Jest react testing

Jest React Testing

A testing skill for React applications using Jest and React Testing Library. It covers component tests, mocking, async testing, hooks testing, and integration testing patterns.

Guardrails

• Start from the behavior under test and the user-visible success criteria.
• Add the smallest test that would fail for the bug or missing behavior. Do not chase coverage for its own sake.
• Prefer real user interactions and accessible queries over implementation details.
• Keep mocks minimal and local. Do not rewrite test infrastructure unless configuration is the task.

When to Use This Skill

Use this skill when:

• Testing React components with Jest and React Testing Library
• Setting up Jest configuration for React projects
• Writing unit tests for components, hooks, and utilities
• Testing user interactions and component behavior
• Mocking modules, functions, API calls, and external dependencies
• Testing asynchronous operations (API calls, timers, promises)
• Testing custom React hooks
• Writing integration tests for complex component trees
• Debugging failing tests or improving test coverage
• Following testing best practices and patterns

Core Concepts

Testing Philosophy

React Testing Library follows these guiding principles:

• Test User Behavior, Not Implementation: Write tests that resemble how users interact with your app
• Accessibility First: Use queries that promote accessible components (getByRole, getByLabelText)
• Avoid Testing Implementation Details: Don't test state, props, or internal methods directly
• Maintainable Tests: Tests should break when behavior changes, not when code refactors
• Confidence Over Coverage: Focus on tests that give confidence, not 100% coverage

Key Testing Concepts

1. Queries: Methods to find elements (getBy, queryBy, findBy)
2. User Events: Simulating user interactions (click, type, select)
3. Async Testing: Testing components with asynchronous operations
4. Mocking: Replacing dependencies with controlled test doubles
5. Assertions: Verifying expected outcomes with matchers

Jest Configuration

Basic Jest Configuration

jest.config.js (JavaScript projects):

/** @type {import('jest').Config} */
const config = {
  // Test environment for DOM testing
  testEnvironment: 'jsdom',

  // Setup files after environment
  setupFilesAfterEnv: ['<rootDir>/src/setupTests.js'],

  // Module paths
  moduleDirectories: ['node_modules', 'src'],

  // Transform files with babel-jest
  transform: {
    '^.+\\.(js|jsx)$': 'babel-jest',
  },

  // Module name mapper for static assets and CSS
  moduleNameMapper: {
    '\\.(css|less|scss|sass)$': 'identity-obj-proxy',
    '\\.(jpg|jpeg|png|gif|svg)$': '<rootDir>/__mocks__/fileMock.js',
  },

  // Coverage configuration
  collectCoverageFrom: [
    'src/**/*.{js,jsx}',
    '!src/index.js',
    '!src/**/*.test.{js,jsx}',
    '!src/**/__tests__/**',
  ],

  // Coverage thresholds
  coverageThreshold: {
    global: {
      branches: 80,
      functions: 80,
      lines: 80,
      statements: 80,
    },
  },
};

module.exports = config;

jest.config.js (TypeScript projects):

import type {Config} from 'jest';

const config: Config = {
  preset: 'ts-jest',
  testEnvironment: 'jsdom',
  setupFilesAfterEnv: ['<rootDir>/src/setupTests.ts'],

  moduleDirectories: ['node_modules', 'src'],

  transform: {
    '^.+\\.tsx?$': 'ts-jest',
  },

  moduleNameMapper: {
    '\\.(css|less|scss|sass)$': 'identity-obj-proxy',
    '\\.(jpg|jpeg|png|gif|svg)$': '<rootDir>/__mocks__/fileMock.ts',
    '^@/(.*)$': '<rootDir>/src/$1',
  },

  collectCoverageFrom: [
    'src/**/*.{ts,tsx}',
    '!src/index.tsx',
    '!src/**/*.test.{ts,tsx}',
    '!src/**/__tests__/**',
    '!src/**/*.d.ts',
  ],

  coverageThreshold: {
    global: {
      branches: 80,
      functions: 80,
      lines: 80,
      statements: 80,
    },
  },
};

export default config;

