Test Behavior, Not Private Plumbing
Choose assertions that verify public behavior instead of coupling tests to private implementation details.
The move: attach tests to stable promises, not temporary plumbing. Testing private methods feels precise, but it often creates a brittle suite. The private method is not the contract; the observable behavior is. If a caller cannot depend on the helper directly, the test should be cautious about depending on it directly. Otherwise the test suite starts defending the current design instead of the required behavior. The practical test is simple: if you could refactor the internals and keep the user-visible result identical, should this test fail? If the answer is no, assert at a higher boundary. If the behavior…
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