Spies (mocks)
๐จโ๐ผ We're calling
console.error in our source code, and we don't want to change
that. We just want to make that do something different in our tests and check
that it was called properly.To do this, we're going to use a feature from vitest called a "spy". A spy is
a function that records how it was called. And we can also change its
implementation. We can create a spy using
the
vi.spyOn utility. Here's a
slightly modified example from their docs:let apples = 0
const cart = {
getApples: () => 13,
}
const spy = vi.spyOn(cart, 'getApples')
spy.mockImplementation(() => apples)
apples = 1
expect(cart.getApples()).toBe(1)
expect(spy).toHaveBeenCalledTimes(1)
// then you can clean things up like so:
spy.mockRestore()
expect(cart.getApples()).toBe(13)
So we're spying on
cart.getApples, and we're changing its implementation to
return the value of apples. Then we change the value of apples and call
cart.getApples. Without the spy, it would return 13, but with the spy, it
returns 1.Then we check that the spy was called once. It's typically important to assert
on the number of times you expect your function to be called, because if it's
called more or less than that, it's probably a bug.
So for this part of the exercise, could you add a spy to the test so we don't
get the error in the console? Make sure to clean up!
You can test it out by adding a
console.error call to the test before making
the assertion or removing it from the source. It's always a good idea to
Make Your Test Fail!npx vitest
