Jest: How to Mock a Class (Complete Guide with Examples)

When you're writing unit tests with Jest, you may need to mock a class—especially when your code depends on services, APIs, or external libraries. Mocking classes allows you to isolate your tests from complex or unreliable dependencies.

In this guide, we’ll show you exactly how to mock a class in Jest, step-by-step.

🧠 Why Mock a Class?

Mocking a class lets you:

• Control the behavior of complex or external dependencies
• Avoid side effects (like HTTP requests, file operations, etc.)
• Simulate edge cases
• Speed up tests

✅ Basic Class Mocking in Jest

🧾 Example Class

// services/emailService.js
export class EmailService {
  sendEmail(to, subject, body) {
    return `Sending email to ${to}`;
  }
}

🔧 How to Mock This Class in Jest

✅ 1. Mock Class with jest.mock() + jest.fn()

// user.test.js
import { EmailService } from './services/emailService';

jest.mock('./services/emailService'); // Automatically replaces class with mocks

test('should call sendEmail', () => {
  EmailService.mockImplementation(() => {
    return {
      sendEmail: jest.fn().mockReturnValue('Mocked email'),
    };
  });

  const emailService = new EmailService();
  const result = emailService.sendEmail('[email protected]', 'Hi', 'Test');

  expect(result).toBe('Mocked email');
  expect(emailService.sendEmail).toHaveBeenCalledWith('[email protected]', 'Hi', 'Test');
});

🔍 What This Does:

• Replaces the real class with a mock constructor
• You control the behavior of methods inside the class

✅ 2. Spy on Class Methods with jest.spyOn()

If you want to mock just one method of a class without mocking the whole class:

import { EmailService } from './services/emailService';

test('should spy on sendEmail', () => {
  const emailService = new EmailService();
  const spy = jest.spyOn(emailService, 'sendEmail').mockReturnValue('Mocked send');

  const result = emailService.sendEmail('[email protected]', 'Hello', 'Body');

  expect(result).toBe('Mocked send');
  expect(spy).toHaveBeenCalledTimes(1);

  spy.mockRestore(); // Restore the real method if needed
});

✅ 3. Mocking a Class from Another Module

jest.mock('./services/emailService', () => {
  return {
    EmailService: jest.fn().mockImplementation(() => ({
      sendEmail: jest.fn().mockReturnValue('Fake send'),
    })),
  };
});

This is useful if you're using require() or want to mock class imports deeply.

✅ 4. Using mockClear() and mockReset()

When you reuse mocks across tests, it's a good idea to clear them:

beforeEach(() => {
  EmailService.mockClear(); // Clear constructor calls
});

🔄 Bonus: Access Class Constructor Calls

Want to make sure your mocked class was instantiated?

expect(EmailService).toHaveBeenCalledTimes(1);

Or check args:

expect(EmailService).toHaveBeenCalledWith(/* args */);

✅ Summary

🚀 Conclusion

Mocking a class in Jest is an essential tool for clean, fast, and reliable unit tests. Whether you're mocking services, utilities, or API clients, the pattern is straightforward and flexible.

🎯 Pro tip: Use class mocking when you don’t care about internal logic—you just want to ensure the interaction is correct.