What does code coverage tell you, and what doesn't it tell you?

Coverage tells you which code ran during the tests. It does not tell you whether the tests check the right things.

There are four common types:

• Line coverage counts executed lines.
• Statement coverage counts executed statements.
• Branch coverage checks whether each outcome of every if, switch case and logical operator (&&, ||) was taken.
• Function coverage counts called functions.

What it is good at: finding untested code. A file at 0%, or an error branch that never ran, is a clear gap.

What it can't tell you:

• Whether the tests check results. A test that calls every function and asserts nothing still gives high coverage.
• Missing code. For example, a validation you never wrote can't show up as uncovered.

So 100% coverage can still miss bugs. Use coverage to find blind spots. Prefer branch coverage to line coverage. Don't treat the number as the goal.