essential-test-design

Unit tests break during refactoring when they assert internal implementation calls rather than observable behavior. This Skill guides authors to verify the caller's contract using boundary assertions, ensuring tests pass when behavior is preserved and fail only on real regressions.

The best way to test debounce and throttle logic is using fake timers to control time progression and asserting observable outcomes at boundaries. This Skill provides practical patterns to verify timing behavior deterministically without relying on real delays.

To rewrite brittle assertions, use this Skill's decision framework to define the contract, identify the regression to catch, and replace internal call spies with boundary assertions. Ask the assistant to review the failing test and rewrite it to verify the observable behavior boundary.

Test retry logic and external API side effects by mocking external boundaries and asserting observable outcomes rather than internal function calls. This Skill recommends using fakes at boundaries with explicit regression scenarios to ensure tests catch real failures without coupling to internals.

Use fake timers in unit tests whenever verifying async delays, debounce, throttle, or retry logic to prevent flaky tests. This Skill applies fake timers to control time progression deterministically, ensuring assertions reflect the caller's contract without relying on real-time execution.