Mastering React Test-Driven Development

Using Jest to spy and stub

So far in this chapter you've built a usable set of spy and stub functions. Jest has built-in functionality that does the same thing but works slightly differently.

Here's a rundown of Jest test double support and how it compares to what you've built. We'll convert our CustomerForm tests to use these new functions in just a moment:

• To create a new spy, call jest.fn(). For example, you might write const fetchSpy = jest.fn().
• To override an existing property, call jest.spyOn(object, property).
• To set a return value, call spy.mockReturnValue(). You can also pass this value directly to the jest.fn() call.
• You can set multiple return values by chaining calls to spy.mockReturnValueOnce().
• To check that your spy was called, ...