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React Testing Library: Writing Reliable...

Q: What is React Testing Library: Writing Reliable Component Tests about?

Master React Testing Library to write user-centric tests that focus on behavior over implementation, ensuring reliable and maintainable test suites.

React Testing Library: Writing Reliable Tests for Components

Testing is a crucial aspect of modern web development that ensures code quality, prevents regressions, and builds confidence in your applications. When it comes to React applications, React Testing Library (RTL) has emerged as the gold standard for testing components in a way that closely resembles how users interact with your application. This comprehensive guide will walk you through everything you need to know about writing reliable tests for React components using RTL.

Introduction to React Testing Library

React Testing Library is a simple and complete testing utility that encourages good testing practices by focusing on testing components as users would interact with them, rather than testing implementation details. Created by Kent C. Dodds, RTL has become the preferred testing framework for React applications due to its philosophy of testing behavior over implementation.

Why Choose React Testing Library?

Unlike other testing utilities that focus on component internals, RTL emphasizes:

- User-centric testing: Tests interact with components the same way users do - Implementation independence: Tests remain valid even when internal component structure changes - Accessibility focus: Encourages finding elements by accessible attributes - Simplicity: Provides a clean, intuitive API for common testing scenarios

Core Philosophy

The guiding principle of RTL is: "The more your tests resemble the way your software is used, the more confidence they can give you." This philosophy drives every design decision in the library, from the query methods available to the way components are rendered and interacted with.

Setting Up React Testing Library

Installation

If you're using Create React App, RTL comes pre-installed. For custom setups, install the necessary packages:

`bash npm install --save-dev @testing-library/react @testing-library/jest-dom @testing-library/user-event `

Configuration

Create a setupTests.js file in your src directory to configure Jest and RTL:

`javascript import '@testing-library/jest-dom'; import { configure } from '@testing-library/react';

// Configure RTL configure({ testIdAttribute: 'data-testid', asyncUtilTimeout: 5000 });

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

// Mock 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(), })), }); `

Basic Test Structure

Here's the basic structure of a React Testing Library test:

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

describe('MyComponent', () => { test('renders correctly', () => { render(); expect(screen.getByText('Hello World')).toBeInTheDocument(); }); }); `

Understanding RTL Queries

React Testing Library provides various query methods to find elements in your components. Understanding when to use each query type is crucial for writing effective tests.

Query Types

RTL offers three types of queries:

1. getBy: Returns the element or throws an error if not found 2. queryBy: Returns the element or null if not found 3. findBy: Returns a promise that resolves when the element is found

Priority Order for Queries

RTL recommends using queries in this priority order:

`javascript // 1. Accessible to everyone screen.getByRole('button', { name: /submit/i }) screen.getByLabelText(/username/i) screen.getByPlaceholderText(/enter username/i) screen.getByText(/welcome/i)

// 2. Semantic queries screen.getByDisplayValue(/john/i) screen.getByAltText(/profile picture/i) screen.getByTitle(/close dialog/i)

// 3. Test IDs (last resort) screen.getByTestId('submit-button') `

Practical Query Examples

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

test('login form queries example', () => { render(); // Finding form elements const usernameInput = screen.getByLabelText(/username/i); const passwordInput = screen.getByLabelText(/password/i); const submitButton = screen.getByRole('button', { name: /sign in/i }); // Finding by text content const welcomeMessage = screen.getByText(/welcome back/i); // Finding by placeholder const searchInput = screen.getByPlaceholderText(/search.../i); // Using queryBy for elements that might not exist const errorMessage = screen.queryByText(/invalid credentials/i); expect(errorMessage).not.toBeInTheDocument(); }); `

Unit Testing React Components

Unit tests focus on testing individual components in isolation. Let's explore how to write comprehensive unit tests for different types of React components.

Testing a Simple Component

`javascript // Button.jsx import React from 'react'; import './Button.css';

const Button = ({ children, variant = 'primary', disabled = false, onClick, type = 'button', ...props }) => { return ( ); };

export default Button; `

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

describe('Button Component', () => { test('renders with correct text', () => { render(); expect(screen.getByRole('button', { name: /click me/i })).toBeInTheDocument(); });

test('applies correct variant class', () => { render(); const button = screen.getByRole('button'); expect(button).toHaveClass('btn--secondary'); });

test('handles disabled state', () => { render(); const button = screen.getByRole('button'); expect(button).toBeDisabled(); });

test('calls onClick handler when clicked', async () => { const user = userEvent.setup(); const handleClick = jest.fn(); render(); await user.click(screen.getByRole('button')); expect(handleClick).toHaveBeenCalledTimes(1); });

test('does not call onClick when disabled', async () => { const user = userEvent.setup(); const handleClick = jest.fn(); render(); await user.click(screen.getByRole('button')); expect(handleClick).not.toHaveBeenCalled(); }); }); `

