6
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 ...
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