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

# 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

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