add useful error when mock is missing exports

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
@@ -41,9 +44,19 @@ vi.mock('../src/log.ts', async () => {
 })
 
 describe('mocking with factory', () => {
-  test('successfuly mocked', () => {
+  test('missing exports on mock', () => {
+    expect(() => example.default).toThrowError('[vitest] No "default" export is defined on the "../src/example" mock defined here:')
+    expect(() => example.boolean).toThrowError('[vitest] No "boolean" export is defined on the "../src/example" mock defined here:')
+    expect(() => example.object).toThrowError('[vitest] No "object" export is defined on the "../src/example" mock defined here:')
+    expect(() => example.array).toThrowError('[vitest] No "array" export is defined on the "../src/example" mock defined here:')
+    expect(() => example.someClasses).toThrowError('[vitest] No "someClasses" export is defined on the "../src/example" mock defined here:')
+  })
+
+  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

@@ -335,7 +335,38 @@ export class VitestMocker {
     return this.request(dep)
   }
 
+  private wrapFactoryInProxy(id: string, importer: string, factory?: () => unknown) {
+    if (!factory)
+      return factory
+
+    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 "${prop}" export is defined on the "${id}" mock defined here: ${importer}`)
+        }
+
+        return val
+      },
+    }
+
+    const factoryHandler = {
+      apply(target: () => Object) {
+        return new Proxy(target(), exportHandler)
+      },
+    }
+
+    return new Proxy(factory, factoryHandler) as () => unknown
+  }
+
   public queueMock(id: string, importer: string, factory?: () => unknown) {
+    factory = this.wrapFactoryInProxy(id, importer, factory)
     VitestMocker.pendingIds.push({ type: 'mock', id, importer, factory })
   }