handle undefined returns of module mocks, and update migration docs

docs/guide/migration.md

@@ -14,6 +14,19 @@ Jest has their [globals API](https://jestjs.io/docs/api) enabled by default. Vit
 
 If you decide to keep globals disabled, be aware that common libraries like [`testing-library`](https://testing-library.com/) will not run auto DOM [cleanup](https://testing-library.com/docs/svelte-testing-library/api/#cleanup).
 
+**Module mocks**
+
+When mocking a module in Jest, if the factory argument returned a primitive, the mock would implicitly mock the default export. In Vitest, the factory argument has to return an object where each export is explicitly defined. For example, the following `jest.mock` would have to be updated as follows:
+
+```diff
+- jest.mock('./some-path', () => 'hello')
++ vi.mock('./some-path', () => ({
++   default: 'hello';
++ })
+```
+
+For more details please refer to the [vi.mock api](/api/#vi-mock)
+
 **Auto-Mocking Behaviour**
 
 Unlike Jest, mocked modules in `<root>/__mocks__` are not loaded unless `vi.mock()` is called. If you need them to be mocked in every test, like in Jest, you can mock them inside [`setupFiles`](/config/#setupfiles).

examples/mocks/test/factory.test.ts

@@ -7,6 +7,7 @@ vi
   .mock('../src/example', () => ({
     mocked: true,
     then: 'a then export',
+    ok: undefined,
     square: (a: any, b: any) => a + b,
     asyncSquare: async (a: any, b: any) => Promise.resolve(a + b),
   }))
@@ -60,6 +61,7 @@ describe('mocking with factory', () => {
   })
 
   test('defined exports on mock', async () => {
+    expect((example as any).ok).toBe(undefined)
     expect((example as any).then).toBe('a then export')
     expect((example as any).mocked).toBe(true)
     expect(example.square(2, 3)).toBe(5)

packages/vitest/src/runtime/mocker.ts

@@ -122,7 +122,7 @@ export class VitestMocker {
           if (target instanceof Promise)
             return target.then.bind(target)
         }
-        else if (val === undefined) {
+        else if (!(prop in target)) {
           throw new Error(`[vitest] No "${prop}" export is defined on the "${dep}"`)
         }