React Testing Library And Jest- The Complete Guide Here

// Query (returns null if not found - no error) screen.queryByText('Missing text')

await user.click(button) expect(button).toHaveTextContent('ON')

// Test error states render(<Component onError=mockError />) // Don't test internal state expect(component.state('isOpen')).toBe(true) // Don't use testid as default screen.getByTestId('submit-button')

// Test const customRender = (ui, providerProps, ...renderOptions ) => return render( <ThemeProvider ...providerProps>ui</ThemeProvider>, renderOptions ) React Testing Library and Jest- The Complete Guide

export default testEnvironment: 'jsdom', setupFilesAfterEnv: ['<rootDir>/src/setupTests.js'], transform: ts,

expect(await screen.findByText('Valid email required')).toBeInTheDocument() ) ✅ DO // Query by accessible name screen.getByRole('button', name: /submit/i ) // Use findBy for async elements expect(await screen.findByText('Loaded')).toBeInTheDocument()

render(<Button onClick=handleClick>Click Me</Button>) // Query (returns null if not found - no error) screen

if (!user) return <div>Loading...</div> return <div>user.name</div>

await user.click(button) expect(button).toHaveTextContent('OFF') ) test('shows error for invalid email', async () => const user = userEvent.setup() render(<SignupForm />) await user.type(screen.getByLabelText(/email/i), 'invalid') await user.click(screen.getByRole('button', name: /submit/i ))

expect(result.current.count).toBe(1) ) Mock functions with Jest const mockFn = jest.fn() mockFn('hello') expect(mockFn).toHaveBeenCalledWith('hello') expect(mockFn).toHaveBeenCalledTimes(1) // Mock return value jest.fn().mockReturnValue('fixed value') jest.fn().mockResolvedValue('async value') jest.fn().mockImplementation(arg => arg * 2) Mock modules // Mock entire module jest.mock('../api', () => ( fetchUser: jest.fn(), )) // Mock with dynamic implementation jest.mock('axios', () => ( get: jest.fn(() => Promise.resolve( data: id: 1 )), )) Mock timers jest.useFakeTimers() test('delayed action', () => render(<DelayedComponent />) Component onError=mockError /&gt

import userEvent from '@testing-library/user-event' test('form submission', async () => const user = userEvent.setup() render(<LoginForm />)

await user.type(screen.getByLabelText(/email/i), 'user@example.com') await user.type(screen.getByLabelText(/password/i), 'secret123') await user.click(screen.getByRole('button', name: /submit/i ))

// Don't test props passed to children expect(ChildComponent).toHaveBeenCalledWith( prop: 'value' )