Explain the test doubles dummy, stub, spy, mock and fake, with a backend example of each.

A test double is anything that takes the place of a real dependency in a test. The name comes from Gerard Meszaros. There are five kinds:

• A dummy only fills a parameter and is never used. Example: a logger that this code path never calls.
• A stub returns fixed, prepared answers. Example: a rates.get() that always resolves 1.1.
• A spy is a stub that also records its calls. Afterwards you can check that mailer.send got the right address.
• A mock is set up with expectations about the calls it should receive. It fails the test if those calls don't happen.
• A fake is a working but simpler implementation. Example: an in-memory UsersRepository that stores data in a Map.

In Jest, jest.fn() can act as a stub, a spy or a mock. It depends on how you use it.

Stubs and fakes help you check state (the result). Spies and mocks help you check interactions (which calls happened).