Is 100% coverage enough?

Definition:

Coverage measures which lines, branches or functions executed while tests ran. It can reveal untested areas, but execution does not prove that the assertions checked correct behavior. A suite can reach every line and still miss invalid inputs, race conditions or a wrong expected value.

Key points:

• No. Executed lines can still have weak assertions or missing edge cases.

• Check important success, failure, permission and concurrency cases.

• Use real database integration tests for queries, constraints and transactions.

• Focus assertions on business invariants, such as stock never becoming negative or one user never reading another tenant's data.

Interview point:

• Use coverage as a signal to inspect gaps, not a target that substitutes for meaningful test design.