Testing Components with State

`javascript // Counter.jsx import React, { useState } from 'react';

const Counter = ({ initialCount = 0, step = 1 }) => { const [count, setCount] = useState(initialCount);

const increment = () => setCount(prev => prev + step); const decrement = () => setCount(prev => prev - step); const reset = () => setCount(initialCount);

return (

Counter: {count}

); };

export default Counter; `

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

describe('Counter Component', () => { test('renders with initial count', () => { render(); expect(screen.getByText('Counter: 5')).toBeInTheDocument(); });

test('increments count when increment button is clicked', async () => { const user = userEvent.setup(); render(); const incrementButton = screen.getByRole('button', { name: /increment/i }); await user.click(incrementButton); expect(screen.getByText('Counter: 1')).toBeInTheDocument(); });

test('decrements count when decrement button is clicked', async () => { const user = userEvent.setup(); render(); const decrementButton = screen.getByRole('button', { name: /decrement/i }); await user.click(decrementButton); expect(screen.getByText('Counter: 4')).toBeInTheDocument(); });

test('resets count when reset button is clicked', async () => { const user = userEvent.setup(); render(); // Change the count first await user.click(screen.getByRole('button', { name: /increment/i })); expect(screen.getByText('Counter: 11')).toBeInTheDocument(); // Then reset await user.click(screen.getByRole('button', { name: /reset/i })); expect(screen.getByText('Counter: 10')).toBeInTheDocument(); });

test('uses custom step value', async () => { const user = userEvent.setup(); render(); await user.click(screen.getByRole('button', { name: /increment/i })); expect(screen.getByText('Counter: 5')).toBeInTheDocument(); }); }); `

Testing Form Components

`javascript // ContactForm.jsx import React, { useState } from 'react';

const ContactForm = ({ onSubmit }) => { const [formData, setFormData] = useState({ name: '', email: '', message: '' }); const [errors, setErrors] = useState({});

return (

Name: {errors.name && {errors.name}}

Email: {errors.email && {errors.email}}

Message:

{errors.message && <span role="alert">{errors.message}</span>}
      </div>
      
      <button type="submit">Send Message</button>
    </form>
  );
};</p><p>export default ContactForm;
<code></code>`</p><p><code></code>`javascript
// ContactForm.test.js
import { render, screen } from '@testing-library/react';
import userEvent from '@testing-library/user-event';
import ContactForm from './ContactForm';</p><p>describe('ContactForm Component', () => {
  const mockOnSubmit = jest.fn();</p><p>  beforeEach(() => {
    mockOnSubmit.mockClear();
  });</p><p>  test('renders all form fields', () => {
    render(<ContactForm onSubmit={mockOnSubmit} />);
    
    expect(screen.getByLabelText(/name/i)).toBeInTheDocument();
    expect(screen.getByLabelText(/email/i)).toBeInTheDocument();
    expect(screen.getByLabelText(/message/i)).toBeInTheDocument();
    expect(screen.getByRole('button', { name: /send message/i })).toBeInTheDocument();
  });</p><p>  test('updates input values when user types', async () => {
    const user = userEvent.setup();
    render(<ContactForm onSubmit={mockOnSubmit} />);
    
    const nameInput = screen.getByLabelText(/name/i);
    const emailInput = screen.getByLabelText(/email/i);
    const messageInput = screen.getByLabelText(/message/i);
    
    await user.type(nameInput, 'John Doe');
    await user.type(emailInput, 'john@example.com');
    await user.type(messageInput, 'Hello world');
    
    expect(nameInput).toHaveValue('John Doe');
    expect(emailInput).toHaveValue('john@example.com');
    expect(messageInput).toHaveValue('Hello world');
  });</p><p>  test('shows validation errors for empty fields', async () => {
    const user = userEvent.setup();
    render(<ContactForm onSubmit={mockOnSubmit} />);
    
    await user.click(screen.getByRole('button', { name: /send message/i }));
    
    expect(screen.getByText('Name is required')).toBeInTheDocument();
    expect(screen.getByText('Email is required')).toBeInTheDocument();
    expect(screen.getByText('Message is required')).toBeInTheDocument();
    expect(mockOnSubmit).not.toHaveBeenCalled();
  });</p><p>  test('shows error for invalid email', async () => {
    const user = userEvent.setup();
    render(<ContactForm onSubmit={mockOnSubmit} />);
    
    await user.type(screen.getByLabelText(/email/i), 'invalid-email');
    await user.click(screen.getByRole('button', { name: /send message/i }));
    
    expect(screen.getByText('Email is invalid')).toBeInTheDocument();
  });</p><p>  test('submits form with valid data', async () => {
    const user = userEvent.setup();
    render(<ContactForm onSubmit={mockOnSubmit} />);
    
    await user.type(screen.getByLabelText(/name/i), 'John Doe');
    await user.type(screen.getByLabelText(/email/i), 'john@example.com');
    await user.type(screen.getByLabelText(/message/i), 'Hello world');
    
    await user.click(screen.getByRole('button', { name: /send message/i }));
    
    expect(mockOnSubmit).toHaveBeenCalledWith({
      name: 'John Doe',
      email: 'john@example.com',
      message: 'Hello world'
    });
  });</p><p>  test('clears errors when user starts typing', async () => {
    const user = userEvent.setup();
    render(<ContactForm onSubmit={mockOnSubmit} />);
    
