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What does code coverage tell you and what does it not?

Tags

#coverage#quality#metrics

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Answer

Coverage shows which lines/branches were executed, but not whether tests assert correct behavior. High coverage doesn’t guarantee quality; low coverage doesn’t mean tests are useless.

Advanced answer

Deep dive

Coverage is a signal, not a goal:

Line/branch coverage shows execution paths.
Mutation testing or assertions quality reflect test strength.
Use coverage to find blind spots, not to hit 100%.

Examples

Good use of coverage:

Coverage report shows no tests for error handling -> add targeted tests

Common pitfalls

Chasing 100% coverage with trivial tests.
Ignoring branch coverage and edge cases.
Treating coverage as a release gate without context.

Interview follow-ups

How would you improve test quality beyond coverage?
When is 60% coverage acceptable?
What is mutation testing and why use it?

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