Test: Reactive spies preserve the this instance

code/lib/test/src/spy.test.ts

@@ -1,5 +1,5 @@
 import { it, vi, expect, beforeEach } from 'vitest';
-import { fn, onMockCall } from './spy';
+import { fn, onMockCall, spyOn } from './spy';
 
 const vitestSpy = vi.fn();
 
@@ -13,3 +13,27 @@ it('mocks are reactive', () => {
   storybookSpy(1);
   expect(vitestSpy).toHaveBeenCalledWith(storybookSpy, [1]);
 });
+
+class Foo {
+  bar = 'bar';
+
+  transform(postfix: string) {
+    return this.bar + postfix;
+  }
+}
+const foo = new Foo();
+
+it('this is correctly binded when making spies reactive', () => {
+  const storybookSpy = spyOn(foo, 'transform');
+  expect(foo.transform('!')).toEqual('bar!');
+  expect(vitestSpy).toHaveBeenCalledWith(storybookSpy, ['!']);
+});
+
+it('this is correctly binded after mock implementation', () => {
+  const storybookSpy = spyOn(foo, 'transform').mockImplementation(function (this: Foo) {
+    return this.bar + 'mocked';
+  });
+
+  expect(foo.transform('!')).toEqual('barmocked');
+  expect(vitestSpy).toHaveBeenCalledWith(storybookSpy, ['!']);
+});

code/lib/test/src/spy.ts

@@ -46,10 +46,10 @@ function reactiveMock(mock: MockInstance) {
 
 function listenWhenCalled(mock: MockInstance) {
   const state = tinyspy.getInternalState(mock as unknown as SpyInternalImpl);
-  const impl = state.impl?.bind(null);
-  state.willCall((...args) => {
+  const impl = state.impl;
+  state.willCall(function (this: unknown, ...args) {
     listeners.forEach((listener) => listener(mock, args));
-    return impl?.(...args);
+    return impl?.apply(this, args);
   });
   return mock;
 }