Fakes, Mocks, and Stubs in Unit Testing

In the blog post Test Doubles — Fakes, Mocks and Stubs., Michał Lipski describes and implements three different types of test doubles: fake, stub, and mock, and the situations in which you would need to use them in your unit testing. He also provides useful examples for each of the three terms.

Here’s a useful image found in his blog that helps to understand the differences between the three:

Fake – object that has a working implementation, but not the same implementation as the ones in production. Usually they take some shortcuts are are simplified versions of the production code. This is useful because you’re able to do integration test of services without starting up a database or performing time consuming requests.

Stub – object that holds predefined data and uses it to answer calls during tests. This is useful when we can’t or don’t want to involve objects that would answer with real data or have undesirable side effects.

Mock – object that registers calls as they are received. This is useful when we don’t want to invoke production code or when there is no easy way to verify that the intended code was executed.

The reason I chose this blog post in particular is because the first time I was exposed to stub functions was earlier in my internship while working on unit tests, and at the time I didn’t have a great understanding of what their purpose was. I was recently reminded of them last week during lecture, so I decided that it’d be a useful topic to read about. At the time I wasn’t aware that there were different types of test doubles, so I found that the examples found in Lipski’s blog are really helpful in understanding the key differences between them, as they share many similarities and are likely to get mixed up. According to Lipski, misunderstanding and mixing test doubles implementations may influence test design and increase fragility of tests, standing in our way to seamless refactoring. So I think that this blog may be a useful resource if you were trying to write clean unit tests.

Source: Test Doubles — Fakes, Mocks and Stubs.

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