What Is Mutation Testing

Mutation testing can highlight gaps in your tests which test coverage tools cannot. At its heart is the premise that if your code's behaviour changes, then at least one test should fail.

Mutation testing involves artificially inserting bugs (or "mutants") into your code and seeing if your tests pick up on them. If a mutant is introduced but no tests fail, then the mutant is said to "survive", indicating a gap in your tests.

You can read more about mutation testing in this post.