    // Trigger validation errors
    await user.click(screen.getByRole('button', { name: /send message/i }));
    expect(screen.getByText('Name is required')).toBeInTheDocument();
    
    // Start typing to clear error
    await user.type(screen.getByLabelText(/name/i), 'John');
    expect(screen.queryByText('Name is required')).not.toBeInTheDocument();
  });
});
<code></code>`</p><p><h2>Integration Testing</h2></p><p>Integration tests verify that multiple components work together correctly. They test the interactions between components and ensure that data flows properly through your application.</p><p><h3>Testing Component Interactions</h3></p><p><code></code>`javascript
// TodoApp.jsx
import React, { useState } from 'react';
import TodoList from './TodoList';
import AddTodoForm from './AddTodoForm';</p><p>const TodoApp = () => {
  const [todos, setTodos] = useState([]);</p><p>  const addTodo = (text) => {
    const newTodo = {
      id: Date.now(),
      text,
      completed: false
    };
    setTodos(prev => [...prev, newTodo]);
  };</p><p>  const toggleTodo = (id) => {
    setTodos(prev => 
      prev.map(todo => 
        todo.id === id ? { ...todo, completed: !todo.completed } : todo
      )
    );
  };</p><p>  const deleteTodo = (id) => {
    setTodos(prev => prev.filter(todo => todo.id !== id));
  };</p><p>  return (
    <div>
      <h1>Todo App</h1>
      <AddTodoForm onAdd={addTodo} />
      <TodoList 
        todos={todos} 
        onToggle={toggleTodo} 
        onDelete={deleteTodo} 
      />
    </div>
  );
};</p><p>export default TodoApp;
<code></code>`</p><p><code></code>`javascript
// AddTodoForm.jsx
import React, { useState } from 'react';</p><p>const AddTodoForm = ({ onAdd }) => {
  const [text, setText] = useState('');</p><p>  const handleSubmit = (e) => {
    e.preventDefault();
    if (text.trim()) {
      onAdd(text.trim());
      setText('');
    }
  };</p><p>  return (
    <form onSubmit={handleSubmit}>
      <input
        type="text"
        value={text}
        onChange={(e) => setText(e.target.value)}
        placeholder="Add a new todo..."
        aria-label="New todo"
      />
      <button type="submit">Add Todo</button>
    </form>
  );
};</p><p>export default AddTodoForm;
<code></code>`</p><p><code></code>`javascript
// TodoList.jsx
import React from 'react';</p><p>const TodoItem = ({ todo, onToggle, onDelete }) => (
  <li>
    <input
      type="checkbox"
      checked={todo.completed}
      onChange={() => onToggle(todo.id)}
      aria-label={<code>Toggle ${todo.text}</code>}
    />
    <span style=#>
      {todo.text}
    </span>
    <button onClick={() => onDelete(todo.id)} aria-label={<code>Delete ${todo.text}</code>}>
      Delete
    </button>
  </li>
);</p><p>const TodoList = ({ todos, onToggle, onDelete }) => {
  if (todos.length === 0) {
    return <p>No todos yet. Add one above!</p>;
  }</p><p>  return (
    <ul>
      {todos.map(todo => (
        <TodoItem
          key={todo.id}
          todo={todo}
          onToggle={onToggle}
          onDelete={onDelete}
        />
      ))}
    </ul>
  );
};</p><p>export default TodoList;
<code></code>`</p><p><code></code>`javascript
// TodoApp.test.js
import { render, screen } from '@testing-library/react';
import userEvent from '@testing-library/user-event';
import TodoApp from './TodoApp';</p><p>describe('TodoApp Integration', () => {
  test('adds new todo when form is submitted', async () => {
    const user = userEvent.setup();
    render(<TodoApp />);
    
    // Initially shows empty state
    expect(screen.getByText('No todos yet. Add one above!')).toBeInTheDocument();
    
    // Add a new todo
    const input = screen.getByLabelText(/new todo/i);
    await user.type(input, 'Learn React Testing Library');
    await user.click(screen.getByRole('button', { name: /add todo/i }));
    
    // Todo appears in the list
    expect(screen.getByText('Learn React Testing Library')).toBeInTheDocument();
    expect(screen.queryByText('No todos yet. Add one above!')).not.toBeInTheDocument();
    
    // Input is cleared
    expect(input).toHaveValue('');
  });</p><p>  test('toggles todo completion status', async () => {
    const user = userEvent.setup();
    render(<TodoApp />);
    
    // Add a todo
    await user.type(screen.getByLabelText(/new todo/i), 'Test todo');
    await user.click(screen.getByRole('button', { name: /add todo/i }));
    
