mutation-testing-js

This skill acts as a specialized assistant for performing mutation testing on JavaScript and TypeScript codebases. By utilizing mutation testing principles, it helps developers move beyond simple line coverage metrics to determine if their test suites actually detect real bugs. It is designed for software engineers, QA automation specialists, and developers focused on maintaining high-quality, reliable test suites for Node.js applications. This skill excels at identifying 'surviving mutants'—code changes that modify behavior without causing existing tests to fail—thereby highlighting gaps in test assertions, edge cases, and boundary conditions.

• Performs systematic branch analysis by examining git diffs to target changed code for manual or automated mutation review.

• Provides a mental model and catalog of mutation operators, including arithmetic, conditional, logical, equality, string, array, and nullish coalescing mutations.

• Offers concrete guidance on integrating mutation testing with the TDD workflow, ensuring that green tests are truly robust.

• Assists in configuring and utilizing Stryker, the industry-standard mutation testing framework for JavaScript and TypeScript.

• Distinguishes between effective tests that kill mutants and weak tests that allow bugs to persist.

• Recommends test strengthening strategies for various logic patterns, such as boundary value testing and specific condition validation.

• To use this skill effectively, initiate an analysis on a feature branch to review recent changes. Provide the code snippets or the target file path to receive suggestions on potential mutants that your current tests might miss. The skill requires an understanding of your testing framework, such as Jest, Mocha, or Vitest, to provide tailored assertion advice. It is best suited for complex business logic where simple coverage might provide a false sense of security. Note that mutation testing is computationally intensive, so it is recommended to focus analysis on critical modules rather than the entire project at once. The output helps prioritize which parts of your test suite require additional assertions or refactoring to ensure they are structure-insensitive and behavior-oriented.