Test coverage and what it can tell us

Short summary

Test coverage metrics can mislead you: 100% statement coverage doesn't guarantee well-tested code, as a single test case can achieve it while missing untested branches. The post demonstrates this with a discount function and explains three coverage types—statement (lines executed), branch (conditional paths), and path coverage (all possible routes)—showing why arbitrary percentage thresholds often miss bugs. Understanding coverage type matters more than hitting a coverage percentage.

• •100% statement coverage can hide untested branches and logic paths
• •Different coverage types measure different things: statement coverage (lines), branch coverage (conditionals), path coverage (all routes)
• •Arbitrary coverage thresholds without understanding the metric type create false confidence in code quality

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