Vue + Vite Unit Testing

Overview

Generate comprehensive, production-ready unit tests for Vue 3 + Vite projects using Vitest framework. Follow industry best practices for testing Vue components, composables, Pinia stores, and TypeScript utilities with proper isolation, mocking, and edge case coverage.

Testing Framework Setup

Primary Stack:

• Vitest: Fast unit test framework built for Vite
• Vue Test Utils: Official testing utility library for Vue components
• @vitest/ui: Optional UI for test visualization

Import pattern:

import { describe, it, expect, beforeEach, afterEach, vi } from 'vitest';
import { mount, shallowMount } from '@vue/test-utils';

Testing Workflow

Follow this systematic approach for all testing tasks:

1. Code Analysis Phase

Before writing any tests:

• Analyze the code structure and identify all public interfaces
• Identify external dependencies (APIs, stores, composables, modules)
• Note all possible code paths, conditions, and edge cases
• Ask clarifying questions about:
  • Missing type definitions or constants
  • Unclear business logic or validation rules
  • External API contracts or data structures
  • Expected error handling behaviors

Only proceed to writing tests after full code understanding.

2. Test Design Phase

Plan test coverage:

• Happy path scenarios (expected inputs and outputs)
• Error handling and failure modes
• Edge cases (empty arrays, null values, boundary conditions)
• Async operations (loading, success, error states)
• User interactions (clicks, inputs, form submissions)
• Lifecycle hooks and reactivity

For Vue components, identify:

• Props validation and default values
• Emitted events and their payloads
• Slots usage and content projection
• Computed properties and watchers
• Component lifecycle behavior

For composables, identify:

• Input parameters and return values
• State management and reactivity
• Side effects (API calls, localStorage, timers)
• Cleanup requirements

Test Structure Standards

Standard Test Template

import { describe, it, expect, beforeEach, afterEach, vi } from 'vitest';

describe('ModuleName or ComponentName', () => {
  // Top-level test variables
  let mockDependency: MockType;
  
  beforeEach(() => {
    // Reset state before each test
    mockDependency = createMockDependency();
  });

  afterEach(() => {
    // Cleanup after each test
    vi.clearAllMocks();
  });

  describe('method or feature name', () => {
    it('should handle happy path scenario', () => {
      // Arrange: Set up test data and mocks
      const input = { /* test data */ };
      
      // Act: Execute the code under test
      const result = functionUnderTest(input);
      
      // Assert: Verify expected outcomes
      expect(result).toBe(expectedValue);
    });

    it('should handle error case', async () => {
      // Arrange
      mockDependency.method.mockRejectedValue(new Error('test error'));
      
      // Act & Assert
      await expect(functionUnderTest()).rejects.toThrow('test error');
    });

    it('should handle edge case: empty input', () => {
      // Test edge cases
      expect(functionUnderTest([])).toEqual([]);
    });
  });
});

AAA Pattern (Arrange-Act-Assert)

Always structure individual tests using AAA:

it('should calculate total price correctly', () => {
  // Arrange: Set up test data
  const items = [
    { price: 100, quantity: 2 },
    { price: 50, quantity: 1 }
  ];
  
  // Act: Execute the function
  const total = calculateTotal(items);
  
  // Assert: Verify the result
  expect(total).toBe(250);
});

Vue-Specific Testing Patterns

1. Component Testing

Decide between mount vs shallowMount:

• Use mount() for integration testing with child components
• Use shallowMount() for isolated unit testing (stubs child components)

import { mount } from '@vue/test-utils';
import MyComponent from './MyComponent.vue';

describe('MyComponent', () => {
  it('should render with props', () => {
    const wrapper = mount(MyComponent, {
      props: {
        title: 'Test Title',
        count: 5
      }
    });
    
    expect(wrapper.find('h1').text()).toBe('Test Title');
    expect(wrapper.find('.count').text()).toBe('5');
  });

  it('should emit event on button click', async () => {
    const wrapper = mount(MyComponent);
    
    await wrapper.find('button').trigger('click');
    
    expect(wrapper.emitted('submit')).toBeTruthy();
    expect(wrapper.emitted('submit')[0]).toEqual([{ data: 'value' }]);
  });

  it('should handle v-model binding', async () => {
    const wrapper = mount(MyComponent, {
      props: {
        modelValue: 'initial'
      }
    });
    
    await wrapper.find('input').setValue('updated');
    
    expect(wrapper.emitted('update:modelValue')[0]).toEqual(['updated']);
  });
});

See references/component-testing.md for complete component testing patterns including slots, provide/inject, and async components.

