What qualities make a unit test worth keeping?

What makes a unit test valuable?

Focuses unit tests on stable behavior, meaningful boundaries, deterministic outcomes, and useful failure messages.

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TL;DR

A valuable unit test protects one observable behavior, runs quickly and deterministically, and fails with a clear reason. It uses representative boundary cases rather than only the happy path. The arrangement should make the business rule understandable, while assertions focus on outcomes instead of private fields or helper calls.

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Practice focus

Behavioral contracts, determinism, boundary cases, readability, failure signal, and refactor safety.

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Core answer
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Why this question matters
The useful distinction is not whether a test is short; it is whether the test gives clear, durable evidence about behavior the team cares about.
A valuable unit test protects one observable behavior, runs quickly and deterministically, and fails with a clear reason. It uses representative boundary cases rather than only the happy path. The arrangement should make the business rule understandable, while assertions focus on outcomes instead of private fields or helper calls.
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Stronger answer
Optional upgrade with examples, trade-offs, and sharper phrasing.
I prefer examples that separate important decisions: values just below and above a limit, missing input, duplicate input, and failure behavior. The test name should describe the scenario and result. Parameterized tests are useful when several inputs express one rule, but I split cases when they need different explanations. A test that survives an internal refactor while catching a changed rule is doing useful work.

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Common mistakesCandidateCommon mistakesMistakes that weaken the answer or signal shallow understanding.Open details

Details

Testing private methods directly.
Asserting every property regardless of relevance.
Using one giant test for several rules.
Depending on the current clock or random data.

What qualities make a unit test worth keeping?

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