Amey-Thakur Legacy code changes
Legacy code is code without tests, so you cannot change it and know you did no harm. The instinct to rewrite is usually a trap: the safe path is to pin the current behavior first, then make the smallest possible cut. Michael Feathers named these moves, and they still hold.
Method
- Characterize before you change. Write tests that capture what the code does now, not what it should do. Feed it real inputs and assert the actual outputs, including the ugly ones. These tests document reality and turn a silent behavior change into a red bar.
- Find a seam to test at. A seam is a place you can alter behavior
without editing the code in question: a function argument, a subclass
override, an injected dependency. Break a hidden
new Clock()or global read into a parameter so the code becomes testable at all. - Sprout new code, do not edit in place. Write the new behavior in a fresh, fully tested method or class, then call it from one line in the old code. The tangle stays untouched while your addition is clean and covered.
- Wrap when new work must bracket the old path. If behavior must happen before or after existing logic, rename the old method and have a new method of the old name call the original plus your addition. The old body moves unchanged.
- Cut the smallest hole. Change the fewest lines that close the ticket. Every extra line touched in untested code is a bet with no test to collect on. Resist tidying the surrounding mess in this change.
- Bank the tests as coverage. Keep the characterization tests after the change. They are now the safety net the next person lacked, converting this file from legacy to merely old.
Signals
- If your change altered behavior by accident, would a characterization test go red?
- Did the new logic land in a tested unit rather than deep inside the untested block?
- Is the diff the minimum that closes the ticket?
Boundaries
When a module is small, dead, or slated for deletion, a full characterization suite can cost more than a careful rewrite: use judgment. This skill covers making one change safely, not deciding whether to retire the system. For staged replacement of a whole system, see strangler-fig.