    // Toggle completion
    const checkbox = screen.getByLabelText(/toggle test todo/i);
    const todoText = screen.getByText('Test todo');
    
    expect(checkbox).not.toBeChecked();
    expect(todoText).not.toHaveStyle('text-decoration: line-through');
    
    await user.click(checkbox);
    
    expect(checkbox).toBeChecked();
    expect(todoText).toHaveStyle('text-decoration: line-through');
  });</p><p>  test('deletes todo when delete button is clicked', async () => {
    const user = userEvent.setup();
    render(<TodoApp />);
    
    // Add a todo
    await user.type(screen.getByLabelText(/new todo/i), 'Todo to delete');
    await user.click(screen.getByRole('button', { name: /add todo/i }));
    
    expect(screen.getByText('Todo to delete')).toBeInTheDocument();
    
    // Delete the todo
    await user.click(screen.getByLabelText(/delete todo to delete/i));
    
    expect(screen.queryByText('Todo to delete')).not.toBeInTheDocument();
    expect(screen.getByText('No todos yet. Add one above!')).toBeInTheDocument();
  });</p><p>  test('manages multiple todos correctly', async () => {
    const user = userEvent.setup();
    render(<TodoApp />);
    
    const input = screen.getByLabelText(/new todo/i);
    const addButton = screen.getByRole('button', { name: /add todo/i });
    
    // Add multiple todos
    const todos = ['First todo', 'Second todo', 'Third todo'];
    
    for (const todoText of todos) {
      await user.type(input, todoText);
      await user.click(addButton);
    }
    
    // All todos are displayed
    todos.forEach(todo => {
      expect(screen.getByText(todo)).toBeInTheDocument();
    });
    
    // Complete the second todo
    await user.click(screen.getByLabelText(/toggle second todo/i));
    expect(screen.getByLabelText(/toggle second todo/i)).toBeChecked();
    
    // Delete the first todo
    await user.click(screen.getByLabelText(/delete first todo/i));
    expect(screen.queryByText('First todo')).not.toBeInTheDocument();
    
    // Remaining todos are still there
    expect(screen.getByText('Second todo')).toBeInTheDocument();
    expect(screen.getByText('Third todo')).toBeInTheDocument();
  });
});
<code></code>`</p><p><h3>Testing with Context</h3></p><p><code></code>`javascript
// ThemeContext.jsx
import React, { createContext, useContext, useState } from 'react';</p><p>const ThemeContext = createContext();</p><p>export const useTheme = () => {
  const context = useContext(ThemeContext);
  if (!context) {
    throw new Error('useTheme must be used within a ThemeProvider');
  }
  return context;
};</p><p>export const ThemeProvider = ({ children }) => {
  const [theme, setTheme] = useState('light');</p><p>  const toggleTheme = () => {
    setTheme(prev => prev === 'light' ? 'dark' : 'light');
  };</p><p>  return (
    <ThemeContext.Provider value=#>
      {children}
    </ThemeContext.Provider>
  );
};
<code></code>`</p><p><code></code>`javascript
// ThemedButton.jsx
import React from 'react';
import { useTheme } from './ThemeContext';</p><p>const ThemedButton = ({ children, ...props }) => {
  const { theme, toggleTheme } = useTheme();</p><p>  return (
    <div className={<code>theme-${theme}</code>}>
      <button 
        onClick={toggleTheme}
        className={<code>btn btn--${theme}</code>}
        {...props}
      >
        {children}
      </button>
      <p>Current theme: {theme}</p>
    </div>
  );
};</p><p>export default ThemedButton;
<code></code>`</p><p><code></code>`javascript
// ThemedButton.test.js
import { render, screen } from '@testing-library/react';
import userEvent from '@testing-library/user-event';
import { ThemeProvider } from './ThemeContext';
import ThemedButton from './ThemedButton';</p><p>// Test helper to render component with context
const renderWithTheme = (ui) => {
  return render(
    <ThemeProvider>
      {ui}
    </ThemeProvider>
  );
};</p><p>describe('ThemedButton with Context', () => {
  test('renders with default light theme', () => {
    renderWithTheme(<ThemedButton>Toggle Theme</ThemedButton>);
    
    expect(screen.getByText('Current theme: light')).toBeInTheDocument();
    expect(screen.getByRole('button')).toHaveClass('btn--light');
  });</p><p>  test('toggles theme when button is clicked', async () => {
    const user = userEvent.setup();
    renderWithTheme(<ThemedButton>Toggle Theme</ThemedButton>);
    
    const button = screen.getByRole('button');
    
    // Initially light theme
    expect(screen.getByText('Current theme: light')).toBeInTheDocument();
    expect(button).toHaveClass('btn--light');
    
    // Click to toggle to dark theme
    await user.click(button);
    
    expect(screen.getByText('Current theme: dark')).toBeInTheDocument();
    expect(button).toHaveClass('btn--dark');
    