Setup Files

src/setupTests.js:

// Add custom jest matchers from jest-dom
import '@testing-library/jest-dom';

// Extend expect with jest-extended matchers (optional)
import * as matchers from 'jest-extended';
expect.extend(matchers);

// Mock window.matchMedia
Object.defineProperty(window, 'matchMedia', {
  writable: true,
  value: jest.fn().mockImplementation(query => ({
    matches: false,
    media: query,
    onchange: null,
    addListener: jest.fn(),
    removeListener: jest.fn(),
    addEventListener: jest.fn(),
    removeEventListener: jest.fn(),
    dispatchEvent: jest.fn(),
  })),
});

// Mock IntersectionObserver
global.IntersectionObserver = class IntersectionObserver {
  constructor() {}
  disconnect() {}
  observe() {}
  takeRecords() {
    return [];
  }
  unobserve() {}
};

// Suppress console errors in tests (optional)
const originalError = console.error;
beforeAll(() => {
  console.error = (...args) => {
    if (
      typeof args[0] === 'string' &&
      args[0].includes('Warning: ReactDOM.render')
    ) {
      return;
    }
    originalError.call(console, ...args);
  };
});

afterAll(() => {
  console.error = originalError;
});

// Reset mocks after each test
afterEach(() => {
  jest.clearAllMocks();
});

File Mocks

mocks/fileMock.js:

module.exports = 'test-file-stub';

mocks/styleMock.js:

module.exports = {};

React Testing Library Queries

Query Types

React Testing Library provides three types of queries:

1. getBy: Returns element or throws error (use for elements that should exist)
2. queryBy: Returns element or null (use for elements that may not exist)
3. findBy: Returns promise that resolves to element (use for async elements)

Query Priority

Recommended Query Order (accessibility-focused):

1. getByRole: Most accessible query

   getByRole('button', { name: /submit/i })
   getByRole('heading', { level: 1 })
   getByRole('textbox', { name: /username/i })
   

2. getByLabelText: For form fields with labels

   getByLabelText(/email address/i)
   getByLabelText('Password')
   

3. getByPlaceholderText: For inputs with placeholders

   getByPlaceholderText(/search/i)
   

4. getByText: For non-interactive elements with text

   getByText(/welcome/i)
   getByText('Error: Invalid credentials')
   

5. getByDisplayValue: For form elements with values

   getByDisplayValue('John Doe')
   

6. getByAltText: For images with alt text

   getByAltText(/profile picture/i)
   

7. getByTitle: For elements with title attribute

   getByTitle(/close/i)
   

8. getByTestId: Last resort when other queries don't work

   getByTestId('custom-element')
   

Query Variants

// Single element queries
screen.getByRole('button')      // Throws if not found or multiple found
screen.queryByRole('button')    // Returns null if not found
await screen.findByRole('button') // Async, waits up to 1000ms

// Multiple element queries
screen.getAllByRole('listitem')      // Throws if none found
screen.queryAllByRole('listitem')    // Returns [] if none found
await screen.findAllByRole('listitem') // Async version

Component Testing Strategies

Basic Component Test

import { render, screen } from '@testing-library/react';
import { Greeting } from './Greeting';

describe('Greeting Component', () => {
  it('renders greeting message', () => {
    render(<Greeting name="Alice" />);

    expect(screen.getByText(/hello, alice/i)).toBeInTheDocument();
  });

  it('renders default greeting when no name provided', () => {
    render(<Greeting />);

    expect(screen.getByText(/hello, guest/i)).toBeInTheDocument();
  });
});

Testing User Interactions

import { render, screen } from '@testing-library/react';
import userEvent from '@testing-library/user-event';
import { Counter } from './Counter';

describe('Counter Component', () => {
  it('increments counter on button click', async () => {
    const user = userEvent.setup();
    render(<Counter />);

    const button = screen.getByRole('button', { name: /increment/i });
    const count = screen.getByText(/count: 0/i);

    expect(count).toBeInTheDocument();

    await user.click(button);

    expect(screen.getByText(/count: 1/i)).toBeInTheDocument();
  });

  it('decrements counter on button click', async () => {
    const user = userEvent.setup();
    render(<Counter initialCount={5} />);

    const decrementBtn = screen.getByRole('button', { name: /decrement/i });

    await user.click(decrementBtn);

    expect(screen.getByText(/count: 4/i)).toBeInTheDocument();
  });
});

