Software Testing & TDD

Pyramid · Test Doubles · Coverage · Red-Green-Refactor

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How to build a reliable safety net for change using the test pyramid, test doubles, code coverage, and the red-green-refactor TDD cycle.

By the end, you can

Explain what tests can and cannot prove, citing Dijkstra's observation.
Describe the three tiers of the test pyramid and explain why the shape matters.
Write a unit test using the Arrange-Act-Assert pattern.
Name and distinguish all five test doubles, and explain when to use a stub versus a mock.
Explain why 100% line coverage does not guarantee correctness, and describe what mutation testing adds.
Carry out a TDD red-green-refactor loop for a new behavior.
Identify common causes of flaky tests and explain the appropriate response.

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