Vitest Testing Patterns Agent Skill

Write tests using Vitest and React Testing Library. Use when creating unit tests, component tests, integration tests, or mocking dependencies. Activates for test file creation, mock patterns, coverage, and testing best practices.

3k tokens

context cost

the whole folder, loaded on every use

files

instructions only

copies elsewhere

how many repositories repackaged it

177

stars on the repo

on the repository, not the skill itself

Install

one command, takes just this skill from the repository

npx skills add https://github.com/curiositech/some_claude_skills --skill vitest-testing-patterns

The instruction itself

19 sections, as written by the author

Vitest Testing Patterns

This skill helps you write effective tests using Vitest and React Testing Library following project conventions.

When to Use

✅ USE this skill for:

Writing unit tests for utilities and functions
Creating component tests with React Testing Library
Setting up mocks for API calls, databases, or external services
Integration testing patterns
Understanding test coverage and CI setup

❌ DO NOT use for:

Jest-specific patterns → similar but check Jest docs for differences
End-to-end testing → use Playwright or Cypress skills
Performance testing → use dedicated performance tools
API contract testing → use OpenAPI/Pact patterns

Test Infrastructure

Configuration: vitest.config.ts

Environment: jsdom
Setup file: src/test/setup.ts
Coverage: v8 provider

Commands:

npm test              # Watch mode
npm run test:run      # Single run
npm run test:coverage # With coverage

File Organization

src/
├── app/api/__tests__/        # API route tests
├── components/__tests__/     # Component tests
├── lib/__tests__/            # Library/utility tests
└── lib/{feature}/__tests__/  # Feature-specific tests

Name tests as {name}.test.ts or {name}.test.tsx.

Core Testing Patterns

1. API Route Tests

import { describe, it, expect, vi, beforeEach } from 'vitest';
import { GET, POST } from '../route';
import { NextRequest } from 'next/server';

// Mock dependencies
vi.mock('@/lib/auth', () => ({
  getSession: vi.fn(),
}));

vi.mock('@/db', () => ({
  db: {
    select: vi.fn().mockReturnThis(),
    from: vi.fn().mockReturnThis(),
    where: vi.fn().mockResolvedValue([]),
  },
}));

describe('GET /api/feature', () => {
  beforeEach(() => {
    vi.clearAllMocks();
  });

  it('returns 401 when not authenticated', async () => {
    vi.mocked(getSession).mockResolvedValue(null);

    const request = new NextRequest('http://localhost/api/feature');
    const response = await GET(request);

    expect(response.status).toBe(401);
  });

  it('returns data when authenticated', async () => {
    vi.mocked(getSession).mockResolvedValue({ userId: 'user-123' });
    vi.mocked(db.select).mockReturnValue({
      from: vi.fn().mockReturnValue({
        where: vi.fn().mockResolvedValue([{ id: '1', name: 'Test' }]),
      }),
    });

    const request = new NextRequest('http://localhost/api/feature');
    const response = await GET(request);
    const data = await response.json();

    expect(response.status).toBe(200);
    expect(data).toHaveLength(1);
  });
});

2. Component Tests

import { describe, it, expect, vi } from 'vitest';
import { render, screen, waitFor } from '@testing-library/react';
import userEvent from '@testing-library/user-event';
import { FeatureComponent } from '../FeatureComponent';

// Mock hooks
vi.mock('@/hooks/useAuth', () => ({
  useAuth: vi.fn().mockReturnValue({
    user: { id: 'user-123', name: 'Test User' },
    isLoading: false,
  }),
}));

describe('FeatureComponent', () => {
  it('renders loading state', () => {
    vi.mocked(useAuth).mockReturnValueOnce({
      user: null,
      isLoading: true,
    });

    render(<FeatureComponent />);
    expect(screen.getByText(/loading/i)).toBeInTheDocument();
  });

  it('handles user interaction', async () => {
    const user = userEvent.setup();
    const onSubmit = vi.fn();

    render(<FeatureComponent onSubmit={onSubmit} />);

    await user.type(screen.getByRole('textbox'), 'Test input');
    await user.click(screen.getByRole('button', { name: /submit/i }));

    expect(onSubmit).toHaveBeenCalledWith('Test input');
  });

  it('displays error state', async () => {
    vi.mocked(fetch).mockRejectedValueOnce(new Error('Network error'));

    render(<FeatureComponent />);

    await waitFor(() => {
      expect(screen.getByRole('alert')).toHaveTextContent(/error/i);
    });
  });
});

