Add test for .thenDo()

Author: sirlancelot

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,

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)