reactjs - jest/enzyme mock function in functional component

Reactjs - jest/enzyme mock function in functional component

Mocking functions in React functional components with Jest and Enzyme involves a few steps to simulate behavior for testing purposes. Here's a basic guide on how to mock a function within a functional component using Jest and Enzyme:

Example Setup

Assume you have a functional component that includes a button triggering a function:

// ButtonComponent.jsx import React from 'react'; const ButtonComponent = ({ onClickHandler }) => { const handleClick = () => { onClickHandler('Button clicked'); }; return ( <button onClick={handleClick}>Click me</button> ); }; export default ButtonComponent;

Testing Setup with Jest and Enzyme

  1. Install Jest and Enzyme: If not already installed, install Jest and Enzyme along with necessary adapters:

    npm install --save-dev jest enzyme enzyme-adapter-react-16

  2. Configure Jest and Enzyme: Set up Jest configuration and Enzyme adapter in your setupTests.js file:

    // setupTests.js import { configure } from 'enzyme'; import Adapter from 'enzyme-adapter-react-16'; configure({ adapter: new Adapter() });

  3. Write the Test: Create a test file for ButtonComponent to mock onClickHandler and simulate button click behavior:

    // ButtonComponent.test.js import React from 'react'; import { shallow } from 'enzyme'; import ButtonComponent from './ButtonComponent'; describe('ButtonComponent', () => { it('should call onClickHandler with correct message', () => { // Mock function const mockOnClickHandler = jest.fn(); // Render the component with mocked onClickHandler const wrapper = shallow(<ButtonComponent onClickHandler={mockOnClickHandler} />); // Simulate button click wrapper.find('button').simulate('click'); // Check if onClickHandler was called with the correct message expect(mockOnClickHandler).toHaveBeenCalledWith('Button clicked'); }); });

Explanation

  • Mocking onClickHandler: Use jest.fn() to create a mock function (mockOnClickHandler) that simulates the behavior of onClickHandler.
  • Shallow Rendering: Use Enzyme's shallow() function to render the ButtonComponent with the mocked onClickHandler.
  • Simulating Click: Use simulate('click') on the button element within wrapper to simulate a user click event.
  • Expectation: Use Jest's toHaveBeenCalledWith() to assert that onClickHandler was called with the expected argument ('Button clicked').

Notes

  • Handling Asynchronous Code: If onClickHandler involves asynchronous operations (like API calls), consider using async/await or jest.useFakeTimers() to handle delays or timeouts in testing.
  • Component Dependencies: If ButtonComponent relies on other context providers or props, mock them as needed for isolated testing.

This example provides a basic framework for mocking functions in React functional components using Jest and Enzyme. Adjust the testing approach based on specific component requirements and functionality.

Examples

  1. Mocking a function call in a React functional component using Jest

    • Description: Mocking a function call inside a React functional component for testing purposes using Jest.
    • Code:
      import React from 'react'; import { render, fireEvent } from '@testing-library/react'; import MyComponent from './MyComponent'; test('Mocking function call on button click', () => { const mockFn = jest.fn(); const { getByText } = render(<MyComponent onClick={mockFn} />); const button = getByText('Click me'); fireEvent.click(button); expect(mockFn).toHaveBeenCalledTimes(1); });
      This code snippet uses Jest's jest.fn() to create a mock function (mockFn) and verifies that it's called once when a button in MyComponent is clicked.

  2. Mocking a function call with parameters in a functional component using Enzyme

    • Description: Mocking a function with parameters called inside a React functional component using Enzyme for testing.
    • Code:
      import React from 'react'; import { shallow } from 'enzyme'; import MyComponent from './MyComponent'; test('Mocking function call with parameters', () => { const mockFn = jest.fn(); const wrapper = shallow(<MyComponent onClick={mockFn} />); wrapper.find('button').simulate('click'); expect(mockFn).toHaveBeenCalledWith('test'); });
      This example uses Enzyme's shallow rendering to simulate a button click and checks that mockFn is called with the parameter 'test'.

  3. Mocking an async function call in a React functional component using Jest

    • Description: Mocking an async function call inside a React functional component using Jest for unit testing.
    • Code:
      import React from 'react'; import { render, waitFor } from '@testing-library/react'; import MyComponent from './MyComponent'; test('Mocking async function call', async () => { const mockAsyncFn = jest.fn().mockResolvedValue('Data'); const { getByText } = render(<MyComponent fetchData={mockAsyncFn} />); await waitFor(() => { expect(getByText('Data')).toBeInTheDocument(); }); });
      This code mocks an async function (fetchData) inside MyComponent using Jest's mockResolvedValue to simulate data fetching and verifies the rendered output.

