Dev refactor

Code Refactoring

Principles

1. Tests pass BEFORE and AFTER
2. Small incremental changes
3. One type of change at a time
4. Commit after each refactoring

Common techniques

Extract Function

// Before
function processOrder(order) {
  // 20 lines of validation
  // 30 lines of calculation
  // 10 lines of sending
}

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

Extract Variable

// Before
if (user.age >= 18 && user.country === 'FR' && !user.banned) { }

// After
const isAdult = user.age >= 18;
const isFrench = user.country === 'FR';
const isActive = !user.banned;
if (isAdult && isFrench && isActive) { }

Replace Conditional with Polymorphism

// Before
function getPrice(type) {
  switch(type) {
    case 'basic': return 10;
    case 'premium': return 20;
  }
}

// After
interface Plan { getPrice(): number }
class BasicPlan implements Plan { getPrice() { return 10; } }
class PremiumPlan implements Plan { getPrice() { return 20; } }

Code Smells to detect

Smell	Refactoring
Long method	Extract Method
Large class	Extract Class
Duplicate code	Extract + Reuse
Long parameter list	Parameter Object
Feature envy	Move Method
Primitive obsession	Value Object

Reducing Entropy (Complexity reduction)

Complexity metrics

Metric	Alert threshold	How to measure
Cyclomatic complexity	> 10 per function	Number of branches (if/else/switch)
Nesting depth	> 3 levels	Nesting of if/for/while
Function length	> 50 lines	Number of lines
Number of parameters	> 4	Function parameters
Afferent/efferent coupling	Unstable ratio	Incoming/outgoing dependencies
File size	> 300 lines	Lines of code

Reduction techniques

Early Return (eliminate nesting)

// BEFORE: deep nesting (high entropy)
function process(user) {
  if (user) {
    if (user.isActive) {
      if (user.hasPermission) {
        return doWork(user);
      }
    }
  }
  return null;
}

// AFTER: early returns (low entropy)
function process(user) {
  if (!user) return null;
  if (!user.isActive) return null;
  if (!user.hasPermission) return null;
  return doWork(user);
}

Break down complex conditions

// BEFORE
if (user.age >= 18 && user.country === 'FR' && !user.banned && user.email.includes('@')) { }

// AFTER
const isEligible = user.age >= 18
  && user.country === 'FR'
  && !user.banned
  && isValidEmail(user.email);
if (isEligible) { }

Eliminate dead code

# Find unused exports
# Find functions never called
# Remove unused imports
# Remove obsolete comments
# Remove orphan files

Consolidate duplications

Rule of 3: refactor on the 3rd duplication, not before.
- 1st occurrence: write the code
- 2nd occurrence: note the duplication (comment)
- 3rd occurrence: extract into a function/module

Workflow

1. MEASURE current complexity (metrics)
2. Identify the code smell
3. Write/verify tests
4. Apply the refactoring
5. MEASURE complexity after (must decrease)
6. Verify tests
7. Commit
8. Repeat