Refactoring

Refactoring Skill

Restructure code safely while preserving behavior.

Workflow

1. Verify Test Coverage

Before refactoring, ensure tests exist:

# Run existing tests
npm test
pytest
go test ./...

If no tests cover the code:

1. Write characterization tests first
2. These capture current behavior (even if buggy)
3. Then refactor with confidence

2. Understand Current Structure

Map the code:

• What are the dependencies?
• Who calls this code?
• What does it depend on?
• What's the data flow?

# Find usages
grep -r "functionName" --include="*.ts"

# Find dependencies
grep -r "import.*from.*module" --include="*.ts"

3. Plan the Refactoring

Identify the target state:

• What's the problem with current code?
• What's the desired structure?
• What's the smallest step toward that?

Choose your approach:

Situation	Approach
Large function	Extract methods
Duplicated code	Extract shared function
Complex conditionals	Replace with polymorphism
Long parameter list	Introduce parameter object
Feature envy	Move method to data's class
Data clump	Extract class

4. Execute in Small Steps

The safe refactoring cycle:

1. Make ONE small change
2. Run tests
3. Commit if green
4. Repeat

Never skip steps:

• Don't combine multiple refactorings
• Don't "fix bugs" while refactoring
• Don't add features while refactoring

5. Verify Behavior Preserved

# Run full test suite
npm test

# Check for regressions
npm run test:e2e

# Manual smoke test if needed

Quick Reference

Safe Refactorings (Automated)

Most IDEs can do these automatically:

Refactoring	Shortcut (VS Code)
Rename	F2
Extract function	Ctrl+Shift+R
Extract variable	Ctrl+Shift+R
Inline variable	Ctrl+Shift+R
Move to file	Drag in explorer

Common Patterns

Extract Function

// Before
function processOrder(order) {
  // validate
  if (!order.items.length) throw new Error('Empty order');
  if (!order.customer) throw new Error('No customer');

  // calculate total
  let total = 0;
  for (const item of order.items) {
    total += item.price * item.quantity;
  }

  // apply discount
  if (order.coupon) {
    total *= (1 - order.coupon.discount);
  }

  return { ...order, total };
}

// After
function processOrder(order) {
  validateOrder(order);
  const total = calculateTotal(order);
  return { ...order, total };
}

function validateOrder(order) {
  if (!order.items.length) throw new Error('Empty order');
  if (!order.customer) throw new Error('No customer');
}

function calculateTotal(order) {
  const subtotal = order.items.reduce(
    (sum, item) => sum + item.price * item.quantity,
    0
  );
  return order.coupon
    ? subtotal * (1 - order.coupon.discount)
    : subtotal;
}

Replace Conditional with Polymorphism

// Before
function getSpeed(vehicle) {
  switch (vehicle.type) {
    case 'car': return vehicle.horsepower * 0.5;
    case 'bike': return vehicle.gears * 5;
    case 'boat': return vehicle.engineSize * 2;
  }
}

// After
class Car {
  getSpeed() { return this.horsepower * 0.5; }
}
class Bike {
  getSpeed() { return this.gears * 5; }
}
class Boat {
  getSpeed() { return this.engineSize * 2; }
}

Introduce Parameter Object

// Before
function createUser(name, email, age, country, role) {
  // ...
}

// After
function createUser({ name, email, age, country, role }) {
  // ...
}

// Or with type
interface CreateUserParams {
  name: string;
  email: string;
  age: number;
  country: string;
  role: string;
}

function createUser(params: CreateUserParams) {
  // ...
}

Code Smells to Address

Smell	Refactoring
Long method	Extract method
Large class	Extract class
Duplicate code	Extract function
Long parameter list	Parameter object
Switch statements	Polymorphism
Feature envy	Move method
Data clump	Extract class
Primitive obsession	Value object
Comments explaining code	Extract well-named method

Anti-Patterns

Don't:

1. Refactor and change behavior simultaneously

   • Refactoring = same behavior, different structure
   • Keep them separate

2. Make big-bang changes

   • Small steps with tests passing between each

3. Refactor without tests

   • Write characterization tests first

4. Over-abstract too early

   • Wait for duplication to appear 3 times

5. Refactor "just in case"

   • Refactor for a reason (readability, extensibility needed now)

See Also

• patterns.md - Detailed refactoring patterns with examples