Refactoring Patterns

Safety Rules

Before any refactoring, verify all of the following:

• Tests exist covering the code you will change
• All tests pass before you start
• Perform one transformation at a time
• Run tests after every single transformation
• Commit after each successful step
• Name every transformation in your commit message (e.g., "refactor: Extract Method -- validateEmail")
• Never change behavior and structure in the same commit
• If a step breaks tests, revert immediately and try a smaller step

Code Smell Catalog

Smell	Symptoms	Typical Refactoring
Long Method	Method over 20 lines; multiple levels of abstraction	Extract Method
Large Class	Class over 300 lines; too many responsibilities	Extract Class
Feature Envy	Method uses more data from another class than its own	Move Method
Data Clump	Same group of fields/params appear together repeatedly	Introduce Parameter Object
Primitive Obsession	Using primitives instead of small domain objects	Replace Primitive with Value Object
Shotgun Surgery	One change requires edits across many files	Move Method, Inline Class
Divergent Change	One class changed for many different reasons	Extract Class
Dead Code	Unreachable code, unused variables, commented-out blocks	Remove Dead Code
Long Parameter List	Method takes more than 3 parameters	Introduce Parameter Object
Switch Statements	Repeated switch/if-else on the same type field	Replace Conditional with Polymorphism
Duplicate Code	Same logic in multiple places	Extract Method, Extract Superclass
Magic Numbers	Unexplained literal values in logic	Replace Magic Number with Named Constant

Smell Examples

Long Method:

function processOrder(order: Order) {
  // validate (10 lines) + calculate totals (15 lines)
  // apply discounts (12 lines) + check inventory (8 lines)
  // create invoice (10 lines) + send notification (6 lines)
}

Feature Envy:

class Order {
  calculateShipping() {
    if (this.customer.address.country === "US") {
      if (this.customer.tier === "premium") return 0;
      return this.customer.address.isRemote ? 15 : 5;
    }
    return 25;
  }
}

Data Clump:

function createUser(name: string, street: string, city: string, state: string, zip: string) {}
function updateAddress(street: string, city: string, state: string, zip: string) {}

Named Refactorings

Refactoring	Input	Output
Extract Method	Code block inside a method	New method + call site
Extract Class	Fields and methods from a class	New class with focused responsibility
Inline Method	Trivial method	Body placed at all call sites
Rename	Unclear name	Intention-revealing name
Move Method	Method in wrong class	Method in the class that owns the data
Introduce Parameter Object	Multiple related parameters	Single object parameter
Replace Conditional with Polymorphism	Type-based switch/if-else	Subclasses with overridden method
Replace Magic Number with Named Constant	Literal number in code	Named constant
Decompose Conditional	Complex boolean expression	Named methods for conditions
Remove Dead Code	Unused code	Deletion

Extract Method

Recipe: (1) Identify code to extract. (2) Create method with intention-revealing name. (3) Copy code in. (4) Local variables become locals; outer scope reads become parameters; modified-and-used-after become return values. (5) Replace original with call. (6) Run tests.

// Before
function printInvoice(invoice: Invoice) {
  console.log("=== Invoice ===");
  let total = 0;
  for (const item of invoice.items) { total += item.price * item.quantity; }
  const tax = total * 0.08;
  console.log(`Total: ${total + tax}`);
}

// After
function printInvoice(invoice: Invoice) {
  console.log("=== Invoice ===");
  console.log(`Total: ${calculateGrandTotal(invoice.items)}`);
}

function calculateGrandTotal(items: InvoiceItem[]): number {
  const subtotal = items.reduce((sum, i) => sum + i.price * i.quantity, 0);
  return subtotal + subtotal * 0.08;
}

Extract Class

Recipe: (1) Identify cohesive fields and methods. (2) Create new class. (3) Move fields. (4) Move methods. (5) Delegate from original. (6) Run tests after each move.

