legacy-code

To safely refactor legacy code, you should first write characterization tests to document the system's current behavior, then introduce seams to isolate dependencies, allowing you to modify the code without unintended side effects.

The Boy Scout Rule in code maintenance involves leaving the codebase cleaner than you found it by making small, incremental improvements continuously, which incrementally reduces technical debt without requiring large rewrite efforts.

To fix a bug in poorly tested legacy systems, apply characterization tests to capture the exact current outputs before making changes, ensuring your bug fix does not inadvertently break other undocumented behaviors.

The best way to add features to a legacy codebase is to use strangulation patterns and clean module extraction, which allows new functionality to be built separately and gradually replace the old untested components safely.

You should use a strangulation pattern for code maintenance when you need to incrementally replace large, critical legacy modules by routing new functionality to modern components while slowly deprecating the old system.

Characterization tests are tests written to document and lock in the actual current behavior of legacy code, and you need them before refactoring or modifying untested systems to prevent regressions.