testing-anti-patterns

Mock usage mistakes occur when tests assert on mock behavior rather than real code. Detect these by reviewing test assertions—if you're verifying that a mock was called instead of validating actual application logic, your test is testing the mock, not production code. This creates false confidence and misses real bugs.

Test-only methods pollute production code and violate separation of concerns, creating accidental side effects and maintenance debt. Keep production and test logic completely separate. If you need to test internal behavior, redesign the code or test through public interfaces instead.

Common anti-patterns include testing mock interactions instead of real behavior, over-mocking dependencies without understanding them, and embedding test-specific code in production. These undermine test reliability and create false positives, letting real bugs slip through.

When reviewing tests, check if assertions verify mock internals or real outcomes, inspect production code for test-only methods, and ensure mocks serve actual isolation needs. Apply these checks to catch quality issues before merging.

Yes, this skill applies to both JavaScript and TypeScript projects. The anti-patterns—mock behavior testing, production pollution, and over-mocking—are language-agnostic and affect test reliability equally in both ecosystems.

Meaningful mocking isolates units to test specific behavior while understanding dependencies. Anti-patterns mock without purpose, test mock internals instead of real code, or add test-specific methods to production. Understanding your dependencies prevents anti-patterns.