AI-Assisted Refactoring - Use AI - ggcoder.com

Refactor Safely: Tests First, Refactor Second

The golden rule of refactoring is that observable behavior must stay identical. AI cannot guarantee that without tests. Before asking AI to refactor anything meaningful, ensure coverage exists — or ask the AI to write characterization tests first.

Safe Refactoring Workflow

1# Step 1 — write characterization tests before touching the code2"Write characterization tests for the UserService class.3Cover every public method. The goal is to document current4behavior, not ideal behavior. Use the existing test patterns5in src/__tests__/."67# Step 2 — refactor in one small step at a time8"Extract the email validation logic from UserService.register()9into a private validateEmail() method. Do not change any other10behavior. Tests must stay green."1112# Step 3 — verify13npm test -- --testPathPattern=UserService

Tip

Refactor in small, verifiable steps. Each step should be independently committable.

Good Refactoring Prompts

Effective refactoring prompts specify the goal (what you want changed) and the constraints (what must not change). Missing constraints are the most common reason AI refactors drift out of scope.

Effective Refactoring Prompts

1# Specify goal + constraints clearly2"Refactor the fetchUser function to use async/await instead of3Promise chains. Do not change the function signature, return type,4or error handling behavior. Existing tests must continue to pass."56# Extract a function7"Extract the discount calculation on lines 55-78 of CartService.ts8into a separate calculateDiscount(items, coupon) function in the9same file. The public API of CartService must not change."1011# Remove duplication12"The validation logic in createUser() and updateUser() is almost13identical. Extract it into a shared validateUserInput(data) helper.14Keep both callers working exactly as before."

Refactoring Tasks Where AI Excels

Renaming Across Files

AI can find and rename a symbol consistently across the entire codebase — including imports, re-exports, and usages in tests. Still review the diff after completion.

Rename Across Files

"Rename the UserDTO type to UserPayload across the entire codebase.
Update all imports, type annotations, and JSDoc references.
Do not rename the database column user_dto — only the TypeScript type."

Extracting Components and Functions

When a component or function grows too large, AI can identify natural boundaries and extract them into smaller, reusable units.

Simplifying Conditionals and Modernizing Syntax

AI is very good at converting nested if/else chains into early returns, replacing callback hell with async/await, or upgrading outdated patterns to idiomatic modern TypeScript.

Conditionals and Modernization

1# Simplify conditionals2"Refactor the nested if/else in processOrder() to use early returns3and guard clauses. The function logic must not change."45# Modernize syntax6"Convert all .then()/.catch() chains in src/api/ to async/await.7Preserve all existing error handling. Do not touch test files."

Removing Dead Code

Ask AI to identify unused exports, parameters, or branches — but verify with your bundler or type checker, as AI may miss dynamic usages.

Reviewing AI Refactors

After every refactor, the review process is non-negotiable. AI is fast but cannot replace human eyes for spotting subtle behavior changes.

Post-Refactor Review Checklist

1# Review checklist after an AI refactor21. git diff — read every changed line, not just the summary32. npm test (or bun test) — all tests must pass43. bun run build — no type errors54. Check edge cases: null inputs, empty arrays, error paths65. Check that error messages and log output are unchanged76. If the change touched async code, verify error propagation

Warning

Pay special attention to behavior drift — when AI restructures logic and accidentally changes the result for an edge case. There is often no error, just silent wrong behavior.

When NOT to Let AI Refactor Unsupervised

AI refactors most safely within small, well-tested scopes. There are situations where you should slow down or decline entirely.

Large blast radius: changes affecting dozens of files or a public API with many callers.
No tests: refactoring without tests is betting on AI confidence, not evidence.
Performance-critical code: AI may replace a hand-tuned loop with a more convenient but slower abstraction.
Complex concurrent or async logic: race conditions and ordering guarantees are easy to break during restructuring.
Security or cryptography code: any change needs review by someone with security expertise.

Summary

Always have tests — or AI-written characterization tests — before refactoring.
Specify both the goal and the constraints in your refactoring prompt: what changes and what must not.
Refactor in small, committable steps — not one large hard-to-review change.
After every refactor: read the diff, run tests and build, check edge cases and error paths.
Watch for behavior drift — silent wrong behavior that causes no error but changes outcomes.
Decline unsupervised refactoring when: large blast radius, no tests, performance-critical paths, or security/concurrent logic.

1# Step 1 — write characterization tests before touching the code2"Write characterization tests for the UserService class.3Cover every public method. The goal is to document current4behavior, not ideal behavior. Use the existing test patterns5in src/__tests__/."67# Step 2 — refactor in one small step at a time8"Extract the email validation logic from UserService.register()9into a private validateEmail() method. Do not change any other10behavior. Tests must stay green."1112# Step 3 — verify13npm test -- --testPathPattern=UserService

1# Specify goal + constraints clearly2"Refactor the fetchUser function to use async/await instead of3Promise chains. Do not change the function signature, return type,4or error handling behavior. Existing tests must continue to pass."56# Extract a function7"Extract the discount calculation on lines 55-78 of CartService.ts8into a separate calculateDiscount(items, coupon) function in the9same file. The public API of CartService must not change."1011# Remove duplication12"The validation logic in createUser() and updateUser() is almost13identical. Extract it into a shared validateUserInput(data) helper.14Keep both callers working exactly as before."

"Rename the UserDTO type to UserPayload across the entire codebase. Update all imports, type annotations, and JSDoc references. Do not rename the database column user_dto — only the TypeScript type."

1# Simplify conditionals2"Refactor the nested if/else in processOrder() to use early returns3and guard clauses. The function logic must not change."45# Modernize syntax6"Convert all .then()/.catch() chains in src/api/ to async/await.7Preserve all existing error handling. Do not touch test files."

1# Review checklist after an AI refactor21. git diff — read every changed line, not just the summary32. npm test (or bun test) — all tests must pass43. bun run build — no type errors54. Check edge cases: null inputs, empty arrays, error paths65. Check that error messages and log output are unchanged76. If the change touched async code, verify error propagation