Testing Forms

import { render, screen } from '@testing-library/react';
import userEvent from '@testing-library/user-event';
import { LoginForm } from './LoginForm';

describe('LoginForm Component', () => {
  it('submits form with username and password', async () => {
    const user = userEvent.setup();
    const handleSubmit = jest.fn();

    render(<LoginForm onSubmit={handleSubmit} />);

    const usernameInput = screen.getByLabelText(/username/i);
    const passwordInput = screen.getByLabelText(/password/i);
    const submitButton = screen.getByRole('button', { name: /submit/i });

    await user.type(usernameInput, 'testuser');
    await user.type(passwordInput, 'password123');
    await user.click(submitButton);

    expect(handleSubmit).toHaveBeenCalledTimes(1);
    expect(handleSubmit).toHaveBeenCalledWith({
      username: 'testuser',
      password: 'password123',
    });
  });

  it('shows validation errors for empty fields', async () => {
    const user = userEvent.setup();
    render(<LoginForm onSubmit={jest.fn()} />);

    const submitButton = screen.getByRole('button', { name: /submit/i });

    await user.click(submitButton);

    expect(screen.getByText(/username is required/i)).toBeInTheDocument();
    expect(screen.getByText(/password is required/i)).toBeInTheDocument();
  });
});

Testing Conditional Rendering

import { render, screen } from '@testing-library/react';
import { UserProfile } from './UserProfile';

describe('UserProfile Component', () => {
  it('shows loading state when loading', () => {
    render(<UserProfile loading={true} />);

    expect(screen.getByText(/loading/i)).toBeInTheDocument();
    expect(screen.queryByRole('heading')).not.toBeInTheDocument();
  });

  it('shows user data when loaded', () => {
    const user = {
      name: 'John Doe',
      email: '[email protected]',
    };

    render(<UserProfile loading={false} user={user} />);

    expect(screen.queryByText(/loading/i)).not.toBeInTheDocument();
    expect(screen.getByRole('heading', { name: /john doe/i })).toBeInTheDocument();
    expect(screen.getByText(/[email protected]/i)).toBeInTheDocument();
  });

  it('shows error message when error occurs', () => {
    render(<UserProfile loading={false} error="Failed to load user" />);

    expect(screen.getByText(/failed to load user/i)).toBeInTheDocument();
    expect(screen.queryByRole('heading')).not.toBeInTheDocument();
  });
});

Mocking Patterns

Mocking Modules

Automatic Mock:

// __mocks__/axios.js
export default {
  get: jest.fn(),
  post: jest.fn(),
  put: jest.fn(),
  delete: jest.fn(),
};

Usage in test:

import axios from 'axios';
import { UserService } from './UserService';

jest.mock('axios');

describe('UserService', () => {
  beforeEach(() => {
    jest.clearAllMocks();
  });

  it('fetches users successfully', async () => {
    const mockUsers = [{ id: 1, name: 'Alice' }];
    axios.get.mockResolvedValue({ data: mockUsers });

    const users = await UserService.getUsers();

    expect(axios.get).toHaveBeenCalledWith('/api/users');
    expect(users).toEqual(mockUsers);
  });

  it('handles fetch error', async () => {
    axios.get.mockRejectedValue(new Error('Network Error'));

    await expect(UserService.getUsers()).rejects.toThrow('Network Error');
  });
});

Mocking Functions

import { render, screen } from '@testing-library/react';
import userEvent from '@testing-library/user-event';
import { Button } from './Button';

describe('Button Component', () => {
  it('calls onClick handler when clicked', async () => {
    const user = userEvent.setup();
    const handleClick = jest.fn();

