How To Approach Legacy Code Without Fear

Many developers avoid legacy projects — but in reality, most software we touch isn’t greenfield. It’s inherited, undocumented, and often misunderstood. But legacy code isn’t automatically bad. It’s code that has proven value — because it’s still in use. My approach isn’t to rewrite everything from scratch. It’s to work with what’s there and improve it with care.

Understand Before You Touch

I never refactor blindly. My first goal: understand what the code does and why it was written that way.

  • I explore the Git history
  • I use Git blame to trace changes
  • I talk to domain experts or former developers
  • I identify assumptions, constraints, and side effects
  • I analyze file structure and autoloading strategy to understand coupling and boundaries

Understanding reduces risk — and increases respect for past decisions.

Lock in the Current Behavior

Before I change anything, I write tests that capture the current behavior — even if it’s messy. These don’t have to be pretty. They’re safety nets. Snapshot tests, high-level integration tests, or simple assertions that make regressions obvious.

In legacy applications with deeply coupled globals, I often start by testing the output of entry points under controlled inputs. In addition to backend-focused testing, I rely on end-to-end (E2E) testing tools like Selenium to validate the application from a user’s perspective. This is especially useful when no unit tests exist, but the UI reflects critical workflows. These tests serve as high-leverage indicators: if the login page, checkout flow or admin dashboard still behave correctly, the system is probably in a good state.

End-to-end tests are often the fastest way to build confidence in risky changes — especially in codebases where no one fully understands all the side effects.

Improve in Small Steps

Refactoring doesn’t mean big rewrites. I follow the boy scout rule:

"Leave the code better than you found it."

Just as important as the technical improvements is the direction you're moving toward. Every small change should contribute to a shared vision of where the codebase is going. Without this, developers risk pulling the code in conflicting directions — improving things locally but creating chaos globally.

Make sure everyone involved understands the architectural direction, even if it's minimal: for example, "We extract business logic from controllers" or "We move toward autoloaded classes." With a common understanding, even small steps build momentum toward something coherent.

Isolate and Extract Risky Code

When I work in a legacy file, I often take the opportunity to clean up and extract:

  • Rename unclear variables
  • Extract logic into functions or methods
  • Add type hints or docblocks
  • Remove duplication when it's safe to do so
  • Separate responsibilities into smaller components

Some parts are especially hard to test or reason about — things like I/O, session handling, and external API calls. I isolate these first into standalone functions or classes with clear inputs and outputs:

  • Move email logic into a MailService
  • Wrap database queries in repository or query classes
  • Isolate 3rd-party API calls from business logic
  • Pull global state interactions into dedicated abstractions

Doing this improves clarity, supports testing, and lays the groundwork for further refactoring. The key is to reduce entanglement and increase predictability — one function at a time.

Communicate Clearly

Refactoring rarely has immediate user-visible value. So I document every change clearly:

  • Why I made the change
  • What I tested
  • What the risk was
  • What conventions or boundaries I introduced (e.g. design pattern, modules, naming)

Clear commit messages and PR descriptions build trust.

Tools I Rely On

Here are some tools I regularly use when working with PHP-based legacy systems:

  • PHPStan / Psalm — for static analysis and type safety improvements
  • Rector — for automated refactoring and upgrades
  • PHP-CS-Fixer / Pint — to enforce consistent code style
  • Composer — to introduce autoloading and manage dependencies
  • PHPUnit / Codeception — for writing regression, integration and end-to-end tests

While many of these tools are PHP-specific, the principles behind them apply in any tech stack:

  • Use version control history to understand past decisions and trace bugs
  • Apply static analysis to uncover structural or type-related issues early
  • Employ test automation tools to guide refactoring confidence
  • Use code formatters and linters to enforce consistency
  • Identify seams or entry points where structure can be gradually introduced

Whether you're working in PHP, JavaScript, Python, Ruby, or Java — visibility, discipline, and small, safe changes are the foundation of successful modernization.

Closing Thoughts

Legacy code doesn’t need to be scary. But it does demand a different mindset than starting from scratch. You’re not designing a new system — you’re intervening in a living, functioning organism, often while it’s under load, in production, and depended onby real users.

That’s why fear-based avoidance doesn’t help. The antidote is:

  • Respect — for the constraints and decisions that came before you
  • Curiosity — to understand before you change
  • Discipline — to improve gradually, not just reactively
  • Communication — so your intent is clear, not hidden in commits

Working with legacy systems is not a punishment — it’s a chance to prove real engineering skill. It’s about finding structure in the chaos, making progress without full certainty, and creating order without rewriting the world.

The key is not to fix everything at once. The key is to find one testable seam, one tangled function, one false assumption — and move it one step closer to clarity. If you do that consistently, legacy code stops being something you survive — and becomes something you shape. And in doing so, you unlock real, tangible value for clients and projects — exactly where it’s most needed.