"Ah, the familiar smell of legacy code waiting to be tackled. Do you like it as well? Wait, what? You don't like handling legacy code? But but it's the kind of code developers work with 80% of the time! It's not surprising at all that developers think badly of legacy code. After all, it has the tendency to be less readable, more prone to break and harder to maintain."
"Martin Fowler defines legacy code as code that's usually quite complex, often lacks good tests and is sometimes simply code written by someone else. That's quite an obscure definition. Michael Feathers, in his book Working Effectively with Legacy Code, offers a closely related but more specific definition: Legacy code is code without tests. No matter which side you're on Fowler or Feathers they both share two things: a surname starting with F, and the same core complaint about legacy code: not enough tests."
Lack of tests is identified as the core cause of legacy code by prominent practitioners. Tests provide safety and maintainability, enabling confident change and preventing breakage. AI agents enable faster identification of defects, automated test generation, and refactoring assistance, making it easier to turn legacy code into evergreen code. Code age alone does not determine legacy status; ease of running and sufficiency of tests determine maintainability. A concrete JavaScript telemetry-system example shows implementation files last updated over a decade ago, warranting inspection of tests and workflows. Adding tests and leveraging automation can restore readability, reduce fragility, and speed future development.
Read at yonatankra.com
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