jest-generator

# Jest Generator Skill

## Purpose

Generate Jest-based unit tests for JavaScript and TypeScript code, following Jest conventions and best practices with proper mocking, describe blocks, and code organization.

## When to Use

- Generate Jest tests for JavaScript/TypeScript modules
- Create test files for React components
- Add missing test coverage to existing code
- Need Jest-specific patterns (mocks, spies, snapshots)

## Test File Naming

**Source to Test Mapping:**
- `src/components/Feature.tsx` → `src/components/Feature.test.tsx`
- `src/utils/validator.ts` → `src/utils/validator.test.ts`
- `src/models/User.ts` → `src/models/User.test.ts`

## Running Tests

```bash
# Preferred: Using bun (faster)
bun test

# Run specific file
bun test src/utils/validator.test.ts

# Run with coverage
bun test --coverage

# Watch mode
bun test --watch

# Alternative: Using npm
npm test
npm test -- --coverage
```

## Jest Test Structure

```typescript
import { functionToTest, ClassToTest } from './Feature'

jest.mock('./dependency')

describe('ModuleName', () => {
  beforeEach(() => {
    jest.clearAllMocks()
  })

  afterEach(() => {
    jest.restoreAllMocks()
  })

  describe('ClassName', () => {
    let instance: ClassToTest

    beforeEach(() => {
      instance = new ClassToTest()
    })

    it('should return expected result with valid input', () => {
      // Arrange
      const input = { key: 'value' }
      const expected = { processed: true }

      // Act
      const result = instance.method(input)

      // Assert
      expect(result).toEqual(expected)
    })

    it('should throw error with invalid input', () => {
      expect(() => instance.method(null)).toThrow('Invalid input')
    })
  })
})
```

## Jest-Specific Patterns

### Mocking

```typescript
// Mock entire module
jest.mock('./api', () => ({
  fetchData: jest.fn(),
}))

// Mock with implementation
const mockFn = jest.fn((x: number) => x * 2)

// Spy on method
const spy = jest.spyOn(obj, 'method').mockReturnV