  4. Mocking a function call inside useEffect in a functional component using Jest

    • Description: Mocking a function call inside useEffect of a React functional component using Jest for testing side effects.
    • Code:
      import React from 'react'; import { render } from '@testing-library/react'; import MyComponent from './MyComponent'; test('Mocking useEffect function call', () => { const mockFn = jest.fn(); jest.spyOn(React, 'useEffect').mockImplementationOnce(mockFn); render(<MyComponent />); expect(mockFn).toHaveBeenCalled(); });
      This example uses Jest's jest.spyOn and mockImplementationOnce to mock a function (mockFn) called inside useEffect in MyComponent.

  5. Mocking a function call with state update in a functional component using Enzyme

    • Description: Mocking a function call that updates state inside a React functional component using Enzyme for testing state changes.
    • Code:
      import React from 'react'; import { mount } from 'enzyme'; import MyComponent from './MyComponent'; test('Mocking function call with state update', () => { const wrapper = mount(<MyComponent />); const instance = wrapper.instance(); jest.spyOn(instance, 'handleClick').mockImplementation(() => { instance.setState({ clicked: true }); }); instance.handleClick(); expect(instance.state.clicked).toBe(true); });
      This code uses Enzyme's mount to render MyComponent, spies on handleClick method, mocks its implementation to update state, and verifies the state change.

  6. Mocking a function call with useState hook in a functional component using Jest

    • Description: Mocking a function call that updates state using useState hook inside a React functional component using Jest for testing.
    • Code:
      import React, { useState } from 'react'; import { render, fireEvent } from '@testing-library/react'; import MyComponent from './MyComponent'; test('Mocking function call with useState hook', () => { const mockSetState = jest.fn(); useState.mockImplementation(init => [init, mockSetState]); const { getByText } = render(<MyComponent />); const button = getByText('Click me'); fireEvent.click(button); expect(mockSetState).toHaveBeenCalledWith(true); });
      This example mocks useState hook to replace its implementation with mockSetState, verifies state update when clicking a button in MyComponent.

  7. Mocking a function call that returns a promise in a functional component using Enzyme

    • Description: Mocking a function call inside a React functional component that returns a promise using Enzyme for asynchronous testing.
    • Code:
      import React from 'react'; import { mount } from 'enzyme'; import MyComponent from './MyComponent'; test('Mocking function call that returns a promise', async () => { const mockAsyncFn = jest.fn().mockResolvedValue('Data'); const wrapper = mount(<MyComponent fetchData={mockAsyncFn} />); await wrapper.instance().fetchData(); wrapper.update(); expect(wrapper.text()).toContain('Data'); });
      This code mocks fetchData function inside MyComponent using Enzyme's mount, resolves its promise, and verifies rendered output contains 'Data'.

  8. Mocking a function call with useCallback hook in a functional component using Jest

    • Description: Mocking a function call inside useCallback hook of a React functional component using Jest for testing memoized callbacks.
    • Code:
      import React from 'react'; import { render, fireEvent } from '@testing-library/react'; import MyComponent from './MyComponent'; test('Mocking function call with useCallback hook', () => { const mockFn = jest.fn(); const { getByText } = render(<MyComponent onClick={React.useCallback(mockFn, [])} />); const button = getByText('Click me'); fireEvent.click(button); expect(mockFn).toHaveBeenCalledTimes(1); });
      This example uses Jest's jest.fn() to create mockFn, mocks its usage inside useCallback hook in MyComponent, and verifies it's called once when clicking a button.

  9. Mocking a function call inside custom hook in a functional component using Enzyme

    • Description: Mocking a function call inside a custom hook used by a React functional component using Enzyme for testing custom hooks.
    • Code:
      import React from 'react'; import { mount } from 'enzyme'; import useCustomHook from './useCustomHook'; import MyComponent from './MyComponent'; jest.mock('./useCustomHook'); test('Mocking function call inside custom hook', () => { const mockFn = jest.fn(); useCustomHook.mockReturnValue({ handleClick: mockFn }); const wrapper = mount(<MyComponent />); wrapper.find('button').simulate('click'); expect(mockFn).toHaveBeenCalledTimes(1); });
      This code mocks useCustomHook to return { handleClick: mockFn }, mounts MyComponent, simulates a button click, and verifies mockFn is called once.

  10. Mocking a function call with useContext hook in a functional component using Jest

    • Description: Mocking a function call inside useContext hook of a React functional component using Jest for testing context-based components.
    • Code:
      import React from 'react'; import { render } from '@testing-library/react'; import MyComponent from './MyComponent'; import { MyContext } from './MyContext'; jest.mock('./MyContext', () => ({ MyContext: { Consumer: ({ children }) => children({ myFn: jest.fn() }), }, })); test('Mocking function call with useContext hook', () => { const { getByText } = render(<MyComponent />); const button = getByText('Click me'); fireEvent.click(button); expect(MyContext.Consumer.mock.calls[0][0].myFn).toHaveBeenCalledTimes(1); });
      This example mocks MyContext.Consumer to provide { myFn: jest.fn() }, renders MyComponent, clicks a button, and verifies myFn is called once.

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