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

    await user.click(screen.getByRole('button'));

    expect(handleClick).toHaveBeenCalledTimes(1);
  });

  it('calls onClick with event object', async () => {
    const user = userEvent.setup();
    const handleClick = jest.fn();

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

    await user.click(screen.getByRole('button'));

    expect(handleClick).toHaveBeenCalledWith(
      expect.objectContaining({
        type: 'click',
      })
    );
  });
});

Mocking API Calls with MSW (Mock Service Worker)

Setup MSW:

// src/mocks/handlers.js
import { rest } from 'msw';

export const handlers = [
  rest.get('/api/users', (req, res, ctx) => {
    return res(
      ctx.status(200),
      ctx.json([
        { id: 1, name: 'Alice' },
        { id: 2, name: 'Bob' },
      ])
    );
  }),

  rest.post('/api/users', async (req, res, ctx) => {
    const { name, email } = await req.json();

    return res(
      ctx.status(201),
      ctx.json({
        id: 3,
        name,
        email,
      })
    );
  }),
];

Setup server:

// src/mocks/server.js
import { setupServer } from 'msw/node';
import { handlers } from './handlers';

export const server = setupServer(...handlers);

Configure in setupTests.js:

import { server } from './mocks/server';

beforeAll(() => server.listen());
afterEach(() => server.resetHandlers());
afterAll(() => server.close());

Usage in tests:

import { render, screen, waitFor } from '@testing-library/react';
import { server } from './mocks/server';
import { rest } from 'msw';
import { UserList } from './UserList';

describe('UserList Component', () => {
  it('fetches and displays users', async () => {
    render(<UserList />);

    expect(screen.getByText(/loading/i)).toBeInTheDocument();

    await waitFor(() => {
      expect(screen.getByText(/alice/i)).toBeInTheDocument();
      expect(screen.getByText(/bob/i)).toBeInTheDocument();
    });
  });

  it('handles server error', async () => {
    server.use(
      rest.get('/api/users', (req, res, ctx) => {
        return res(
          ctx.status(500),
          ctx.json({ message: 'Internal Server Error' })
        );
      })
    );

    render(<UserList />);

    await waitFor(() => {
      expect(screen.getByText(/error loading users/i)).toBeInTheDocument();
    });
  });
});

Mocking Context

import { render, screen } from '@testing-library/react';
import { AuthContext } from './AuthContext';
import { ProtectedComponent } from './ProtectedComponent';

const mockAuthContext = (overrides = {}) => ({
  user: { id: 1, name: 'Test User' },
  isAuthenticated: true,
  login: jest.fn(),
  logout: jest.fn(),
  ...overrides,
});

describe('ProtectedComponent', () => {
  it('renders content for authenticated user', () => {
    const contextValue = mockAuthContext();

    render(
      <AuthContext.Provider value={contextValue}>
        <ProtectedComponent />
      </AuthContext.Provider>
    );

    expect(screen.getByText(/welcome, test user/i)).toBeInTheDocument();
  });

  it('renders login prompt for unauthenticated user', () => {
    const contextValue = mockAuthContext({
      user: null,
      isAuthenticated: false,
    });

    render(
      <AuthContext.Provider value={contextValue}>
        <ProtectedComponent />
      </AuthContext.Provider>
    );

    expect(screen.getByText(/please log in/i)).toBeInTheDocument();
  });
});

Mocking Child Components

import { render, screen } from '@testing-library/react';
import { ParentComponent } from './ParentComponent';

// Mock the child component
jest.mock('./ChildComponent', () => ({
  ChildComponent: ({ title, onAction }) => (
    <div>
      <h2>{title}</h2>
      <button onClick={onAction}>Mocked Action</button>
    </div>
  ),
}));

describe('ParentComponent', () => {
  it('renders with mocked child', () => {
    render(<ParentComponent />);

    expect(screen.getByText(/mocked action/i)).toBeInTheDocument();
  });
});

