Can you recognise which one is the real Jack Sparrow and which is a... double? 🤫

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- "Bartosz! You were supposed to teach me about coding! 😳"

- "Reader, this IS about coding... Continue reading."

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Ok. So what does this have to do with testing?

A lot

When you want to write a unit test, you want to test a single unit of behavior of your application in an isolated manner. The key here is isolation, because you want your unit test to be fast and independent from other tests.

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The graphic below presents a unit test of a service class that collaborates with another class to produce some result, but it's isolated from the repository class... with a TEST DOUBLE.

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When running unit tests, you don't want to establish connections to any external dependencies, especially a database (there are other tests, like integration or e2e tests, for such cases).

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Dependency on a database would ruin the performance of your unit tests and entangle the results with some external state.

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We can divide test doubles into two high-level groups

• stubs - to simulate inputs for the tested unit
• mocks - to simulate and examine outputs

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Ok... That's all for today. Pills are supposed to be short. 😁 But... remember that unit testing is a really broad topic with a lot of surprising and complex parts. 🙄

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All the best,
Bartosz