✔️ Legacy code

Michael Feathers defines legacy code as code without tests. He further elaborates in his book:

“Code without tests is bad code. It doesn’t matter how well written it is; it doesn’t matter how pretty or object-oriented or how well-encapsulated it is. With tests, we can change the behavior of our code quickly and verifiably. Without them, we really don’t know if our code is getting better or worse.”

Without automated tests, any modification to the codebase is risky, as there is no reliable way to know if a change introduces defects. This lack of feedback forces us to rely on manual testing or institutional knowledge -- often lost over time -- which makes maintenance difficult and error-prone. It is not about the age of the code or who wrote it, but about whether the code can be changed safely. If you cannot verify behavior through tests, the code is "legacy" regardless of its age or origin.