Mock functions

In the emitter.test.ts, I will start from creating the listener function but it won't be a regular JavaScript function. Instead, I will use the vi.fn() API from Vitest, which creates a special kind of function.

const listener = vi.fn()

📜 Learn more about vi.fn() from the Vitest documentation.

Calling vi.fn() returns a function imbued with superpowers, one of which is the ability to know when and how it is being called!

Note that for this test, I will create an empty mock function because what the listener itself does is irrelevant to the point of the test (asserting that it has been called).

Now that the mock function is ready, I will use it as a listener argument for the hello event:

emitter.on('hello', listener)

Everything up to this point was the setup for this test. The action here would be calling the .emit() method to emit the hello event because the listeners are only called when the respective event gets emitted.

emitter.emit('hello', 'John')

I expect two things to happen once the hello event is emitted:

The listener function gets called, and gets called once;
The listener function gets called with the right data—the 'John' string.

The expect() function from Vitest comes with handy assertions to describe both of those expectations!

First, I will use the .toHaveBeenCalledOnce() assertion:

expect(listener).toHaveBeenCalledOnce()

This will only pass if the listener function has been called exactly once. If it gets called any other number of times, it's a bug, and the test will fail.

In the same fashion, I will apply the .toHaveBeenCalledWith() assertion to check that the listener function gets called with the right data:

expect(listener).toHaveBeenCalledWith('John')

02. Functions/01. Mock functions (🏁 solution)

Mock functions