Refactor

Execute safe refactoring operations with verification at each step. Refactoring changes code structure without altering external behavior.

Core Principle

Preserve behavior. Every refactoring must pass tests before and after. If behavior changes, it's not refactoring—it's a rewrite.

Refactoring Operations

Rename

Rename variables, functions, classes, files, or directories while maintaining all references.

When to use:

• Identifier name is unclear or violates naming conventions
• Code review feedback on naming
• Aligning names with business domain

Steps:

1. Find all occurrences (definitions + usages)
2. Verify the scope (global vs local)
3. Rename atomically (all at once)
4. Run tests to confirm behavior unchanged

bash
# Find all usages before renaming
grep -r "oldName" --include="*.ts" --include="*.js" --include="*.py"

Extract

Pull out a code block into a new function, method, class, or module.

When to use:

• Function is too long (>30 lines)
• Code block is duplicated elsewhere
• Logic can be reused in other contexts
• Improving readability by naming complex operations

Steps:

1. Identify the code block to extract
2. Determine inputs (parameters) and outputs (return value)
3. Create the new function/module
4. Replace original code with a call to the new function
5. Verify tests still pass

Extraction targets:

• Function/method extraction
• Class extraction
• Module/file extraction
• Interface/type extraction

Inline

Replace a function call with the function body.

When to use:

• Function is trivial (one-liner)
• Function is called only once
• Indirection adds unnecessary complexity
• Making code easier to follow

Steps:

1. Verify the function has no side effects
2. Copy function body to call site
3. Replace arguments with actual values
4. Remove the now-unused function
5. Run tests

Move

Relocate code to a more appropriate location in the codebase.

When to use:

• Code belongs in a different module/package
• Organizing by feature instead of type
• Reducing coupling between modules
• Aligning with project architecture

Steps:

1. Identify the target location
2. Update all import paths
3. Ensure no circular dependencies
4. Move the file/code
5. Update tests and documentation

Safety Checklist

Before any refactoring, verify:

bash
# 1. Ensure clean git state
git status
git stash  # or commit work-in-progress

# 2. Run existing tests to establish baseline
# Pick the appropriate test command for your project
npm test 2>/dev/null || go test ./... 2>/dev/null || pytest 2>/dev/null || cargo test 2>/dev/null

# 3. Create a checkpoint
git checkout -b refactor/<description>

Pre-refactoring checklist:

• ] All tests pass on current code
• ] Git working directory is clean (or changes stashed)
• ] You understand the code's current behavior
• ] You've identified all usages of code being refactored
• ] You have a rollback plan (git)

Step-by-Step Workflow

Phase 1: Preparation

1. Understand current behavior

• Read the code thoroughly
• Run existing tests
• Note edge cases and dependencies

1. Plan the refactoring

• Choose the operation type (rename/extract/inline/move)
• Identify all affected files
• Estimate risk level

1. Set up safety net

bash # Ensure tests exist # If no tests, consider adding characterization tests first

# Create refactoring branch git checkout -b refactor/<description>

Phase 2: Execution

1. Make the change incrementally

• One logical change per commit
• Small, reviewable steps
• Never mix refactoring with feature changes

1. Verify after each step

bash # Run tests npm test 2>/dev/null || go test ./...

# Verify build succeeds npm run build 2>/dev/null || go build ./...

1. Commit frequently

bash git add -A git commit -m "ref: <what changed>"

Phase 3: Verification

1. Run full test suite

bash # Comprehensive verification npm test 2>/dev/null && npm run lint 2>/dev/null

1. Manual smoke test (if applicable)

• Start the application
• Verify critical paths work
• Check edge cases

1. Clean up

bash # Remove any dead code # Update documentation if needed # Merge or create PR

Common Patterns

Pattern: Extract Function

typescript
// Before
function processOrder(order: Order) {
  // 50 lines of validation
  // 30 lines of calculation
  // 20 lines of persistence
}

// After
function processOrder(order: Order) {
  const validated = validateOrder(order);
  const calculated = calculateTotals(validated);
  return persistOrder(calculated);
}

Pattern: Move to Module

bash
# Before: utils.js has 500 lines
# After: Split into organized modules

mkdir -p src/utils/validation
mkdir -p src/utils/formatting

# Move validation functions
mv src/utils/validateUser.js src/utils/validation/
mv src/utils/validateOrder.js src/utils/validation/

# Update imports
find . -name "*.ts" -exec sed -i 's|from.*utils/validateUser|from../utils/validation/validateUser|g' {} \;

Pattern: Rename with IDE Support

bash
# For large codebases, use language server
# TypeScript
npx tsserver --rename <file> <oldName> <newName>

# Or use find + sed for simpler cases
grep -rl "oldFunctionName" src/ | xargs sed -i 's/oldFunctionName/newFunctionName/g'

Edge Cases

Circular Dependencies

When moving code creates circular imports:

1. Extract shared types/interfaces to a separate module
2. Use dependency injection
3. Create a facade module

Breaking Changes

If refactoring must change public API:

1. Create new API alongside old
2. Deprecate old API
3. Migrate callers gradually
4. Remove old API in future release

Large-Scale Refactoring

For codebase-wide changes:

1. Script the change (sed, ast-grep, codemods)
2. Run on entire codebase at once
3. Commit as single atomic change
4. Verify everything still works

Troubleshooting

Tests Fail After Refactoring

bash
# 1. Check what changed
git diff HEAD~1

# 2. Verify test expectations
# Look for tests that assert implementation details

# 3. Consider if test needs updating (rare)
# Only if refactoring exposed test as brittle

Build Errors

bash
# TypeScript: Check for missing imports
npx tsc --noEmit

# Go: Check for unused imports
goimports -l .

# General: Search for references to old names
grep -r "oldName" --include="*.ts" --include="*.js" --include="*.go"

Performance Regression

bash
# Profile before and after
# Compare key metrics
# Ensure refactoring didn't introduce N+1 queries or unnecessary copies

Best Practices

1. Small steps — One logical change per commit
2. Tests first — Never refactor without passing tests
3. Version control — Use branches for risky refactoring
4. No behavior changes — Refactor only, then change behavior separately
5. Document intent — Commit messages explain WHY, not WHAT
6. Review carefully — Refactoring bugs are subtle
7. Incremental rollout — For large changes, merge progressively