testing-anti-patterns

Testing mock behavior instead of actual system behavior creates false confidence. Tests that assert only on mock method calls don't validate whether your code actually works—they validate the test's own setup. Real bugs slip through because mocks don't replicate production behavior, dependencies, or integration points where failures occur.

Look for red flags: tests that break on refactoring, high mock-to-assertion ratios, test-only methods added to production code, and missing error-path coverage. These patterns indicate tests verify implementation details rather than behavior. Review whether tests assert on actual system outputs and whether mocks are necessary or masking real issues.

Unit tests isolate components to verify individual logic; integration tests validate how components interact in realistic conditions. Unit tests alone miss integration failures, data flow problems, and dependency issues. Combining both ensures tests catch bugs that occur only when components work together, not just in isolation.

Mock only external dependencies you don't control (third-party APIs, databases) after understanding their behavior. Never mock to avoid writing integration tests or to test behavior you don't understand. If you're mocking because the code is hard to test, refactor the design. Direct testing catches integration bugs that mocks hide.

Error-path testing validates that your code handles failures correctly. Write tests for each failure scenario: network timeouts, invalid inputs, missing resources, and dependency failures. Don't rely on happy-path tests; deliberately trigger errors and assert the code responds appropriately, including recovery logic and error messages.

Test-only methods expose internal implementation to tests, making tests brittle and tightly coupled to code structure. When you refactor, these tests break even if behavior hasn't changed. Tests should verify external behavior, not internal structure, ensuring they remain stable through refactoring and catch real regressions.