mutation-testing-advisor

Purpose

Evaluate the true fault-detection capability of a test suite by simulating code mutations (inverting conditionals, altering arithmetic, removing function calls) and highlighting tests that pass despite broken logic.

When to Use

• A codebase has 90%+ line coverage but bugs still regularly slip into production.
• Reviewing unit tests that assert on execution without verifying state changes.
• Setting up Stryker or Mutmut mutation testing in critical packages.

What to Analyze

1. Mutant Categories:
   • Conditional Boundary Mutations (> swapped for >=).
   • Inverted Logic (if (isValid) swapped for if (!isValid)).
   • Return Value Mutations (returning null or undefined).
   • Statement Deletions (removing side-effect calls).
2. Surviving Mutants: Why didn't any test fail when this line was corrupted?
3. Weak Assertion Diagnosis: Identifying tests with no expect() or vague assertions (e.g. expect(res).toBeDefined()).
4. Targeted Assertion Improvements: Writing specific assertions that kill surviving mutants.

Output Format

• Mutation Score Assessment: High-risk areas of false test confidence.
• Surviving Mutant Table: [File | Mutation | Why Test Passed | Fix].
• Hardened Test Code: Rewritten unit tests with strict state and error assertions.

Avoid

• Chasing 100% line coverage while leaving assertions superficial.
• Testing mock implementation details instead of real behavior.