Testing Patterns

For verifying test effectiveness through mutation analysis, load the mutation-testing skill. Use its mutator rules while planning and writing tests, not only after tests exist. For evaluating test quality against Dave Farley's properties, load the test-design-reviewer skill.

Core Principle

Test behavior, not implementation. 100% coverage through business behavior, not implementation details.

Example: Validation code in payment-validator.ts gets 100% coverage by testing processPayment() behavior, NOT by directly testing validator functions.

Mutation-Aware Test Planning

When planning or writing tests, automatically scan the intended behavior and changed production code against the mutator rules from the mutation-testing skill's resources/mutator-rules.md resource. A good test should fail if a realistic mutant changes the behavior.

Load that resource when the code under test includes conditionals, arithmetic, equality, boolean logic, array/string operations, optional chaining, or meaningful side effects. Use it to identify likely surviving mutants before the Stryker run.

When the scan finds an obvious gap, add or strengthen a behavior test immediately. When the gap depends on product or domain judgment, use the harness's ask-question facility before choosing a test. Ask one concise question with concrete choices, explain the potential mutant, and state the tradeoff.