Mocks.md

The TL;DR of mocking is that sometimes we need to swap out things we don't control with things that we do. For example, it might be outside of the scope of our test to make sure that a third-party API goes down. Or, if that API isn't free, you don't necessarily want to run up a bill every time you run your test suite, right?

You can create a mock function using vi.fn() (or, jest.fn()), which takes a callback function. If you you don't provide one, it'll just use an empty function as the implementation (e.g. () => undefined).

const getNumber = vi.fn(() => 5000);

const number = getNumber();

expect(number).toBe(5000);
expect(getNumber).toHaveBeenCalled();
expect(number).toHaveReturnedWith(5000);

Methods

• mockImplementation: Takes a function that you want your mock function to call whenever it's called.
• mockImplementationOnce: Accepts a function that will only be used the next time a function is called.
• withImplementation: Overrides the original mock implementation temporarily while the callback is being executed. Calls the function immediately.
• mockReturnValue: Nevermind the implementation, we just know we want it to return whatever value.
• mockReturnValueOnce: Set the return value—but only the next time it's called.
• mockResolvedValue: Sets the value of the promise when it resolves.
• mockResolvedValueOnce: Set the resolved value of a promise next time it resolves.
• mockRejectedValue: Rejects a promise with the error provided.
• mockRejectedValueOnce: Rejects a promise with the error provided next time.
• mockReturnThis: Sets the value of this.