Your project has 100% line coverage. What can you conclude?

Options

1. There are no bugs left anywhere in the code
2. Every line ran, but edge cases may be unchecked (Correct option)
3. Mutation testing is no longer needed at all
4. The tests run fast on every machine and in CI

Answer

Every line ran during tests, but not that the behavior was checked or that all branches and edge cases were covered

Why

Coverage measures which lines were EXECUTED during tests, not whether the results were checked. A test with no assertions still produces coverage.

It also misses combinations: a line can run while one important branch or input never gets tested.

Coverage is good for finding untested code, not for proving correctness.

Remember

Coverage shows what ran, not what was verified.

Key terms

• Test coverage
• Edge case
• Mutation testing
• Unit test
• Regression
• Assertion