Global methods

I start by going to the beforeAll() hook and adding a spy on the console.log method using the vi.spyOn() utility:

beforeAll(() => {
  vi.spyOn(console, 'log').mockImplementation(() => {})
})

I'm overriding the implementation of console.log for this test suite by using .mockImplementation() and providing it an empty function. With this, I can still assert on the console.log calls without it actually printing anything to the terminal output.

From this moment forward, all console.log calls are recorded and I can assert on them in any test!

Next, I need to reset the recorded state of the spy function for each test run. I will use vi.resetAllMocks() for that:

afterEach(() => {
  vi.resetAllMocks()
})

vi.resetAllMocks() targets all mocks created in this test suite, not just the spy for console.log. Instead of resetting an individual spy, I prefer resetting all mocks because none of them should leak their state to irrelevant tests.

And, finally, restore the original implementation of console.log once the tests are done:

afterAll(() => {
  vi.restoreAllMocks()
})

This concludes the testing setup changes but we still have some assertions to adjust.

Since I am spying on a global method, I can write assertions on that global method directly:

expect(console.log).toHaveBeenCalledWith(
  'Sever is listening at http://127.0.0.1/',
)
expect(console.log).toHaveBeenCalledOnce()

expect(console.log).toHaveBeenCalledWith(
  'Sever is listening at http://127.0.0.1:5639/',
)
expect(console.log).toHaveBeenCalledOnce()

The only thing remaining is to run the tests and make sure they are passing:

 ✓ print-server-url.test.ts (2)
   ✓ prints the server message for url with host and no port
   ✓ prints the server message for url with host and port

Bonus: Type-safe global spies

Although Vitest reassigns the value of console.log to a spy function on runtime, TypeScript doesn't know about that. In fact, if you attempt to read some of the spy function's properties, you'll be greeted with a type error:

console.log.mock.calls
// Property 'mock' does not exist on type '...'

To fix this, you need to extend the type of Console['log'] in your test and mark it as a mock instance.

import { MockInstance } from 'vitest'

declare namespace console {
  var log: Console['log'] &
    MockInstance<Parameters<Console['log']>, ReturnType<Console['log']>>
}

console.log.mock.calls
// [["Server is listening at http://localhost:5639/"]]

The way you extend the global type will depend on the declaration for that type. The type of console.log happened to be declared in a global namespace console so that's the namespace we extend above. Inspect the type declaration of the global you want to extend to ensure it's done correctly.

04. Globals/01. Global methods (🏁 solution)

Global methods

Bonus: Type-safe global spies