mutation-testing

Mutation testing measures test effectiveness by injecting small faults into production code and verifying whether the test suite detects them. Mutants that tests catch are killed, while undetected ones survive, exposing test gaps.

In mutation testing, killed mutants indicate your tests successfully detected the injected code fault, whereas survived mutants reveal test gaps. Equivalent mutants are syntactically changed but behaviorally identical, requiring manual review to exclude.

Mutation testing applies across Java, Rust, Python, JavaScript/TypeScript, .NET, and Scala ecosystems. It uses language-specific tools like Stryker for JavaScript, mutmut for Python, cargo-mutants for Rust, and PIT for Java.

Mutation testing supports CI-ready workflows and incremental analysis practices to keep feedback fast. Incremental analysis isolates and mutates only recently modified code rather than executing full suite scans across the entire project.

To start measuring test effectiveness, install a mutation testing tool appropriate for your project's language, such as PIT or Stryker. Run a mutation test to identify where your tests fail to detect injected faults and improve them.

Code coverage only measures if code is executed by tests, not if tests actually assert correct behavior. Mutation testing injects small faults into production code to evaluate if the test suite detects them, revealing untested logic branches.