refactoring-patterns

Safe transformations — same behavior, better structure.

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name	refactoring-patterns
description	Safe transformations — same behavior, better structure.
tier	core
applyTo	*/refactor,/extract,/rename,/inline*

Refactoring Patterns Skill

Safe transformations — same behavior, better structure.

Golden Rule

Tests pass before AND after. Never refactor and add features in the same commit.

When to Refactor

Trigger	Action
Feature is hard to add	Refactor first, then add feature
Same bug twice	Refactor to prevent recurrence
"I don't understand"	Refactor for clarity
Duplicate code	Extract and reuse
Long function (>30 lines)	Extract logical units

When NOT to Refactor

No tests + time pressure
Code won't change again
Right before release (deadline pressure)
Should rewrite instead (>70% changes needed)
Exploratory/prototype code

Core Refactoring Moves

Extract Function

When a block does one logical thing, give it a name.


 () {
  
   (!order..)   ();
   (!order.)   ();
   (order. < )   ();
  
  
   taxRate = order. ===  ?  : ;
   tax = order. * taxRate;
  
  
   discount = order.. ?  : ;
   finalTotal = order. + tax - (order. * discount);
  
   finalTotal;
}


 () {
  (order);
   tax = (order);
   discount = (order);
   order. + tax - discount;
}

 ():  {
   (!order..)   ();
   (!order.)   ();
   (order. < )   ();
}

 ():  {
   taxRate = order. ===  ?  : ;
   order. * taxRate;
}

 ():  {
   order.. ? order. *  : ;
}

// Before
if (user.age >= 18 && user.country === 'US' && !user.banned && user.emailVerified) {
  allowAccess();
}

// After
const isAdult = user.age >= 18;
const isUSResident = user.country === 'US';
const isInGoodStanding = !user.banned && user.emailVerified;
const canAccess = isAdult && isUSResident && isInGoodStanding;

if (canAccess) {
  allowAccess();
}

// Before
const d = new Date().getTime() - start;
const arr = users.filter(u => u.a);

// After
const elapsedMs = new Date().getTime() - startTime;
const activeUsers = users.filter(user => user.isActive);

// Before
function calculatePay(employee: Employee): number {
  switch (employee.type) {
    case 'hourly':
      return employee.hours * employee.rate;
    case 'salaried':
      return employee.salary / 12;
    case 'commission':
      return employee.sales * employee.commissionRate + employee.basePay;
  }
}

// After
interface PayStrategy {
  calculate(employee: Employee): number;
}

class HourlyPay implements PayStrategy {
  calculate(emp: Employee): number {
    return emp.hours * emp.rate;
  }
}

class SalariedPay implements PayStrategy {
  calculate(emp: Employee): number {
    return emp.salary / 12;
  }
}

class CommissionPay implements PayStrategy {
  calculate(emp: Employee): number {
    return emp.sales * emp.commissionRate + emp.basePay;
  }
}

// Before
function getPayAmount(employee: Employee): number {
  let result: number;
  if (employee.isSeparated) {
    result = 0;
  } else {
    if (employee.isRetired) {
      result = employee.pension;
    } else {
      result = employee.salary;
    }
  }
  return result;
}

// After
function getPayAmount(employee: Employee): number {
  if (employee.isSeparated) return 0;
  if (employee.isRetired) return employee.pension;
  return employee.salary;
}

Smell	Symptoms	Refactoring
Long function	>30 lines, multiple comments explaining sections	Extract Function
Long parameter list	>4 parameters	Introduce Parameter Object
Duplicate code	Same logic in 2+ places	Extract Function, Pull Up Method
Feature envy	Method uses another object's data more than its own	Move Function
Large class	Class does too many things	Extract Class
Primitive obsession	Using primitives instead of small objects	Replace Primitive with Object
Data clumps	Same group of variables appear together	Introduce Parameter Object
Switch statements	Type-based conditionals	Replace Conditional with Polymorphism
Temporary field	Field only used sometimes	Extract Class
Refused bequest	Subclass ignores inherited methods	Replace Inheritance with Delegation

Refactor	Rewrite
Core design is sound	Fundamental design is wrong
Tests exist and pass	Code is untestable
<30% of code changes	>70% of code changes
Incremental improvement	Complete replacement
Low risk	Higher risk
Keep shipping features	Pause feature work

1. Commit current state (safety net)
2. Run all tests (establish baseline)
3. Make ONE small change
4. Run tests
5. Commit with descriptive message
6. Repeat steps 3-5

Refactoring	VS Code Shortcut
Rename Symbol	F2
Extract Function	Ctrl+Shift+R → Extract Function
Extract Variable	Ctrl+Shift+R → Extract Variable
Inline Variable	Ctrl+Shift+R → Inline Variable
Move to File	Ctrl+Shift+R → Move to new file