3. Library/Utility Tests

import { describe, it, expect, vi, beforeEach, afterEach } from 'vitest';
import { processData, formatDate } from '../utils';

describe('processData', () => {
  it('transforms input correctly', () => {
    const input = { raw: 'data' };
    const result = processData(input);

    expect(result).toEqual({
      processed: true,
      data: 'DATA',
    });
  });

  it('throws on invalid input', () => {
    expect(() => processData(null)).toThrow('Invalid input');
  });
});

describe('formatDate', () => {
  beforeEach(() => {
    vi.useFakeTimers();
    vi.setSystemTime(new Date('2025-01-15T10:00:00Z'));
  });

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

  it('formats relative dates', () => {
    const yesterday = new Date('2025-01-14T10:00:00Z');
    expect(formatDate(yesterday)).toBe('yesterday');
  });
});

Mocking Patterns

Module Mocking

// Mock entire module
vi.mock('@/lib/auth', () => ({
  getSession: vi.fn(),
  requireAuth: vi.fn(),
}));

// Mock with partial implementation
vi.mock('date-fns', async () => {
  const actual = await vi.importActual('date-fns');
  return {
    ...actual,
    format: vi.fn(() => '2025-01-15'),
  };
});

// Mock default export (like Anthropic SDK)
vi.mock('@anthropic-ai/sdk', () => ({
  default: class MockAnthropic {
    messages = {
      create: vi.fn().mockResolvedValue({
        content: [{ type: 'text', text: 'Mock response' }],
        usage: { input_tokens: 10, output_tokens: 20 },
      }),
    };
  },
}));

Function Mocking

// Create mock function
const mockFn = vi.fn();

// Set return values
mockFn.mockReturnValue('sync value');
mockFn.mockResolvedValue('async value');
mockFn.mockRejectedValue(new Error('Failed'));

// One-time behavior
mockFn.mockReturnValueOnce('first call only');

// Custom implementation
mockFn.mockImplementation((arg) => arg.toUpperCase());

// Verify calls
expect(mockFn).toHaveBeenCalled();
expect(mockFn).toHaveBeenCalledTimes(2);
expect(mockFn).toHaveBeenCalledWith('expected', 'args');

Chained Mock Pattern (Drizzle ORM)

vi.mock('@/db', () => ({
  db: {
    select: vi.fn().mockReturnValue({
      from: vi.fn().mockReturnValue({
        where: vi.fn().mockReturnValue({
          orderBy: vi.fn().mockReturnValue({
            limit: vi.fn().mockResolvedValue([{ id: '1' }]),
          }),
        }),
      }),
    }),
    insert: vi.fn().mockReturnValue({
      values: vi.fn().mockReturnValue({
        returning: vi.fn().mockResolvedValue([{ id: 'new-1' }]),
      }),
    }),
  },
}));

Timer Mocking

describe('debounced function', () => {
  beforeEach(() => {
    vi.useFakeTimers();
  });

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

  it('debounces calls', async () => {
    const callback = vi.fn();
    const debounced = debounce(callback, 300);

    debounced();
    debounced();
    debounced();

    expect(callback).not.toHaveBeenCalled();

    vi.advanceTimersByTime(300);

    expect(callback).toHaveBeenCalledTimes(1);
  });
});

Query Priorities

Use queries in this order (most to least preferred):

getByRole - Accessible queries (buttons, links, headings)
getByLabelText - Form fields with labels
getByPlaceholderText - Inputs with placeholders
getByText - Non-interactive elements
getByTestId - Last resort (data-testid)

// Preferred
screen.getByRole('button', { name: /submit/i });
screen.getByRole('heading', { level: 1 });
screen.getByLabelText(/email/i);

// Avoid unless necessary
screen.getByTestId('submit-button');

Async Patterns

// Wait for element to appear
await waitFor(() => {
  expect(screen.getByText('Loaded')).toBeInTheDocument();
});

// Find (built-in waitFor)
const element = await screen.findByText('Loaded');

// Wait for element to disappear
await waitFor(() => {
  expect(screen.queryByText('Loading')).not.toBeInTheDocument();
});

Test Cleanup

import { cleanup } from '@testing-library/react';

afterEach(() => {
  cleanup();            // React cleanup (automatic with setup.ts)
  vi.clearAllMocks();   // Reset mock call counts
  vi.resetAllMocks();   // Reset mocks to initial state
  vi.restoreAllMocks(); // Restore original implementations
});

Accessibility Testing

import { axe, toHaveNoViolations } from 'jest-axe';

expect.extend(toHaveNoViolations);

it('has no accessibility violations', async () => {
  const { container } = render(<Component />);
  const results = await axe(container);
  expect(results).toHaveNoViolations();
});

Common Matchers

// jest-dom matchers (from setup.ts)
expect(element).toBeInTheDocument();
expect(element).toBeVisible();
expect(element).toBeDisabled();
expect(element).toHaveTextContent('text');
expect(element).toHaveAttribute('href', '/path');
expect(element).toHaveClass('active');
expect(input).toHaveValue('input value');

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How to use it

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