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 Mocking // 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' ) . mockReturnValue ( 'mocked' ) spy . mockRestore ( ) Async Testing it ( 'should resolve with data' , async ( ) => { const result = await asyncFunction ( ) expect ( result ) . toBeDefined ( ) } ) it ( 'should reject with error' , async ( ) => { await expect ( asyncFunction ( ) ) . rejects . toThrow ( 'Error' ) } ) Timers beforeEach ( ( ) => jest . useFakeTimers ( ) ) afterEach ( ( ) => jest . useRealTimers ( ) ) it ( 'should execute after timeout' , ( ) => { const callback = jest . fn ( ) setTimeout ( callback , 1000 ) jest . advanceTimersByTime ( 1000 ) expect ( callback ) . toHaveBeenCalled ( ) } ) Common Matchers // Equality expect ( value ) . toBe ( expected ) expect ( value ) . toEqual ( expected ) // Truthiness expect ( value ) . toBeTruthy ( ) expect ( value ) . toBeNull ( ) expect ( value ) . toBeDefined ( ) // Arrays/Objects expect ( array ) . toContain ( item ) expect ( obj ) . toHaveProperty ( 'key' ) // Functions/Promises expect ( fn ) . toThrow ( 'error' ) await expect ( promise ) . resolves . toBe ( value ) // Mock functions expect ( mockFn ) . toHaveBeenCalled ( ) expect ( mockFn ) . toHaveBeenCalledWith ( arg ) React Component Testing import { render , screen , fireEvent } from '@testing-library/react' import '@testing-library/jest-dom' import { Button } from './Button' describe ( 'Button' , ( ) => { it ( 'should call onClick when clicked' , ( ) => { const handleClick = jest . fn ( ) render ( < Button onClick = { handleClick }

Click me < / Button

) fireEvent . click ( screen . getByText ( 'Click me' ) ) expect ( handleClick ) . toHaveBeenCalledTimes ( 1 ) } ) } ) Quality Checklist Test file properly named ( .test.ts ) All exported functions/classes tested Happy path and edge cases included Error conditions tested External dependencies mocked Tests organized with describe blocks beforeEach/afterEach for setup/cleanup Descriptive test names Coverage ≥ 80% Best Practices Use descriptive test names: should when Organize with describe blocks Mock external dependencies only Test user behavior for components Use test.each for parametrized tests Keep tests independent Test error conditions Aim for 80%+ coverage