Refactoring: Improve Code Without Changing What It Does

Refactoring is the practice of restructuring existing code so it is easier to read, change and test, without changing what the software does for its users. You do it in small steps, with a safety net, while the product keeps running.

I’m a Brazilian engineer who has spent seven years, since 2019, building and maintaining live web and mobile products, and the pattern I see most often is this: teams either never refactor until everything is painful, or they try to fix everything at once and break things. This guide covers the middle path.

What is refactoring?

The term was popularized by Martin Fowler’s book Refactoring. The core idea is simple: a refactoring is a small transformation that preserves observable behavior. Rename a variable. Extract a block of code into a function. Move a function to the module where it belongs. Replace a nested conditional with clearer logic.

Each of those leaves the product doing exactly what it did before. A customer clicking through the app cannot tell the difference. Your next developer can.

What refactoring is not

• Not a bug fix. Fixing a bug changes behavior, on purpose.
• Not a new feature. Same reason.
• Not a performance project. Optimizing can change timing and resource use, so treat it as its own task.
• Not a dependency upgrade or a framework migration. Those often need refactoring first, but they are separate changes.

How is refactoring different from rewriting?

Refactoring keeps the existing system and improves it in small, releasable steps. Rewriting builds a new system and switches over later. Both can fix a messy codebase, but their risk profiles are very different.

The hidden-behavior row is the part founders underestimate. Old code often contains years of quiet decisions: an edge case for one customer, a payment rule added after an incident. A rewrite has to rediscover all of it. Refactoring keeps it intact while you clean around it.

When the problem is bigger than code structure, such as an end-of-life stack, refactoring alone will not be enough. Then the safer route is usually an incremental migration, where you put a switch in front of the old system and move traffic piece by piece, rather than a big-bang rewrite. That is how I approached leading the migration of a live job-application platform from a legacy stack to a new one while people kept using it every day.

When is refactoring worth the time, and when is it not?

Refactoring is an investment, and like any investment it only pays off if you use the result. My rule of thumb: refactor code you will touch again soon.

It is usually worth it when

• You are about to add a feature in an area that is hard to understand. Clean it first, then build.
• The same part of the code produces bug after bug.
• Simple changes take far longer than they should because of tangled logic.
• New developers keep getting stuck in the same files.
• You want to add tests to code that is currently impossible to test.

It is usually not worth it when

• The code is ugly but stable and nobody needs to change it. Leave it alone.
• The feature is about to be removed or replaced.
• You have no way to verify behavior and no time to build that safety net. Refactoring blind is gambling.
• The goal is purely aesthetic, such as matching a style preference.

The question I ask is: what will be easier next month because of this change? If I can’t answer in one sentence, I skip it.

What makes refactoring safe: tests, small steps and reviews?

Safe refactoring rests on three habits. They work together, and dropping one makes the others weaker.

1. Tests that pin down current behavior

You need a way to know behavior did not change. If the code already has good automated tests, run them after every step. If it doesn’t, write characterization tests first: tests that record what the code does today, even if that behavior is odd. They are not a statement of what is correct. They are a tripwire.

Prioritize the money paths: checkout, billing, signup, anything where a silent change costs real money or trust. Having worked on the cart and checkout team at Total Wine & More, I always start there. If automated tests are not realistic yet, a short manual checklist for those flows is better than nothing.

2. Small steps, each one releasable

Make one change, run the checks, commit. Then the next. A good refactoring step is small enough that you can describe it in a single line, and if something breaks you can revert it without losing a day of work.

For larger changes, put the new path behind a feature flag or keep the old function alongside the new one until you trust it. That lets you switch back quickly instead of debugging under pressure.

3. Review that separates structure from behavior

Keep refactoring commits and pull requests separate from feature work. A reviewer can then ask one question: does this change only structure? Reviewing a mixed pull request is much harder, and bugs hide in the mix.

On my TypeScript projects I lean on the tools: editor refactorings (rename, extract, move), linters and a strict compiler are more reliable than editing by hand.

Which refactorings pay off first in a growing product?

You do not need exotic techniques. In the products I’ve worked on, a handful of basic moves delivered most of the value.

1. Rename things honestly. A function called process that actually validates and charges a card is a trap. Good names are the cheapest documentation.
2. Split huge functions and components. A single 500-line function is hard to test and hard to change. Extract named pieces with one job each.
3. Remove duplication where it is real. If the same business rule lives in three places, a change will eventually miss one. Do not merge code that only looks similar by coincidence.
4. Isolate external dependencies. Put payment providers, email services and third-party APIs behind a thin wrapper of your own. It makes testing easier and future swaps cheaper.
5. Delete dead code. Unused code costs reading time and hides real logic. Version control remembers it if you need it back.
6. Move business rules out of the UI and out of database queries. Rules in one clear place are easier to test and change.

Notice that none of these require a new framework or a big plan. They also build the foundation for bigger moves later. At Inovaula I built v2 of the web platform and then led the team, which reached the steadiest delivery flow it had had; in my experience, small and consistent habits like these make that kind of predictability easier to reach.

How do you fit refactoring into a delivery schedule?

The approach that works best is not a dedicated refactoring sprint. Those get cut the first time a deadline slips. Instead, make refactoring part of how features are built.

• Prepare, then change. Before adding a feature, spend a little time making the area easier to change. Then add the feature. Include both in the estimate.
• Leave it a bit cleaner. When you touch a file for any reason, make one small improvement nearby. It adds up without a project.
• Keep a short list. Write down painful areas as you meet them, with a note on why they hurt. When something becomes a blocker, you already have the evidence.
• Time-box bigger efforts. If an area needs real restructuring, agree on a limit up front, ship it in slices, and stop when the limit is reached.
• Explain it in business terms. Tie each effort to an outcome: fewer incidents in checkout, faster onboarding for new developers, or lower risk before a launch.

When a team I lead is under heavy deadline pressure, I still protect two things: tests around the money paths and small refactors inside feature work. Larger cleanups can wait for a calmer window.

If the codebase is inherited and you do not yet know what is risky, audit before refactoring anything. When I take over a codebase, I go in this order: access first, then the build, then the money paths, then the risks. Only then do I pick targets.

Want a second opinion on your codebase?

If you are deciding what to refactor in your own product, or whether it needs more than refactoring, email me@filipeeduardo.dev with a short description of the situation. I’m glad to take a look and tell you what I’d check first.