Async Testing Patterns

Testing with waitFor

import { render, screen, waitFor } from '@testing-library/react';
import { AsyncComponent } from './AsyncComponent';

describe('AsyncComponent', () => {
  it('loads and displays data', async () => {
    render(<AsyncComponent />);

    expect(screen.getByText(/loading/i)).toBeInTheDocument();

    await waitFor(() => {
      expect(screen.getByText(/data loaded/i)).toBeInTheDocument();
    });
  });

  it('waits for specific condition', async () => {
    render(<AsyncComponent />);

    await waitFor(
      () => {
        expect(screen.queryByText(/loading/i)).not.toBeInTheDocument();
      },
      { timeout: 3000 }
    );
  });
});

Testing with findBy Queries

import { render, screen } from '@testing-library/react';
import { DataFetcher } from './DataFetcher';

describe('DataFetcher Component', () => {
  it('displays fetched data', async () => {
    render(<DataFetcher />);

    // findBy automatically waits for element to appear
    const heading = await screen.findByRole('heading', { name: /data/i });
    expect(heading).toBeInTheDocument();
  });

  it('handles timeout for missing elements', async () => {
    render(<DataFetcher url="/api/missing" />);

    await expect(
      screen.findByText(/success/i, {}, { timeout: 500 })
    ).rejects.toThrow();
  });
});

Testing Promises

import { render, screen } from '@testing-library/react';
import userEvent from '@testing-library/user-event';
import { AsyncForm } from './AsyncForm';

describe('AsyncForm Component', () => {
  it('submits form and shows success message', async () => {
    const user = userEvent.setup();
    render(<AsyncForm />);

    const input = screen.getByLabelText(/name/i);
    const submitBtn = screen.getByRole('button', { name: /submit/i });

    await user.type(input, 'John Doe');
    await user.click(submitBtn);

    const successMsg = await screen.findByText(/submitted successfully/i);
    expect(successMsg).toBeInTheDocument();
  });

  it('shows error message on failure', async () => {
    const user = userEvent.setup();
    render(<AsyncForm shouldFail={true} />);

    const submitBtn = screen.getByRole('button', { name: /submit/i });
    await user.click(submitBtn);

    const errorMsg = await screen.findByRole('alert');
    expect(errorMsg).toHaveTextContent(/submission failed/i);
  });
});

Testing with Fake Timers

import { render, screen, act } from '@testing-library/react';
import { Timer } from './Timer';

describe('Timer Component', () => {
  beforeEach(() => {
    jest.useFakeTimers();
  });

  afterEach(() => {
    jest.useRealTimers();
  });

  it('updates timer every second', () => {
    render(<Timer />);

    expect(screen.getByText(/0 seconds/i)).toBeInTheDocument();

    act(() => {
      jest.advanceTimersByTime(1000);
    });

    expect(screen.getByText(/1 second/i)).toBeInTheDocument();

    act(() => {
      jest.advanceTimersByTime(3000);
    });

    expect(screen.getByText(/4 seconds/i)).toBeInTheDocument();
  });

  it('cleans up timer on unmount', () => {
    const { unmount } = render(<Timer />);

    const clearIntervalSpy = jest.spyOn(global, 'clearInterval');

    unmount();

    expect(clearIntervalSpy).toHaveBeenCalled();
  });
});

Testing Custom Hooks

Basic Hook Testing

import { renderHook } from '@testing-library/react';
import { useCounter } from './useCounter';

describe('useCounter Hook', () => {
  it('initializes with default value', () => {
    const { result } = renderHook(() => useCounter());

    expect(result.current.count).toBe(0);
  });

  it('initializes with provided value', () => {
    const { result } = renderHook(() => useCounter(10));

    expect(result.current.count).toBe(10);
  });

  it('increments count', () => {
    const { result } = renderHook(() => useCounter());

    act(() => {
      result.current.increment();
    });

    expect(result.current.count).toBe(1);
  });

  it('decrements count', () => {
    const { result } = renderHook(() => useCounter(5));

    act(() => {
      result.current.decrement();
    });

    expect(result.current.count).toBe(4);
  });
});