    // Click again to toggle back to light theme
    await user.click(button);
    
    expect(screen.getByText('Current theme: light')).toBeInTheDocument();
    expect(button).toHaveClass('btn--light');
  });</p><p>  test('multiple components share the same theme state', async () => {
    const user = userEvent.setup();
    renderWithTheme(
      <div>
        <ThemedButton>First Button</ThemedButton>
        <ThemedButton>Second Button</ThemedButton>
      </div>
    );
    
    const buttons = screen.getAllByRole('button');
    const themeTexts = screen.getAllByText(/current theme:/i);
    
    // Both components start with light theme
    themeTexts.forEach(text => {
      expect(text).toHaveTextContent('light');
    });
    
    // Click first button to toggle theme
    await user.click(buttons[0]);
    
    // Both components update to dark theme
    themeTexts.forEach(text => {
      expect(text).toHaveTextContent('dark');
    });
  });
});
<code></code>`</p><p><h2>Mocking APIs and External Dependencies</h2></p><p>Real-world applications often depend on external APIs, third-party libraries, and other services. Testing these interactions requires effective mocking strategies.</p><p><h3>Mocking HTTP Requests</h3></p><p><code></code>`javascript
// userService.js
export const fetchUser = async (userId) => {
  const response = await fetch(<code>/api/users/${userId}</code>);
  if (!response.ok) {
    throw new Error('Failed to fetch user');
  }
  return response.json();
};</p><p>export const updateUser = async (userId, userData) => {
  const response = await fetch(<code>/api/users/${userId}</code>, {
    method: 'PUT',
    headers: {
      'Content-Type': 'application/json',
    },
    body: JSON.stringify(userData),
  });
  
  if (!response.ok) {
    throw new Error('Failed to update user');
  }
  
  return response.json();
};
<code></code>`</p><p><code></code>`javascript
// UserProfile.jsx
import React, { useState, useEffect } from 'react';
import { fetchUser, updateUser } from './userService';</p><p>const UserProfile = ({ userId }) => {
  const [user, setUser] = useState(null);
  const [loading, setLoading] = useState(true);
  const [error, setError] = useState(null);
  const [editing, setEditing] = useState(false);
  const [formData, setFormData] = useState({});</p><p>  useEffect(() => {
    const loadUser = async () => {
      try {
        setLoading(true);
        const userData = await fetchUser(userId);
        setUser(userData);
        setFormData(userData);
      } catch (err) {
        setError(err.message);
      } finally {
        setLoading(false);
      }
    };</p><p>    if (userId) {
      loadUser();
    }
  }, [userId]);</p><p>  const handleSave = async () => {
    try {
      setLoading(true);
      const updatedUser = await updateUser(userId, formData);
      setUser(updatedUser);
      setEditing(false);
    } catch (err) {
      setError(err.message);
    } finally {
      setLoading(false);
    }
  };</p><p>  if (loading) return <div>Loading...</div>;
  if (error) return <div role="alert">Error: {error}</div>;
  if (!user) return <div>No user found</div>;</p><p>  return (
    <div>
      <h1>User Profile</h1>
      {editing ? (
        <div>
          <input
            value={formData.name || ''}
            onChange={(e) => setFormData(prev => ({ ...prev, name: e.target.value }))}
            placeholder="Name"
            aria-label="Name"
          />
          <input
            value={formData.email || ''}
            onChange={(e) => setFormData(prev => ({ ...prev, email: e.target.value }))}
            placeholder="Email"
            aria-label="Email"
          />
          <button onClick={handleSave} disabled={loading}>
            Save
          </button>
          <button onClick={() => setEditing(false)}>
            Cancel
          </button>
        </div>
      ) : (
        <div>
          <p>Name: {user.name}</p>
          <p>Email: {user.email}</p>
          <button onClick={() => setEditing(true)}>
            Edit
          </button>
        </div>
      )}
    </div>
  );
};</p><p>export default UserProfile;
<code></code>`</p><p><code></code>`javascript
// UserProfile.test.js
import { render, screen, waitFor } from '@testing-library/react';
import userEvent from '@testing-library/user-event';
import UserProfile from './UserProfile';
import * as userService from './userService';</p><p>// Mock the entire userService module
jest.mock('./userService');</p><p>const mockUserService = userService;</p><p>describe('UserProfile Component', () => {
  const mockUser = {
    id: 1,
    name: 'John Doe',
    email: 'john@example.com'
  };</p><p>  beforeEach(() => {
    jest.clearAllMocks();
  });</p><p>  test('loads and displays user data', async () => {
    mockUserService.fetchUser.mockResolvedValue(mockUser);</p><p>    render(<UserProfile userId={1} />);</p><p>    // Shows loading initially
    expect(screen.getByText('Loading...')).toBeInTheDocument();</p><p>    // Wait for user data to load
    await waitFor(() => {
      expect(screen.getByText('Name: John Doe')).toBeInTheDocument();
    });</p><p>    expect(screen.getByText('Email: john@example.com')).toBeInTheDocument();
    expect(mockUserService.fetchUser).toHaveBeenCalledWith(1);
  });</p><p>  test('handles API error', async () => {
    mockUserService.fetchUser.mockRejectedValue(new Error('Failed to fetch user'));</p><p>    render(<UserProfile userId={1} />);</p><p>    await waitFor(() => {
      expect(screen.getByRole('alert')).toHaveTextContent('Error: Failed to fetch user');
    });
  });</p><p>  test('enters edit mode when edit button is clicked', async () => {
    const user = userEvent.setup();
    mockUserService.fetchUser.mockResolvedValue(mockUser);</p><p>    render(<UserProfile userId={1} />);</p><p>    await waitFor(() => {
      expect(screen.getByText('Name: John Doe')).toBeInTheDocument();
    });</p><p>    await user.click(screen.getByRole('button', { name: /edit/i }));</p><p>    expect(screen.getByDisplayValue('John Doe')).toBeInTheDocument();
    expect(screen.getByDisplayValue('john@example.com')).toBeInTheDocument();
    expect(screen.getByRole('button', { name: /save/i })).toBeInTheDocument();
    expect(screen.getByRole('button', { name: /cancel/i })).toBeInTheDocument();
  });</p><p>  test('saves updated user data', async () => {
    const user = userEvent.setup();
    const updatedUser = { ...mockUser, name: 'Jane Doe' };
    
