Mocking composables and stores in Vue

Introduction

Mocking composables and stores helps you test Vue components without relying on real data or side effects. It makes tests faster and more reliable.

When you want to test a component that uses a composable but avoid calling its real logic.

When you want to simulate different store states in a component test.

When you want to isolate component behavior from external dependencies.

When you want to control the data returned by composables or stores during tests.

When you want to avoid network calls or complex logic in unit tests.

Syntax

Vue

import { vi } from 'vitest'
import * as composableModule from '@/composables/useExample'

vi.mock('@/composables/useExample', () => ({
  useExample: () => ({
    data: 'mocked data',
    method: vi.fn()
  })
}))

Use vi.mock() to replace the real composable or store module with a mock version.

Return the mocked functions or data you want your component to use during tests.

Examples

This mocks a Pinia store composable to provide a fake user and a mocked login method.

Vue

vi.mock('@/stores/userStore', () => ({
  useUserStore: () => ({
    user: { name: 'Test User' },
    login: vi.fn()
  })
}))

This mocks a composable that returns reactive data and loading state.

Vue

import { ref } from 'vue'

vi.mock('@/composables/useFetchData', () => ({
  useFetchData: () => ({
    data: ref('mocked data'),
    loading: ref(false)
  })
}))

Sample Program

This test mocks the useCounter composable to return a fixed count and a mocked increment function. It mounts MyComponent which uses this composable. Then it logs the rendered text and checks how many times increment was called.

Vue

import { mount } from '@vue/test-utils'
import { vi } from 'vitest'
import { ref } from 'vue'
import MyComponent from '@/components/MyComponent.vue'

// Mock the composable
vi.mock('@/composables/useCounter', () => {
  const count = ref(10)
  const increment = vi.fn()
  return {
    useCounter: () => ({
      count,
      increment
    })
  }
})

import { useCounter } from '@/composables/useCounter'

// Test
const wrapper = mount(MyComponent)

console.log(wrapper.text())

// Call increment method
useCounter().increment()

console.log(useCounter().increment.mock.calls.length)

Important Notes

Always import the mocked composable after calling vi.mock() to ensure the mock is applied.

Use vi.fn() to create mock functions you can track in tests.

Mocking helps keep tests fast and focused on the component, not external logic.

Summary

Mock composables and stores to isolate component tests from real logic.

Use vi.mock() to replace modules with fake versions.

Mock functions let you check calls and control behavior in tests.

Practice

1. What is the main reason to mock composables or stores in Vue component tests?

easy

A. To automatically generate UI snapshots

B. To improve the component's runtime performance

C. To isolate the component test from real logic and external dependencies

D. To enable two-way data binding in tests

Solution

1. Step 1: Understand the purpose of mocking

   Mocking replaces real implementations with fake ones to isolate tests from external logic.

2. Step 2: Apply this to composables and stores

   Mocking composables and stores prevents side effects and focuses tests on the component itself.

3. Final Answer:

   To isolate the component test from real logic and external dependencies -> Option C

4. Quick Check:

   Mocking isolates tests = A [OK]

Hint: Mock to isolate component logic from external code [OK]

Common Mistakes:

• Thinking mocking improves runtime performance
• Confusing mocking with UI snapshot generation
• Believing mocking enables data binding

2. Which syntax correctly mocks a composable module named useAuth using Vitest's vi.mock()?

easy

A. vi.mock('./useAuth', () => ({ useAuth: () => ({ isLoggedIn: true }) }))

B. vi.mock('useAuth', () => ({ isLoggedIn: true }))

C. vi.mock('useAuth', () => ({ useAuth: () => ({ isLoggedIn: true }) }))

D. vi.mock('./useAuth', () => ({ isLoggedIn: true }))

Solution

1. Step 1: Identify the module path

   Modules are mocked by their relative or absolute path, so './useAuth' is correct if the file is local.

2. Step 2: Provide a factory returning the mocked composable function

   The factory must return an object with the composable function that returns the mocked state.

3. Final Answer:

   vi.mock('./useAuth', () => ({ useAuth: () => ({ isLoggedIn: true }) })) -> Option A

4. Quick Check:

   Correct path and factory function = C [OK]

Hint: Mock with correct path and return a function for composables [OK]

Common Mistakes:

• Using module name without path when path is required
• Returning state object directly instead of a function
• Mocking without a factory function

3. Given this mocked store code snippet:

vi.mock('./store', () => ({ useStore: () => ({ count: 5, increment: vi.fn() }) }))

const store = useStore();
store.increment();
console.log(store.count);

What will be logged to the console?

medium

A. undefined

B. 5

C. Error: increment is not a function

D. 0

Solution

1. Step 1: Analyze the mocked store return values

   The mocked useStore returns an object with count: 5 and increment as a mock function.

2. Step 2: Check the console.log output

   The count property is 5 and is not changed by calling increment() because it's a mock function without implementation.

3. Final Answer:

   5 -> Option B

4. Quick Check:

   Mocked count value logged = 5 [OK]

Hint: Mocked state stays same unless mock function changes it [OK]

Common Mistakes:

• Assuming mock function changes state automatically
• Expecting undefined because increment is mocked
• Confusing mock function with real implementation

4. Identify the error in this test mocking a composable:

vi.mock('./useCounter', () => ({
  useCounter: {
    count: 0,
    increment: vi.fn()
  }
}))

medium

A. The mock should return a function, not an object directly

B. The vi.fn() should be replaced with a real function

C. The module path './useCounter' is incorrect

D. The mock should not include the increment function

Solution

1. Step 1: Understand composable mocking pattern

   Composable modules export functions, so mocks must return a function, not an object.

2. Step 2: Identify the incorrect mock structure

   The code returns an object directly instead of a function returning the object.

3. Final Answer:

   The mock should return a function, not an object directly -> Option A

4. Quick Check:

   Composable mocks return functions = D [OK]

Hint: Mock composables as functions returning objects [OK]

Common Mistakes:

• Returning object directly instead of function
• Replacing vi.fn() unnecessarily
• Incorrect module path assumptions

5. You want to test a Vue component that uses a Pinia store with a fetchData action. How do you mock the store to verify fetchData is called without running its real logic?

hard

A. Replace fetchData with an empty async function in the real store

B. Import the real store and spy on fetchData without mocking

C. Mock the component instead of the store

D. Use vi.mock() to replace the store module with a mock where fetchData is a vi.fn()

Solution

1. Step 1: Understand mocking for store actions

   To isolate tests, replace the store module with a mock that has fetchData as a mock function.

2. Step 2: Use vi.mock() to replace the store module

   This allows you to track calls to fetchData without executing its real code.

3. Final Answer:

   Use vi.mock() to replace the store module with a mock where fetchData is a vi.fn() -> Option D

4. Quick Check:

   Mock store action with vi.fn() = A [OK]

Hint: Mock store actions with vi.fn() to track calls without side effects [OK]

Common Mistakes:

• Not mocking the store and calling real logic
• Mocking the component instead of the store
• Replacing functions with empty but untracked implementations