Testing Hooks with Props

import { renderHook } from '@testing-library/react';
import { useFetch } from './useFetch';

describe('useFetch Hook', () => {
  it('fetches data for given URL', async () => {
    const { result } = renderHook(() => useFetch('/api/users'));

    expect(result.current.loading).toBe(true);

    await waitFor(() => {
      expect(result.current.loading).toBe(false);
    });

    expect(result.current.data).toBeDefined();
    expect(result.current.error).toBeNull();
  });

  it('refetches when URL changes', async () => {
    const { result, rerender } = renderHook(
      ({ url }) => useFetch(url),
      { initialProps: { url: '/api/users' } }
    );

    await waitFor(() => {
      expect(result.current.loading).toBe(false);
    });

    const firstData = result.current.data;

    rerender({ url: '/api/posts' });

    await waitFor(() => {
      expect(result.current.data).not.toEqual(firstData);
    });
  });
});

Testing Hooks with Context

import { renderHook } from '@testing-library/react';
import { ThemeProvider } from './ThemeContext';
import { useTheme } from './useTheme';

describe('useTheme Hook', () => {
  const wrapper = ({ children }) => (
    <ThemeProvider initialTheme="light">
      {children}
    </ThemeProvider>
  );

  it('returns current theme', () => {
    const { result } = renderHook(() => useTheme(), { wrapper });

    expect(result.current.theme).toBe('light');
  });

  it('toggles theme', () => {
    const { result } = renderHook(() => useTheme(), { wrapper });

    act(() => {
      result.current.toggleTheme();
    });

    expect(result.current.theme).toBe('dark');
  });
});

Testing Async Hooks

import { renderHook, waitFor } from '@testing-library/react';
import { useAsyncData } from './useAsyncData';

describe('useAsyncData Hook', () => {
  it('loads data asynchronously', async () => {
    const { result } = renderHook(() => useAsyncData('/api/data'));

    expect(result.current.loading).toBe(true);
    expect(result.current.data).toBeNull();

    await waitFor(() => {
      expect(result.current.loading).toBe(false);
    });

    expect(result.current.data).toBeDefined();
  });

  it('handles errors', async () => {
    const { result } = renderHook(() => useAsyncData('/api/error'));

    await waitFor(() => {
      expect(result.current.loading).toBe(false);
    });

    expect(result.current.error).toBeDefined();
    expect(result.current.data).toBeNull();
  });
});

Integration Testing Patterns

Testing Component Integration

import { render, screen } from '@testing-library/react';
import userEvent from '@testing-library/user-event';
import { App } from './App';

describe('App Integration', () => {
  it('navigates between pages', async () => {
    const user = userEvent.setup();
    render(<App />);

    expect(screen.getByText(/home page/i)).toBeInTheDocument();

    const aboutLink = screen.getByRole('link', { name: /about/i });
    await user.click(aboutLink);

    expect(screen.getByText(/about page/i)).toBeInTheDocument();
  });

  it('completes full user flow', async () => {
    const user = userEvent.setup();
    render(<App />);

    // Navigate to signup
    await user.click(screen.getByRole('link', { name: /sign up/i }));

    // Fill out form
    await user.type(screen.getByLabelText(/email/i), '[email protected]');
    await user.type(screen.getByLabelText(/password/i), 'password123');

    // Submit form
    await user.click(screen.getByRole('button', { name: /submit/i }));

    // Verify success
    expect(await screen.findByText(/welcome/i)).toBeInTheDocument();
  });
});