2. Composables Testing

import { composableUnderTest } from './useFeature';

describe('useFeature composable', () => {
  it('should initialize with default state', () => {
    const { state, count } = composableUnderTest();
    
    expect(state.value).toBe('idle');
    expect(count.value).toBe(0);
  });

  it('should update reactive state', () => {
    const { increment, count } = composableUnderTest();
    
    increment();
    
    expect(count.value).toBe(1);
  });

  it('should handle async operations', async () => {
    const { fetchData, data, loading } = composableUnderTest();
    
    expect(loading.value).toBe(false);
    
    const promise = fetchData();
    expect(loading.value).toBe(true);
    
    await promise;
    expect(loading.value).toBe(false);
    expect(data.value).toBeDefined();
  });
});

See references/composables-testing.md for advanced composable testing patterns including side effects and cleanup.

3. Pinia Store Testing

import { setActivePinia, createPinia } from 'pinia';
import { useMyStore } from './myStore';

describe('myStore', () => {
  beforeEach(() => {
    setActivePinia(createPinia());
  });

  it('should initialize with default state', () => {
    const store = useMyStore();
    
    expect(store.items).toEqual([]);
    expect(store.loading).toBe(false);
  });

  it('should add item to store', () => {
    const store = useMyStore();
    const newItem = { id: 1, name: 'Test' };
    
    store.addItem(newItem);
    
    expect(store.items).toContainEqual(newItem);
  });

  it('should handle async actions', async () => {
    const store = useMyStore();
    
    await store.fetchItems();
    
    expect(store.loading).toBe(false);
    expect(store.items.length).toBeGreaterThan(0);
  });
});

See references/store-testing.md for Pinia store testing patterns including getters, mutations, and actions.

Mocking Strategies

External Dependencies

// Mock API calls
vi.mock('@/api/users', () => ({
  fetchUsers: vi.fn(),
  createUser: vi.fn()
}));

// Mock composables
vi.mock('@/composables/useAuth', () => ({
  useAuth: vi.fn(() => ({
    user: { id: 1, name: 'Test User' },
    isAuthenticated: true,
    login: vi.fn(),
    logout: vi.fn()
  }))
}));

// Mock Vue Router
const mockRouter = {
  push: vi.fn(),
  replace: vi.fn()
};

const wrapper = mount(Component, {
  global: {
    mocks: {
      $router: mockRouter
    }
  }
});

Timers and Delays

import { vi } from 'vitest';

describe('setTimeout behavior', () => {
  beforeEach(() => {
    vi.useFakeTimers();
  });

  afterEach(() => {
    vi.useRealTimers();
  });

  it('should execute callback after delay', () => {
    const callback = vi.fn();
    
    setTimeout(callback, 1000);
    
    expect(callback).not.toHaveBeenCalled();
    
    vi.advanceTimersByTime(1000);
    
    expect(callback).toHaveBeenCalledOnce();
  });
});

Testing Best Practices

1. Test Isolation

• Each test should be independent and not rely on other tests
• Use beforeEach to reset state
• Clean up mocks with vi.clearAllMocks() or vi.resetAllMocks()

2. Descriptive Test Names

// ✅ Good: Clear and descriptive
it('should display error message when API request fails', () => {});

// ❌ Bad: Vague and unclear
it('should work', () => {});

3. Avoid Test Logic

// ❌ Bad: Contains loops and conditions
it('should validate all items', () => {
  for (const item of items) {
    if (item.type === 'special') {
      expect(validate(item)).toBe(true);
    }
  }
});

// ✅ Good: Simple and direct
it('should validate special item', () => {
  const specialItem = { type: 'special', value: 100 };
  expect(validate(specialItem)).toBe(true);
});

it('should validate normal item', () => {
  const normalItem = { type: 'normal', value: 50 };
  expect(validate(normalItem)).toBe(true);
});

4. Test Coverage Priority

1. Critical business logic (payment, authentication, data validation)
2. Complex algorithms (calculations, transformations)
3. Error handling (edge cases, failure modes)
4. User interactions (forms, buttons, navigation)
5. Integration points (API calls, external services)

5. Async Testing

// ✅ Properly handle async operations
it('should fetch data successfully', async () => {
  const result = await fetchData();
  expect(result).toBeDefined();
});

// ✅ Use resolves/rejects for promises
await expect(fetchData()).resolves.toEqual(expectedData);
await expect(failingOperation()).rejects.toThrow('Error message');

Complete Test Deliverables

When generating tests, always provide:

1. Complete test suites - No placeholders or "// TODO" comments
2. All edge cases covered - Empty inputs, null values, boundaries
3. Proper imports - All necessary test utilities and dependencies
4. Appropriate mocks - For external dependencies and side effects
5. Clear test descriptions - Self-documenting test names
6. Proper cleanup - afterEach hooks where needed

Reference Files

For detailed examples and advanced patterns:

• references/component-testing.md - Comprehensive component testing patterns (slots, teleport, provide/inject, async components)
• references/composables-testing.md - Advanced composable testing (side effects, watchers, cleanup)
• references/store-testing.md - Pinia store testing patterns (getters, actions, state management)