Legacy Code Gets Worse With Age

Legacy code does not get better over time. It gets worse.

Last week, I spent three hours fixing a bug. It should have taken 20 minutes. The issue was a validation module from 2019. Everyone ignored it because "it works." It did not work. I found a TODO comment from 2020 inside the file.

Many people treat legacy code like optional debt. They think they can pay it back when they have time. Legacy code is more like mold. It spreads. It contaminates other parts of your system. The longer you ignore it, the more expensive it becomes to clean.

This creates a bad cycle:

  • You inherit a messy project.
  • You add one more if statement to make your feature work.
  • Six months later, someone else does the same.
  • One year later, the file has 800 lines and zero tests.

This "working" code has hidden costs:

  • Development speed drops. You spend more time reading context than writing code.
  • Bugs increase. Fixing one thing breaks something else.
  • Onboarding becomes hard. New developers struggle to understand why logic is duplicated everywhere.

Watch for these red flags:

  • Useless or lying comments.
  • Duplicated business logic in different files.
  • Circular dependencies and high coupling.

Do not try to rewrite everything. Full rewrites fail 80% of the time. You spend months rebuilding what already exists while the business waits for new features.

Use incremental refactoring with characterization tests:

  • Capture the current behavior with tests, even if it is weird.
  • Refactor without changing that behavior.
  • Repeat until the code is readable.
  • Only then change the behavior with real tests.

Follow these rules to avoid traps:

  • Never refactor without tests.
  • Do not change behavior during a refactor. A bug might be an undocumented feature a client relies on.
  • Leave quiet code alone. If a module has not changed in three years and causes no issues, leave it.

Focus your energy on the code you touch frequently.

Source: https://dev.to/taina_costa_f/legacy-code-nao-envelhece-como-vinho-quanto-mais-espera-pior-fica-132h