Testing with Router

import { render, screen } from '@testing-library/react';
import { MemoryRouter } from 'react-router-dom';
import userEvent from '@testing-library/user-event';
import { AppRoutes } from './AppRoutes';

const renderWithRouter = (ui, { initialEntries = ['/'] } = {}) => {
  return render(
    <MemoryRouter initialEntries={initialEntries}>
      {ui}
    </MemoryRouter>
  );
};

describe('AppRoutes Integration', () => {
  it('renders home page by default', () => {
    renderWithRouter(<AppRoutes />);

    expect(screen.getByText(/home/i)).toBeInTheDocument();
  });

  it('renders user page at /users/:id', () => {
    renderWithRouter(<AppRoutes />, { initialEntries: ['/users/123'] });

    expect(screen.getByText(/user profile/i)).toBeInTheDocument();
  });

  it('navigates programmatically', async () => {
    const user = userEvent.setup();
    renderWithRouter(<AppRoutes />);

    const navButton = screen.getByRole('button', { name: /go to profile/i });
    await user.click(navButton);

    expect(screen.getByText(/profile page/i)).toBeInTheDocument();
  });
});

Testing with Redux

import { render, screen } from '@testing-library/react';
import { Provider } from 'react-redux';
import { configureStore } from '@reduxjs/toolkit';
import userEvent from '@testing-library/user-event';
import { TodoList } from './TodoList';
import todosReducer from './todosSlice';

const createMockStore = (initialState = {}) => {
  return configureStore({
    reducer: {
      todos: todosReducer,
    },
    preloadedState: initialState,
  });
};

const renderWithStore = (ui, { store = createMockStore() } = {}) => {
  return render(<Provider store={store}>{ui}</Provider>);
};

describe('TodoList Integration', () => {
  it('adds new todo', async () => {
    const user = userEvent.setup();
    renderWithStore(<TodoList />);

    const input = screen.getByPlaceholderText(/new todo/i);
    const addButton = screen.getByRole('button', { name: /add/i });

    await user.type(input, 'Buy groceries');
    await user.click(addButton);

    expect(screen.getByText(/buy groceries/i)).toBeInTheDocument();
  });

  it('renders initial todos from store', () => {
    const initialState = {
      todos: {
        items: [
          { id: 1, text: 'Existing todo', completed: false },
        ],
      },
    };

    renderWithStore(<TodoList />, { store: createMockStore(initialState) });

    expect(screen.getByText(/existing todo/i)).toBeInTheDocument();
  });
});

Jest DOM Matchers

Common Matchers

// Element presence
expect(element).toBeInTheDocument();
expect(element).not.toBeInTheDocument();

// Visibility
expect(element).toBeVisible();
expect(element).not.toBeVisible();

// Text content
expect(element).toHaveTextContent('Hello World');
expect(element).toHaveTextContent(/hello/i);

// Attributes
expect(element).toHaveAttribute('type', 'submit');
expect(element).toHaveAttribute('disabled');

// Classes
expect(element).toHaveClass('active');
expect(element).toHaveClass('btn', 'btn-primary');

// Styles
expect(element).toHaveStyle({ color: 'red' });
expect(element).toHaveStyle('display: none');

// Forms
expect(input).toHaveValue('test');
expect(input).toHaveDisplayValue('Test');
expect(checkbox).toBeChecked();
expect(checkbox).not.toBeChecked();
expect(input).toBeDisabled();
expect(input).toBeEnabled();
expect(input).toBeRequired();
expect(input).toBeInvalid();
expect(input).toBeValid();

// Focus
expect(element).toHaveFocus();

// Accessibility
expect(element).toHaveAccessibleName('Submit button');
expect(element).toHaveAccessibleDescription('Click to submit form');

// Contains
expect(container).toContainElement(child);
expect(container).toContainHTML('<span>Text</span>');

Best Practices

Test Organization

1. Group Related Tests: Use describe blocks to organize tests

   describe('UserProfile', () => {
     describe('when loading', () => {
       it('shows loading spinner', () => {});
     });
   
     describe('when loaded', () => {
       it('displays user information', () => {});
       it('shows profile picture', () => {});
     });
   });
   

2. Use Descriptive Test Names: Test names should describe behavior

   // Good
   it('displays error message when login fails', () => {});
   
   // Bad
   it('test login', () => {});
   

3. Follow AAA Pattern: Arrange, Act, Assert

   it('increments counter', async () => {
     // Arrange
     const user = userEvent.setup();
     render(<Counter />);
   
     // Act
     await user.click(screen.getByRole('button', { name: /increment/i }));
   
