Exploring Test Doubles
In this chapter, we’ll look at the most involved piece of the TDD puzzle: test doubles.
Jest has a set of convenience functions for test doubles, such as jest.spyOn and jest.fn. Unfortunately, using test doubles well is a bit of a dark art. If you don’t know what you’re doing, you can end up with complicated, brittle tests. Maybe this is why Jest doesn’t promote them as a first-class feature of its framework.
Don’t be turned off: test doubles are a highly effective and versatile tool. The trick is to restrict your usage to a small set of well-defined patterns, which you’ll learn about in the next few chapters.
In this chapter, we will build our own set of hand-crafted test double functions. They work pretty much just how Jest functions do, but with a simpler (and more clunky) interface. The aim is to take the magic out of these functions, showing you how they are built and how they can be used to simplify your...
