Mutation Testing: Covering Your Code With the Right Test Cases (Part 1)

Mutation Testing Basics

The concept of mutation testing is to modify code in a small way and verify that tests detect that modification. Undetected modification gives a hint to what test is likely missing.

Mutation Procedure Next Steps:

1. Modify code in small way (creating mutant)
2. Execute existing test suites (killing mutant)
3. Verify that at least one existent test failed (mutant killed)
4. If all test succeded (mutant survived), there is likely a missing test case

But why do we have to consider testing the result of the modified code? Let's take a look at an example: