Refactoring With AI Only When You Have Tests

Refactoring means changing how code is written without changing what it does. AI is great at it: condensing loops, extracting helpers, renaming variables. But the only way to *prove* the behavior didn't change is to run tests after.

Some examples

• You ask Claude to refactor a 40-line function into smaller pieces — then run your test suite to confirm green.
• You let Cursor rewrite a class to use composition over inheritance, with tests as the safety net.
• You skip a refactor on untested legacy code because there's no way to verify nothing broke.
• You ask ChatGPT to rewrite a function and write tests *first* before accepting the refactor.

Try it!

Find a small function in your project. Write 3 tests. Then ask AI to refactor. Confirm tests still pass.

A find-and-replace breaks code when names collide. Asking AI to rename a symbol — not a string — keeps the right things and skips the wrong ones.

Some examples

• You ask Cursor 'rename the local variable user to currentUser in this file only' and it skips the unrelated user.id from the API.
• Claude renames a function getData to fetchInvoices and updates every caller, but leaves the string 'getData' inside a comment alone.
• You ask Copilot Chat to rename a prop and it updates the destructure, the JSX, and the type — three places in one move.
• Codex catches that 'item' appears in two scopes and asks which one you meant before renaming.

Try it!

Pick a vague name like data or item in your code, ask Cursor to rename just that symbol in one file, then diff to see what changed.

AI can break up giant functions into smaller named ones — but only if you tell it what the code is supposed to do.

Some examples

• Paste the function and say 'split this into smaller functions named after what they do.'
• Ask for the same behavior, not new features.
• Run your tests (or click around) after — refactors break things silently.

Try it!

Find the ugliest function in something you wrote. Ask AI to refactor it without changing behavior. Diff it.