fix: add error message when mock is missing export

examples/mocks/src/default.ts

@@ -0,0 +1 @@
+export default 'a default'

examples/mocks/src/example.ts

@@ -25,3 +25,4 @@ export const number = 123
 export const string = 'baz'
 export const boolean = true
 export const symbol = Symbol.for('a.b.c')
+export default 'a default'

examples/mocks/test/factory.test.ts

@@ -6,6 +6,9 @@ import logger from '../src/log'
 vi
   .mock('../src/example', () => ({
     mocked: true,
+    then: 'a then export',
+    square: (a: any, b: any) => a + b,
+    asyncSquare: async (a: any, b: any) => Promise.resolve(a + b),
   }))
 
 // doesn't think comments are mocks
@@ -40,10 +43,27 @@ vi.mock('../src/log.ts', async () => {
   }
 })
 
+vi.mock('../src/default.ts', () => null)
+
 describe('mocking with factory', () => {
-  test('successfuly mocked', () => {
+  test('missing exports on mock', () => {
+    expect(() => example.default).toThrowError('[vitest] No "default" export is defined on the "mock:/src/example.ts"')
+    expect(() => example.boolean).toThrowError('[vitest] No "boolean" export is defined on the "mock:/src/example.ts"')
+    expect(() => example.object).toThrowError('[vitest] No "object" export is defined on the "mock:/src/example.ts"')
+    expect(() => example.array).toThrowError('[vitest] No "array" export is defined on the "mock:/src/example.ts"')
+    expect(() => example.someClasses).toThrowError('[vitest] No "someClasses" export is defined on the "mock:/src/example.ts"')
+  })
+
+  it('non-object return on factory gives error', async () => {
+    await expect(() => import('../src/default').then(m => m.default)).rejects
+      .toThrowError('[vitest] vi.mock(path: string, factory?: () => unknown) is not returning an object. Did you mean to return an object with a "default" key?')
+  })
+
+  test('defined exports on mock', async () => {
+    expect((example as any).then).toBe('a then export')
     expect((example as any).mocked).toBe(true)
-    expect(example.boolean).toBeUndefined()
+    expect(example.square(2, 3)).toBe(5)
+    expect(example.asyncSquare(2, 3)).resolves.toBe(5)
   })
 
   test('successfuly with actual', () => {

packages/vitest/src/runtime/mocker.ts

@@ -107,8 +107,30 @@ export class VitestMocker {
     if (cached)
       return cached
     const exports = await mock()
+
+    if (exports === null || typeof exports !== 'object')
+      throw new Error('[vitest] vi.mock(path: string, factory?: () => unknown) is not returning an object. Did you mean to return an object with a "default" key?')
+
     this.moduleCache.set(dep, { exports })
-    return exports
+
+    const exportHandler = {
+      get(target: Record<string, any>, prop: any) {
+        const val = target[prop]
+
+        // 'then' can exist on non-Promise objects, need nested instanceof check for logic to work
+        if (prop === 'then') {
+          if (target instanceof Promise)
+            return target.then.bind(target)
+        }
+        else if (val === undefined) {
+          throw new Error(`[vitest] No "${prop}" export is defined on the "${dep}"`)
+        }
+
+        return val
+      },
+    }
+
+    return new Proxy(exports, exportHandler)
   }
 
   private getMockPath(dep: string) {