fix: independently mock each instance's methods for mocked class

packages/vitest/src/runtime/mocker.ts

@@ -328,7 +328,20 @@ export class VitestMocker {
           const spyModule = this.spyModule
           if (!spyModule)
             throw this.createError('[vitest] `spyModule` is not defined. This is Vitest error. Please open a new issue with reproduction.')
-          const mock = spyModule.spyOn(newContainer, property).mockImplementation(() => undefined)
+          const mock = spyModule.spyOn(newContainer, property).mockImplementation(function (this: any) {
+            // do similar to ject? https://github.com/jestjs/jest/blob/2c3d2409879952157433de215ae0eee5188a4384/packages/jest-mock/src/index.ts#L678-L691
+            // check constructor call
+            if (this instanceof newContainer[property]) {
+              // mock each class instance's method
+              // TODO: more sound way to loop prorotype methods?
+              for (const key in this) {
+                if (typeof this[key] === 'function') {
+                  // TODO: ability to restore?
+                  spyModule.spyOn(this, key).mockImplementation(() => undefined)
+                }
+              }
+            }
+          })
           mock.mockRestore = () => {
             mock.mockReset()
             mock.mockImplementation(() => undefined)

test/core/src/mockedE.ts

@@ -0,0 +1,5 @@
+export class MockedE {
+  public doSomething() {
+    return 'hello'
+  }
+}

test/core/test/mocked-wip.test.ts

@@ -0,0 +1,22 @@
+import { expect, test, vi } from 'vitest'
+
+import { MockedE } from '../src/mockedE'
+
+vi.mock('../src/mockedE')
+
+// TODO: move to mocked.test.ts
+
+test('mock each instance method separately', () => {
+  expect(MockedE).toBeTypeOf('function')
+
+  const instance1 = new MockedE()
+  const instance2 = new MockedE()
+  expect(instance1).not.toBe(instance2)
+  expect(instance1.doSomething).not.toBe(instance2.doSomething)
+  expect(vi.mocked(instance1.doSomething).mock).not.toBe(vi.mocked(instance2.doSomething).mock)
+
+  instance1.doSomething()
+  expect(instance1.doSomething).toBeCalledTimes(1)
+  expect(instance2.doSomething).toBeCalledTimes(0)
+  expect(MockedE.prototype.doSomething).toBeCalledTimes(0)
+})

test/core/test/mocked.test.ts

@@ -76,7 +76,8 @@ describe('mocked classes', () => {
     expect(instance.doSomething).toBeTypeOf('function')
     expect(instance.doSomething()).not.toBe('A')
 
-    expect(MockedC.prototype.doSomething).toHaveBeenCalledOnce()
+    // TODO: check what jest does
+    expect(MockedC.prototype.doSomething).not.toHaveBeenCalledOnce()
     expect(MockedC.prototype.doSomething).not.toHaveReturnedWith('A')
   })