    mockUserService.fetchUser.mockResolvedValue(mockUser);
    mockUserService.updateUser.mockResolvedValue(updatedUser);</p><p>    render(<UserProfile userId={1} />);</p><p>    // Wait for initial load
    await waitFor(() => {
      expect(screen.getByText('Name: John Doe')).toBeInTheDocument();
    });</p><p>    // Enter edit mode
    await user.click(screen.getByRole('button', { name: /edit/i }));</p><p>    // Update name
    const nameInput = screen.getByLabelText(/name/i);
    await user.clear(nameInput);
    await user.type(nameInput, 'Jane Doe');</p><p>    // Save changes
    await user.click(screen.getByRole('button', { name: /save/i }));</p><p>    // Verify API call
    expect(mockUserService.updateUser).toHaveBeenCalledWith(1, {
      ...mockUser,
      name: 'Jane Doe'
    });</p><p>    // Verify UI update
    await waitFor(() => {
      expect(screen.getByText('Name: Jane Doe')).toBeInTheDocument();
    });</p><p>    // Should exit edit mode
    expect(screen.queryByLabelText(/name/i)).not.toBeInTheDocument();
  });</p><p>  test('handles update error', async () => {
    const user = userEvent.setup();
    
    mockUserService.fetchUser.mockResolvedValue(mockUser);
    mockUserService.updateUser.mockRejectedValue(new Error('Failed to update user'));</p><p>    render(<UserProfile userId={1} />);</p><p>    await waitFor(() => {
      expect(screen.getByText('Name: John Doe')).toBeInTheDocument();
    });</p><p>    await user.click(screen.getByRole('button', { name: /edit/i }));
    await user.click(screen.getByRole('button', { name: /save/i }));</p><p>    await waitFor(() => {
      expect(screen.getByRole('alert')).toHaveTextContent('Error: Failed to update user');
    });
  });</p><p>  test('cancels edit mode without saving', async () => {
    const user = userEvent.setup();
    mockUserService.fetchUser.mockResolvedValue(mockUser);</p><p>    render(<UserProfile userId={1} />);</p><p>    await waitFor(() => {
      expect(screen.getByText('Name: John Doe')).toBeInTheDocument();
    });</p><p>    await user.click(screen.getByRole('button', { name: /edit/i }));</p><p>    // Make changes
    const nameInput = screen.getByLabelText(/name/i);
    await user.clear(nameInput);
    await user.type(nameInput, 'Changed Name');</p><p>    // Cancel without saving
    await user.click(screen.getByRole('button', { name: /cancel/i }));</p><p>    // Should show original data
    expect(screen.getByText('Name: John Doe')).toBeInTheDocument();
    expect(screen.queryByLabelText(/name/i)).not.toBeInTheDocument();
    expect(mockUserService.updateUser).not.toHaveBeenCalled();
  });
});
<code></code>`</p><p><h3>Using Mock Service Worker (MSW)</h3></p><p>For more realistic API mocking, consider using Mock Service Worker:</p><p><code></code>`javascript
// mocks/handlers.js
import { rest } from 'msw';</p><p>export const handlers = [
  rest.get('/api/users/:userId', (req, res, ctx) => {
    const { userId } = req.params;
    
    return res(
      ctx.json({
        id: parseInt(userId),
        name: 'John Doe',
        email: 'john@example.com'
      })
    );
  }),</p><p>  rest.put('/api/users/:userId', async (req, res, ctx) => {
    const { userId } = req.params;
    const userData = await req.json();
    
