How do Mocks and Spies work in Jest (jest.fn, jest.spyOn)?

Explanation & Concept

• `jest.fn()`: Creates a mock function that tracks calls, arguments, and return values. • `jest.spyOn()`: Wraps an existing method on an object to track calls while optionally preserving or overriding its implementation.

Code Example

javascript

import * as userService from "./userService";

test("sends welcome email on registration", async () => {
  // Spy on email sending method and mock implementation
  const emailSpy = jest.spyOn(userService, "sendEmail").mockResolvedValue(true);

  await userService.registerUser({ email: "test@example.com" });

  // Assertions
  expect(emailSpy).toHaveBeenCalledTimes(1);
  expect(emailSpy).toHaveBeenCalledWith("test@example.com", expect.any(String));

  emailSpy.mockRestore(); // Restore original implementation
});

How It Works & Key Takeaways

Spies allow asserting that side-effect functions (like email senders or analytics trackers) were invoked correctly without sending real emails in unit tests.

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javascript

How do Mocks and Spies work in Jest (jest.fn, jest.spyOn)?

Explanation & Concept

Code Example

javascript

import * as userService from "./userService";

test("sends welcome email on registration", async () => {
  // Spy on email sending method and mock implementation
  const emailSpy = jest.spyOn(userService, "sendEmail").mockResolvedValue(true);

  await userService.registerUser({ email: "test@example.com" });

  // Assertions
  expect(emailSpy).toHaveBeenCalledTimes(1);
  expect(emailSpy).toHaveBeenCalledWith("test@example.com", expect.any(String));

  emailSpy.mockRestore(); // Restore original implementation
});

How It Works & Key Takeaways

Spies allow asserting that side-effect functions (like email senders or analytics trackers) were invoked correctly without sending real emails in unit tests.

Mock Test · 1 Questions · Optional

Ready for Snapshot Testing?

Take a quick test to evaluate yourself, or move straight to the next module.

Beginner3 questions

Skip