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.claude/skills/testing/vitest/SKILL.md

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+# Vitest
+
+## Description
+
+Modern JavaScript/TypeScript testing with Vitest including mocking and coverage.
+
+## When to Use
+
+- Testing JavaScript/TypeScript
+- React component testing
+- Unit and integration tests
+
+---
+
+## Core Patterns
+
+### Basic Tests
+
+```typescript
+import { describe, it, expect } from 'vitest';
+
+describe('math', () => {
+  it('should add numbers', () => {
+    expect(1 + 1).toBe(2);
+  });
+
+  it('should throw on invalid input', () => {
+    expect(() => divide(1, 0)).toThrow('Division by zero');
+  });
+});
+```
+
+### Mocking
+
+```typescript
+import { vi, describe, it, expect } from 'vitest';
+
+// Mock module
+vi.mock('./api', () => ({
+  fetchUser: vi.fn().mockResolvedValue({ id: 1 })
+}));
+
+// Mock function
+const callback = vi.fn();
+callback('arg');
+expect(callback).toHaveBeenCalledWith('arg');
+```
+
+### Async Tests
+
+```typescript
+it('should fetch data', async () => {
+  const data = await fetchData();
+  expect(data).toEqual({ id: 1 });
+});
+
+it('should reject on error', async () => {
+  await expect(fetchData()).rejects.toThrow('Error');
+});
+```
+
+### React Testing
+
+```typescript
+import { render, screen } from '@testing-library/react';
+import { userEvent } from '@testing-library/user-event';
+
+it('should handle click', async () => {
+  const onClick = vi.fn();
+  render(<Button onClick={onClick}>Click</Button>);
+
+  await userEvent.click(screen.getByRole('button'));
+  expect(onClick).toHaveBeenCalled();
+});
+```
+
+## Best Practices
+
+1. Use describe blocks for grouping
+2. Prefer async/await for async tests
+3. Use userEvent over fireEvent
+4. Mock at module boundaries
+5. Clean up after tests
+
+## Common Pitfalls
+
+- **Not awaiting async**: Always await promises
+- **Stale mocks**: Clear mocks between tests
+- **Testing implementation**: Test behavior