    return res(
      ctx.json({
        id: parseInt(userId),
        ...userData
      })
    );
  }),
];
<code></code>`</p><p><code></code>`javascript
// mocks/server.js
import { setupServer } from 'msw/node';
import { handlers } from './handlers';</p><p>export const server = setupServer(...handlers);
<code></code>`</p><p><code></code>`javascript
// setupTests.js (updated)
import '@testing-library/jest-dom';
import { server } from './mocks/server';</p><p>// Enable API mocking before tests
beforeAll(() => server.listen());</p><p>// Reset handlers after each test
afterEach(() => server.resetHandlers());</p><p>// Clean up after tests
afterAll(() => server.close());
<code></code>`</p><p><h3>Mocking Third-Party Libraries</h3></p><p><code></code>`javascript
// LocationPicker.jsx
import React, { useState, useEffect } from 'react';
import { GoogleMap, Marker } from '@react-google-maps/api';</p><p>const LocationPicker = ({ onLocationSelect, initialLocation }) => {
  const [selectedLocation, setSelectedLocation] = useState(initialLocation);
  const [map, setMap] = useState(null);</p><p>  const handleMapClick = (event) => {
    const location = {
      lat: event.latLng.lat(),
      lng: event.latLng.lng()
    };
    setSelectedLocation(location);
    onLocationSelect(location);
  };</p><p>  return (
    <div>
      <h2>Select Location</h2>
      <GoogleMap
        zoom={10}
        center={selectedLocation}
        onClick={handleMapClick}
        onLoad={setMap}
        mapContainerStyle=#
      >
        {selectedLocation && (
          <Marker position={selectedLocation} />
        )}
      </GoogleMap>
      {selectedLocation && (
        <div data-testid="selected-location">
          Selected: {selectedLocation.lat.toFixed(4)}, {selectedLocation.lng.toFixed(4)}
        </div>
      )}
    </div>
  );
};</p><p>export default LocationPicker;
<code></code>`</p><p><code></code>`javascript
// LocationPicker.test.js
import { render, screen } from '@testing-library/react';
import userEvent from '@testing-library/user-event';
import LocationPicker from './LocationPicker';</p><p>// Mock the Google Maps library
jest.mock('@react-google-maps/api', () => ({
  GoogleMap: ({ children, onClick, onLoad }) => {
    // Simulate map load
    React.useEffect(() => {
      onLoad && onLoad({});
    }, [onLoad]);</p><p>    return (
      <div 
        data-testid="google-map"
        onClick={() => {
          // Simulate map click with mock coordinates
          onClick({
            latLng: {
              lat: () => 40.7128,
              lng: () => -74.0060
            }
          });
        }}
      >
        {children}
      </div>
    );
  },
  Marker: ({ position }) => (
    <div data-testid="marker">
      Marker at {position.lat}, {position.lng}
    </div>
  )
}));</p><p>describe('LocationPicker Component', () => {
  const mockOnLocationSelect = jest.fn();
  const initialLocation = { lat: 37.7749, lng: -122.4194 };</p><p>  beforeEach(() => {
    mockOnLocationSelect.mockClear();
  });</p><p>  test('renders map with initial location', () => {
    render(
      <LocationPicker 
        onLocationSelect={mockOnLocationSelect}
        initialLocation={initialLocation}
      />
    );</p><p>    expect(screen.getByTestId('google-map')).toBeInTheDocument();
    expect(screen.getByTestId('marker')).toBeInTheDocument();
    expect(screen.getByTestId('selected-location')).toHaveTextContent(
      'Selected: 37.7749, -122.4194'
    );
  });</p><p>  test('updates location when map is clicked', async () => {
    const user = userEvent.setup();
    render(
      <LocationPicker 
        onLocationSelect={mockOnLocationSelect}
        initialLocation={initialLocation}
      />
    );</p><p>    await user.click(screen.getByTestId('google-map'));</p><p>    expect(mockOnLocationSelect).toHaveBeenCalledWith({
      lat: 40.7128,
      lng: -74.0060
    });</p><p>    expect(screen.getByTestId('selected-location')).toHaveTextContent(
      'Selected: 40.7128, -74.0060'
    );
  });
});
<code></code>`</p><p><h2>Best Practices and Advanced Techniques</h2></p><p><h3>Custom Render Function</h3></p><p>Create a custom render function for consistent test setup:</p><p><code></code>`javascript
// test-utils.js
import React from 'react';
import { render } from '@testing-library/react';
import { BrowserRouter } from 'react-router-dom';
import { ThemeProvider } from './contexts/ThemeContext';
import { AuthProvider } from './contexts/AuthContext';</p><p>const AllTheProviders = ({ children }) => {
  return (
    <BrowserRouter>
      <ThemeProvider>
        <AuthProvider>
          {children}
        </AuthProvider>
      </ThemeProvider>
    </BrowserRouter>
  );
};</p><p>const customRender = (ui, options) =>
  render(ui, { wrapper: AllTheProviders, ...options });</p><p>// Re-export everything
export * from '@testing-library/react';</p><p>// Override render method
export { customRender as render };
<code></code>`</p><p><h3>Testing Custom Hooks</h3></p><p><code></code>`javascript
// useCounter.js
import { useState, useCallback } from 'react';</p><p>export const useCounter = (initialValue = 0) => {
  const [count, setCount] = useState(initialValue);</p><p>  const increment = useCallback(() => {
    setCount(prev => prev + 1);
  }, []);</p><p>  const decrement = useCallback(() => {
    setCount(prev => prev - 1);
  }, []);</p><p>  const reset = useCallback(() => {
    setCount(initialValue);
  }, [initialValue]);</p><p>  return { count, increment, decrement, reset };
};
<code></code>`</p><p><code></code>`javascript
// useCounter.test.js
import { renderHook, act } from '@testing-library/react';
import { useCounter } from './useCounter';</p><p>describe('useCounter Hook', () => {
  test('initializes with default value', () => {
    const { result } = renderHook(() => useCounter());
    expect(result.current.count).toBe(0);
  });</p><p>  test('initializes with custom value', () => {
    const { result } = renderHook(() => useCounter(10));
    expect(result.current.count).toBe(10);
  });</p><p>  test('increments count', () => {
    const { result } = renderHook(() => useCounter());
    
