Spies

Let's start by creating a spy for the logger.log() method using the vi.spyOn() function from Vitest:

const logger = new Logger()
vi.spyOn(logger, 'log')
const service = new UserService(logger)

The spyOn() function has a bit of unusual call signature since it will not accept the logger.log method reference directly. Instead, it expects the object that owns the method first, and then the method name as a string:

vi.spyOn(target, method)

🦉 The reason behind this call signature is how .spyOn() works under the hood. It redefines the method on the target object with a spied version of that method! You can think of it as target[method] = methodSpy.

Now that I have the spy ready and recording, I can write an assertion on that method being called with the correct arguments using the .toHaveBeenCalledWith() assertion:

expect(logger.log).toHaveBeenCalledWith('createUser', { id: 'abc-123' })

Finally, I will make sure that logger.log() has been called exactly once:

expect(logger.log).toHaveBeenCalledOnce()

To validate the test, I will run npm test:

 ✓ user-service.test.ts (1)
   ✓ logs out the "createUser" event when creating a new user

02. Functions/02. Spies (🏁 solution)

Spies