Unit Testing Best Practices
Central idea
Valuable tests protect important behavior while remaining stable when implementation details change.
Summary
A test suite should be judged by bug protection, resistance to refactoring, speed of feedback, and maintenance cost. The crucial distinction is observable behavior versus implementation detail. Tests coupled to private methods, internal call sequences, or class boundaries create false alarms during safe refactoring. Test domain behavior heavily and orchestration more selectively. A unit is a coherent behavior, not necessarily a single class. Use mocks mainly at external contracts where an interaction is itself part of the observable outcome.
Key ideas
Test outcomes, not wiring
Assert what users or neighboring systems can observe rather than how internal objects collaborated.
Refactoring resilience matters
A test that fails after a behavior-preserving change consumes attention without protecting value.
Mocks have a narrow role
Mock stable boundaries you truly own or must observe, not every dependency in the object graph.
Four dimensions
Protection from meaningful regressions.
Resistance to refactoring.
Fast feedback.
Low maintenance cost.
Why it matters now
AI can generate many tests, but quantity is not confidence. The design of the test oracle and its coupling still requires engineering judgment.
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