Hooks

Loading "Handle Test Side Effects (🏁 solution)"

First, I add the beforeAll() and afterAll() function declarations to the global namespace in TypeScript:

interface Assertions {
	toBe(expected: unknown): void
}

declare global {
	var expect: (actual: unknown) => Assertions
	var test: (title: string, callback: () => void) => void
	var beforeAll: (callback: () => void) => void
	var afterAll: (callback: () => void) => void
}

Then, I implement the beforeAll() function, which invokes the given callback immediately.

globalThis.beforeAll = function (callback) {
	callback()
}

Here, I'm relying on the fact that the beforeAll() function will be called before any individual tests. Actual testing frameworks usually have a runner responsible for internally orchestrating hooks and tests regardless of the invocation order.

The afterAll function will be a bit different. To invoke the callback once the tests are done, I will utilize the beforeExit event of a Node.js process to let me know when the test run is about to exit.

globalThis.afterAll = function (callback) {
	process.on('beforeExit', () => {
		callback()
	})
}

Then, I go to the greet.test.ts and add the beforeAll() hook that patches the global Date constructor and uses the stored OriginalDate class to create a fixed date.

const OriginalDate = globalThis.Date

beforeAll(() => {
	globalThis.Date = new Proxy(globalThis.Date, {
		construct: () => new OriginalDate('2024-01-01'),
	})
})

I recommend providing an entire UTC date, including an explicit timezone, as the value of the mocked date to have a resilient test setup: new OriginalDate('2024-01-01 00:00:00.000Z')

Similarly, I make sure to clean up this Date mock in the afterAll() hook:

afterAll(() => {
	globalThis.Date = OriginalDate
})

Remember the rule of clean hooks: have a cleanup for each side effect in your setup (e.g. spawn a server → close the server; patch a global → restore the global).

02. Test Structure/04. Hooks (🏁 solution)

Hooks