testing-patterns

Characterization tests document the actual behavior of existing code before refactoring. You write them by capturing current outputs as assertions, creating a safety net to verify behavior remains unchanged after modifying the source.

Seams are specific locations in code where you can alter behavior without editing the source file. They enable dependency injection and test doubles, allowing you to isolate modules and break hard dependencies for safer unit testing.

Yes, these testing patterns apply across both object-oriented and functional codebases. The strategies for adding tests, breaking dependencies, and characterizing existing behavior are designed to be universally applicable.

Yes, the dependency breaking strategies include concrete examples tailored for JavaScript and TypeScript projects. You can immediately apply patterns for seams, characterization tests, and test doubles in your JS/TS codebase.

Start by identifying a legacy module, then outline a plan to introduce seams. Write characterization tests to document current behavior, and replace hard dependencies with test doubles to enable safe changes.

Use characterization tests when you need to safely evolve legacy code with unknown or undocumented behavior. Unlike standard unit tests that define expected behavior, characterization tests capture actual current behavior to prevent regressions during refactoring.