     // Assert
     expect(screen.getByText(/count: 1/i)).toBeInTheDocument();
   });
   

Query Best Practices

1. Prefer Accessible Queries: Use getByRole, getByLabelText
2. Use Screen Queries: Import from screen instead of destructuring render
3. Avoid getByTestId: Use it as last resort only
4. Use Regular Expressions: More flexible than exact strings

Async Testing Best Practices

1. Use findBy for Async: Prefer findBy over getBy + waitFor
2. Set Proper Timeouts: Configure waitFor timeouts for slow operations
3. Avoid act() Warnings: Use userEvent, waitFor, findBy appropriately
4. Clean Up Timers: Use jest.useFakeTimers() and cleanup properly

Mocking Best Practices

1. Mock at Right Level: Mock external dependencies, not internal logic
2. Reset Mocks: Clear mocks between tests
3. Use MSW for API: Prefer MSW over mocking axios/fetch directly
4. Avoid Over-Mocking: Don't mock what you're testing

Coverage Best Practices

1. Focus on Behavior: Test user-facing behavior, not implementation
2. Don't Chase 100%: Focus on critical paths
3. Test Error States: Include error handling tests
4. Test Edge Cases: Include boundary conditions

Common Testing Patterns

Testing Lists and Iterations

it('renders list of items', () => {
  const items = ['Apple', 'Banana', 'Cherry'];
  render(<ItemList items={items} />);

  items.forEach(item => {
    expect(screen.getByText(item)).toBeInTheDocument();
  });
});

Testing Accessibility

it('has accessible form', () => {
  render(<ContactForm />);

  const nameInput = screen.getByLabelText(/name/i);
  expect(nameInput).toHaveAccessibleName('Name');
  expect(nameInput).toBeRequired();

  const submitButton = screen.getByRole('button', { name: /submit/i });
  expect(submitButton).toHaveAttribute('type', 'submit');
});

Testing Error Boundaries

it('catches errors and displays fallback', () => {
  const ThrowError = () => {
    throw new Error('Test error');
  };

  // Suppress console.error for this test
  const spy = jest.spyOn(console, 'error').mockImplementation(() => {});

  render(
    <ErrorBoundary fallback={<div>Something went wrong</div>}>
      <ThrowError />
    </ErrorBoundary>
  );

  expect(screen.getByText(/something went wrong/i)).toBeInTheDocument();

  spy.mockRestore();
});

Testing Portals

it('renders modal in portal', () => {
  render(<Modal isOpen={true}>Modal Content</Modal>);

  const modal = screen.getByText(/modal content/i);
  expect(modal).toBeInTheDocument();

  // Modal should be in document.body, not in the component tree
  expect(modal.parentElement).toBe(document.body);
});

Troubleshooting

Common Issues

Issue: "Unable to find element"

• Solution: Use screen.debug() to see DOM, check query type, wait for async updates

Issue: "Act warnings"

• Solution: Use userEvent instead of fireEvent, wrap state updates in act(), use waitFor/findBy

Issue: "Jest timeout"

• Solution: Increase timeout, check for infinite loops, ensure async operations complete

Issue: "Cannot read property of undefined"

• Solution: Check mocks are set up correctly, ensure components receive required props

Issue: "Multiple elements found"

• Solution: Make queries more specific, use getAllBy for multiple elements

Resources

• Jest Documentation: https://jestjs.io/
• React Testing Library: https://testing-library.com/react
• Testing Library Queries: https://testing-library.com/docs/queries/about
• Jest DOM Matchers: https://github.com/testing-library/jest-dom
• MSW Documentation: https://mswjs.io/
• Common Mistakes: https://kentcdodds.com/blog/common-mistakes-with-react-testing-library

---

Skill Version: 1.0.0 Last Updated: October 2025 Skill Category: Testing, React, Quality Assurance Compatible With: Jest 29+, React Testing Library 13+, React 16.8+

Install: npx skills add ChristopherAlphonse/calphonse-skills --skill jest-react-testing