// Before
class User {
  name: string; email: string;
  street: string; city: string; state: string; zip: string;
  fullAddress(): string { return `${this.street}, ${this.city}, ${this.state} ${this.zip}`; }
}

// After
class Address {
  constructor(public street: string, public city: string, public state: string, public zip: string) {}
  full(): string { return `${this.street}, ${this.city}, ${this.state} ${this.zip}`; }
}

class User {
  name: string; email: string; address: Address;
}

Introduce Parameter Object

Recipe: (1) Create type for the parameter group. (2) Add object parameter. (3) Update callers. (4) Remove old params. (5) Run tests.

// Before
function searchProducts(minPrice: number, maxPrice: number, category: string, inStock: boolean) {}

// After
interface ProductFilter { minPrice: number; maxPrice: number; category: string; inStock: boolean; }
function searchProducts(filter: ProductFilter) {}

Replace Conditional with Polymorphism

Recipe: (1) Create base interface with the varying method. (2) Create subclass per case. (3) Move case logic into subclass. (4) Replace conditional with method call. (5) Run tests.

// Before
function calculateArea(shape: Shape): number {
  switch (shape.type) {
    case "circle":    return Math.PI * shape.radius ** 2;
    case "rectangle": return shape.width * shape.height;
    case "triangle":  return (shape.base * shape.height) / 2;
  }
}

// After
interface Shape { area(): number; }
class Circle implements Shape {
  constructor(private radius: number) {}
  area() { return Math.PI * this.radius ** 2; }
}
class Rectangle implements Shape {
  constructor(private width: number, private height: number) {}
  area() { return this.width * this.height; }
}
class Triangle implements Shape {
  constructor(private base: number, private height: number) {}
  area() { return (this.base * this.height) / 2; }
}

Decompose Conditional

Recipe: (1) Extract condition into a named method. (2) Extract then-branch if non-trivial. (3) Extract else-branch if non-trivial. (4) Run tests.

// Before
if (date.getMonth() >= 5 && date.getMonth() <= 8 && !isHoliday(date)) {
  rate = quantity * summerRate;
} else {
  rate = quantity * regularRate + serviceFee;
}

// After
if (isSummerBusinessDay(date)) { rate = summerCharge(quantity); }
else { rate = regularCharge(quantity); }

Remove Dead Code

Recipe: (1) Verify truly unused (search references, check exports). (2) Delete. (3) Run tests. (4) Commit as refactor: Remove Dead Code -- <what>.

Dead code includes: unreachable branches, unused functions, commented-out blocks, unused variables and imports, removed feature flag code. Never comment out code "for later" -- version control preserves history.

Strangler Fig Pattern

For large-scale refactors that cannot be completed in a single step:

1. Identify the boundary of the legacy code to replace
2. Build the new implementation alongside the old one
3. Route traffic incrementally using a facade or feature flag
4. Verify each increment in production
5. Remove the old implementation once all traffic uses the new code

// Step 1: Facade delegates to old code
class PaymentProcessor {
  process(payment: Payment): Result { return this.legacyProcessor.process(payment); }
}

// Step 2-3: Route with feature flag
class PaymentProcessor {
  process(payment: Payment): Result {
    if (featureFlag.isEnabled("new-payment-processor"))
      return this.newProcessor.process(payment);
    return this.legacyProcessor.process(payment);
  }
}

// Step 4-5: After verification, remove old code
class PaymentProcessor {
  process(payment: Payment): Result { return this.newProcessor.process(payment); }
}

Strangler Fig Checklist

• Both old and new paths are tested
• Rollback is possible at every stage by toggling the flag
• Monitoring and alerts cover both paths
• Old code is deleted only after the new path is proven in production

Refactoring Workflow Summary

1. Identify the smell
2. Choose the named refactoring
3. Verify tests pass (write them if missing)
4. Apply one transformation
5. Run tests
6. Commit: "refactor: <Refactoring Name> -- <target>"
7. Repeat from step 4 until the smell is resolved