React Testing Library And Jest- The Complete Guide «Tested & Working»

test('loads and displays user', async () => const mockUser = name: 'John Doe' fetch.mockResolvedValueOnce( json: async () => mockUser, )

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 />) React Testing Library and Jest- The Complete Guide

jest.useRealTimers() // restore Controlled component const Toggle = () => const [on, setOn] = useState(false) return ( <button onClick=() => setOn(!on)> on ? 'ON' : 'OFF' </button> ) test('loads and displays user', async () =&gt; const

act(() => result.current.increment() )

// Test behavior, not implementation expect(screen.getByText('Welcome John')).toBeInTheDocument() test('loads and displays user'

render(<Button onClick=handleClick>Click Me</Button>)

expect(screen.getByText('Done')).toBeInTheDocument() )