    act(() => {
      result.current.increment();
    });
    
    expect(result.current.count).toBe(1);
  });</p><p>  test('decrements count', () => {
    const { result } = renderHook(() => useCounter(5));
    
    act(() => {
      result.current.decrement();
    });
    
    expect(result.current.count).toBe(4);
  });</p><p>  test('resets to initial value', () => {
    const { result } = renderHook(() => useCounter(10));
    
    act(() => {
      result.current.increment();
      result.current.increment();
    });
    
    expect(result.current.count).toBe(12);
    
    act(() => {
      result.current.reset();
    });
    
    expect(result.current.count).toBe(10);
  });
});
<code></code>`</p><p><h3>Testing Error Boundaries</h3></p><p><code></code>`javascript
// ErrorBoundary.jsx
import React from 'react';</p><p>class ErrorBoundary extends React.Component {
  constructor(props) {
    super(props);
    this.state = { hasError: false, error: null };
  }</p><p>  static getDerivedStateFromError(error) {
    return { hasError: true, error };
  }</p><p>  componentDidCatch(error, errorInfo) {
    console.error('Error caught by boundary:', error, errorInfo);
  }</p><p>  render() {
    if (this.state.hasError) {
      return (
        <div role="alert">
          <h2>Something went wrong.</h2>
          <p>{this.state.error?.message}</p>
        </div>
      );
    }</p><p>    return this.props.children;
  }
}</p><p>export default ErrorBoundary;
<code></code>`</p><p><code></code>`javascript
// ErrorBoundary.test.js
import { render, screen } from '@testing-library/react';
import ErrorBoundary from './ErrorBoundary';</p><p>const ThrowError = ({ shouldThrow }) => {
  if (shouldThrow) {
    throw new Error('Test error');
  }
  return <div>No error</div>;
};</p><p>describe('ErrorBoundary', () => {
  // Suppress console.error for these tests
  const originalError = console.error;
  beforeAll(() => {
    console.error = jest.fn();
  });
  
  afterAll(() => {
    console.error = originalError;
  });</p><p>  test('renders children when there is no error', () => {
    render(
      <ErrorBoundary>
        <ThrowError shouldThrow={false} />
      </ErrorBoundary>
    );</p><p>    expect(screen.getByText('No error')).toBeInTheDocument();
  });</p><p>  test('renders error message when child component throws', () => {
    render(
      <ErrorBoundary>
        <ThrowError shouldThrow={true} />
      </ErrorBoundary>
    );</p><p>    expect(screen.getByRole('alert')).toBeInTheDocument();
    expect(screen.getByText('Something went wrong.')).toBeInTheDocument();
    expect(screen.getByText('Test error')).toBeInTheDocument();
  });
});
<code></code>`</p><p><h2>Conclusion</h2></p><p>React Testing Library provides a powerful and intuitive way to test React components by focusing on user behavior rather than implementation details. By following the practices outlined in this guide, you can create a comprehensive test suite that:</p><p>- Provides confidence in your application's functionality
- Catches regressions early in the development process
- Documents how your components are intended to be used
- Remains maintainable as your codebase evolves</p><p>Remember these key principles when writing tests with RTL:</p><p>1. <strong>Test behavior, not implementation</strong>: Focus on what the user sees and does
2. <strong>Use semantic queries</strong>: Prefer accessible queries that reflect real user interactions
3. <strong>Write integration tests</strong>: Test how components work together
4. <strong>Mock external dependencies</strong>: Keep tests isolated and predictable
5. <strong>Keep tests simple and focused</strong>: Each test should verify one specific behavior</p><p>With these foundations in place, you'll be well-equipped to build robust, well-tested React applications that provide excellent user experiences and maintain high code quality over time.</p><p>The investment in comprehensive testing pays dividends in reduced debugging time, increased confidence in deployments, and improved code maintainability. As your application grows in complexity, a solid test suite becomes invaluable for ensuring that new features don't break existing functionality and that your components continue to work as expected across different scenarios and user interactions.</p>
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