use(args)?.behavior ?? {
       type: BehaviorType.DO,
       callback: fallbackImplementation,
@@ -59,7 +59,7 @@ export const configureMock = <TFunc extends AnyMockable>(
 
       case BehaviorType.DO: {
         // eslint-disable-next-line @typescript-eslint/no-unsafe-return
-        return behavior.callback?.(...args)
+        return behavior.callback?.call(this, ...args)
       }
     }
   }

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 {
@@ -129,6 +130,26 @@ describe('vitest-when', () => {
     expect(callback).toHaveBeenCalledTimes(1)
   })
 
+  it('should mock a constructor via thenDo', () => {
+    const Spy = subject
+      .when(vi.fn<typeof SimpleClass>())
+      .calledWith(42)
+      .thenDo(function (this: SimpleClass) {
+        this.simpleMethod = () => 'hello'
+      })
+
+    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', () => {
     const spy = subject.when(vi.fn()).calledWith(1, 2, 3).thenReturn(4, 5, 6)