Best Practices for Testing React Applications with Jest and React Testing Library
Testing is an essential part of software development, ensuring that your React application behaves as expected and remains maintainable as it grows. Jest and React Testing Library (RTL) are popular tools for testing React applications. While Jest provides a robust testing framework, RTL focuses on testing components from the user’s perspective, making your tests more reliable and less brittle. This article will guide you through the best practices for testing React applications using Jest and React Testing Library.
M Zeeshan
8/16/20244 min read
Why Test Your React Applications?
Testing provides several benefits:
Catch Bugs Early: Automated tests help catch bugs before they reach production, reducing the risk of breaking changes.
Documentation: Tests serve as documentation, showing how components are expected to behave.
Confidence in Refactoring: With a solid test suite, you can refactor code confidently, knowing that tests will catch regressions.
Improved Code Quality: Writing tests encourages better design and modular code, as tightly coupled code is harder to test.
Setting Up Jest and React Testing Library
Before diving into best practices, let’s set up Jest and React Testing Library in your React project.
Install Jest and React Testing Library:
If you’re using Create React App (CRA), Jest and RTL come pre-installed. Otherwise, you can install them with:
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npm install --save-dev jest @testing-library/react @testing-library/jest-dom
Configure Jest:
Jest requires minimal configuration, but you can customize it by adding a jest.config.js file in your project’s root directory if needed. CRA projects already come with Jest configured out of the box.
Best Practices for Testing with Jest and RTL
1. Write Tests That Mimic User Behavior
React Testing Library encourages you to test components the way a user would interact with them, rather than focusing on implementation details.
Example:
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import { render, screen, fireEvent } from '@testing-library/react'; import '@testing-library/jest-dom/extend-expect'; import App from './App'; test('increments counter when button is clicked', () => { render(<App />); const button = screen.getByRole('button', { name: /increment/i }); fireEvent.click(button); expect(screen.getByText(/count: 1/i)).toBeInTheDocument(); });
In this test, the fireEvent.click simulates a user clicking the button, and the expectation checks that the counter has incremented. This approach ensures your tests are focused on what the user actually sees and does.
2. Use Descriptive Test Names
Clear and descriptive test names make it easier to understand what’s being tested and why. A good test name should describe the expected behavior in plain language.
Example:
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test('displays an error message when the form is submitted with empty fields', () => { // Test implementation });
This test name clearly communicates what the test is checking, making it easier for others (or future you) to understand.
3. Test Components in Isolation
When testing individual components, mock any dependencies that aren’t relevant to the component’s behavior. This allows you to test the component in isolation and avoid brittle tests.
Example:
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import { render, screen } from '@testing-library/react'; import userEvent from '@testing-library/user-event'; import Login from './Login'; // Mock the API call jest.mock('../api/auth', () => ({ login: jest.fn().mockResolvedValue({ token: 'fake-token' }), })); test('calls login API when form is submitted', async () => { render(<Login />); userEvent.type(screen.getByLabelText(/username/i), 'testuser'); userEvent.type(screen.getByLabelText(/password/i), 'password'); userEvent.click(screen.getByRole('button', { name: /log in/i })); expect(await screen.findByText(/welcome back, testuser/i)).toBeInTheDocument(); });
By mocking the API call, you ensure that the test focuses on the component’s behavior, not on the API’s response.
4. Use screen for Queries
React Testing Library provides a screen object, which is a recommended way to query DOM elements. It improves readability and reduces the need to pass container around.
Example:
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// Using screen expect(screen.getByText(/hello world/i)).toBeInTheDocument();
Using screen also makes it easier to understand what’s being queried in your tests, enhancing readability.
5. Avoid Testing Implementation Details
Testing implementation details can lead to fragile tests that break when refactoring, even if the component’s behavior hasn’t changed. Focus on testing the output and behavior from the user’s perspective.
Example:
Instead of testing the internal state of a component:
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// Avoid this expect(component.state('count')).toBe(1);
Test the rendered output:
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// Do this instead expect(screen.getByText(/count: 1/i)).toBeInTheDocument();
6. Leverage Jest Mock Functions
Mock functions (also known as spies or stubs) in Jest allow you to test interactions between components and their dependencies, such as API calls or event handlers.
Example:
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import { render, screen } from '@testing-library/react'; import userEvent from '@testing-library/user-event'; import Button from './Button'; test('calls onClick handler when clicked', () => { const handleClick = jest.fn(); render(<Button onClick={handleClick}>Click Me</Button>); userEvent.click(screen.getByRole('button', { name: /click me/i })); expect(handleClick).toHaveBeenCalledTimes(1); });
In this test, jest.fn() is used to mock the click handler, allowing you to verify that it was called when the button was clicked.
7. Use act to Handle Asynchronous Code
When testing components that involve asynchronous updates (e.g., fetching data), use act to ensure all updates are processed before making assertions.
Example:
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import { render, screen, waitFor } from '@testing-library/react'; import FetchComponent from './FetchComponent'; test('displays data after fetching', async () => { render(<FetchComponent />); await waitFor(() => expect(screen.getByText(/data fetched/i)).toBeInTheDocument()); });
Using waitFor ensures that the assertion waits for the data to be fetched and displayed.
8. Test Edge Cases and Error Handling
Don’t just test the happy path; make sure to cover edge cases and error scenarios. This ensures your components handle unexpected situations gracefully.
Example:
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test('displays error message when API call fails', async () => { api.fetchData.mockRejectedValueOnce(new Error('API Error')); render(<FetchComponent />); expect(await screen.findByText(/error fetching data/i)).toBeInTheDocument(); });
In this test, the API call is mocked to fail, and the component is tested for proper error handling.
9. Use Custom Render Function
If your components rely on context providers, themes, or routing, consider creating a custom render function that wraps components with these dependencies.
Example:
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import { render } from '@testing-library/react'; import { ThemeProvider } from 'styled-components'; import theme from '../theme'; const customRender = (ui, options) => render(<ThemeProvider theme={theme}>{ui}</ThemeProvider>, options); export * from '@testing-library/react'; export { customRender as render };
Using customRender, you can easily test components that rely on themes or other providers without repeating the setup code.
10. Keep Tests Fast and Independent
Tests should run quickly and independently. Slow tests can lead to frustration and neglect, while dependent tests can cause flaky test results. Ensure your tests don’t rely on global state or other tests.
Example:
If a test requires a specific state, set it up explicitly within that test:
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beforeEach(() => { jest.clearAllMocks(); localStorage.clear(); });
This practice ensures that tests don’t interfere with each other and can be run in any order.