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HTTP Mocking

Remember our guiding principle for testing?
The more your tests resemble the way your software is used, the more confidence they can give you. - @kentcdodds
Making fake versions of things is the opposite of that. But as we've discussed a few times already, sometimes you just can't avoid this. Often you have to fake things out. If you haven't already read The Merits of Mocking, then make sure you get that on your reading list.
In the case of third party integrations you typically do want to mock these. For a few reasons:
  1. If their system goes down, your tests will be completely broken.
  2. During development, you're reliant on their progress if they're actively working on their end of the integration.
  3. You can't work offline if you're reliant on their system.
That said you should have some sort of process for verifying the integration works. Some folks say you shouldn't include other people's code in your tests because they're outside of your control. I disagree. The point of tests is to tell you whether things broke, not whether only your stuff broke. The user doesn't care whose stuff broke after all.
However, the problem with including third party services in your tests is that those kinds of tests are often extremely flaky which leads to developers not trusting the tests and pushing forward anyway.
It's much better to have an established process (or production monitoring) to identify when the integration is broken. That way you can safely mock for local development and testing while still being confident it will work in production.

One-off mocks

We've already gone through HTTP mocks a few times in these workshops. But now we're going to add another element to this and that is one-off HTTP mocks. With MSW, you have your established mocks which should resemble the API as closely as possible. But often you want to test what happens when things are in a bad state. For example, what happens when the API returns a 500 error? Or maybe you want to test a specific return type.
MSW has support for this using the server.use API. Here's an example modified from the MSW docks:
import { rest, HttpResponse } from 'msw'
import { setupServer } from 'msw/node'

const { json } = HttpResponse

const server = setupServer(
	http.get('https://example.com/book/:bookId', () => {
		return json({ title: 'Lord of the Rings' })
	}),
)

beforeAll(() => {
	server.listen()
})

afterEach(() => {
	// Resets all runtime request handlers added via `server.use`
	server.resetHandlers()
})

afterAll(() => {
	server.close()
})

test('handles 500 errors from the API', () => {
	server.use(
		// Adds a runtime request handler for posting a new book review.
		http.get('https://example.com/book/:bookId', () => {
			return json({ error: 'Something went wrong' }, { status: 500 })
		}),
	)

	// Performing a "GET /book/:bookId" request
	// will get the mocked response from our runtime request handler.
})

test('asserts another scenario', () => {
	// Since we call `server.resetHandlers()` after each test,
	// this test suite will use the default GET /book/:bookId response.
})
This allows specific tests to override the existing handlers so we can test specific scenarios.
So your default handlers can just have the happy path and then you can add specific handlers for specific tests. The server.resetHandlers bit in the afterEach is what's responsible for this. It resets the handlers to the default handlers (our "happy path").