Refactoring

Refactoring Skill

Core Principle

Refactoring changes code structure without changing behavior.

Refactoring is NOT about adding features or fixing bugs. It's about making code easier to understand, maintain, and extend. Every refactoring should be:

• Safe — Behavior remains unchanged
• Testable — Tests verify behavior preservation
• Incremental — Small steps, frequent commits
• Reversible — Can roll back if needed

When to Refactor

Good Times to Refactor

✅ Before adding a feature — Make room for new functionality ✅ During code review — Improve code before merge ✅ When you touch existing code — Leave it better than you found it ✅ When you see duplication — DRY principle ✅ When code is hard to test — Improve testability ✅ When you struggle to understand code — Simplify for future readers

Bad Times to Refactor

❌ While debugging — Fix the bug first, refactor later ❌ Near a deadline — Risk vs. reward is poor ❌ Without tests — Can't verify behavior preservation ❌ In unfamiliar code — Understand first, refactor later ❌ Just because — Must have a clear purpose

Refactoring Patterns

1. Extract Function

Problem: Long function doing multiple things

Solution: Extract logical blocks into named functions

Before:

def process_order(order):
    # Validate
    if not order.items:
        raise ValueError("Order has no items")
    if order.total < 0:
        raise ValueError("Invalid total")

    # Calculate
    subtotal = sum(item.price * item.quantity for item in order.items)
    tax = subtotal * 0.08
    total = subtotal + tax

    # Save
    db.save(order)
    send_confirmation_email(order.customer.email, order.id)

After:

def process_order(order):
    validate_order(order)
    calculate_totals(order)
    save_and_notify(order)

def validate_order(order):
    if not order.items:
        raise ValueError("Order has no items")
    if order.total < 0:
        raise ValueError("Invalid total")

def calculate_totals(order):
    subtotal = sum(item.price * item.quantity for item in order.items)
    order.tax = subtotal * 0.08
    order.total = subtotal + order.tax

def save_and_notify(order):
    db.save(order)
    send_confirmation_email(order.customer.email, order.id)

Benefits: Easier to test, understand, and modify each piece independently

2. Extract Variable

Problem: Complex expression that's hard to understand

Solution: Break into named intermediate variables

Before:

if (user.age >= 18 && user.country === 'US' &&
    (user.verified || user.trustScore > 0.8)) {
    allowPurchase();
}

After:

const isAdult = user.age >= 18;
const isUSUser = user.country === 'US';
const isTrusted = user.verified || user.trustScore > 0.8;
const canPurchase = isAdult && isUSUser && isTrusted;

if (canPurchase) {
    allowPurchase();
}

Benefits: Self-documenting, easier to debug, reusable logic

3. Replace Magic Numbers

Problem: Unexplained numeric literals throughout code

Solution: Named constants with clear meaning

Before:

def calculate_shipping(weight):
    if weight < 5:
        return 4.99
    elif weight < 20:
        return 9.99
    else:
        return 14.99

After:

LIGHT_PACKAGE_THRESHOLD = 5  # pounds
MEDIUM_PACKAGE_THRESHOLD = 20  # pounds
LIGHT_SHIPPING_COST = 4.99
MEDIUM_SHIPPING_COST = 9.99
HEAVY_SHIPPING_COST = 14.99

def calculate_shipping(weight):
    if weight < LIGHT_PACKAGE_THRESHOLD:
        return LIGHT_SHIPPING_COST
    elif weight < MEDIUM_PACKAGE_THRESHOLD:
        return MEDIUM_SHIPPING_COST
    else:
        return HEAVY_SHIPPING_COST

Benefits: Clear meaning, easy to update, self-documenting

4. Simplify Conditional Logic

Problem: Nested or complex conditionals

Solution: Early returns, guard clauses, or strategy pattern

Before:

function calculateDiscount(user, order) {
    if (user) {
        if (user.isPremium) {
            if (order.total > 100) {
                return order.total * 0.2;
            } else {
                return order.total * 0.1;
            }
        } else {
            if (order.total > 100) {
                return order.total * 0.05;
            }
        }
    }
    return 0;
}

After:

function calculateDiscount(user, order) {
    if (!user) return 0;

    const discountRate = getDiscountRate(user, order);
    return order.total * discountRate;
}

function getDiscountRate(user, order) {
    if (user.isPremium) {
        return order.total > 100 ? 0.2 : 0.1;
    }
    return order.total > 100 ? 0.05 : 0;
}

Benefits: Reduced nesting, clearer logic flow, easier to extend

5. Replace Duplication with Abstraction

Problem: Same code repeated in multiple places

Solution: Extract to shared function/class

Before:

def send_welcome_email(user):
    subject = f"Welcome {user.name}!"
    body = render_template("welcome.html", user=user)
    smtp.send(user.email, subject, body)
    log.info(f"Sent welcome email to {user.email}")

def send_reset_email(user, token):
    subject = "Password Reset Request"
    body = render_template("reset.html", user=user, token=token)
    smtp.send(user.email, subject, body)
    log.info(f"Sent reset email to {user.email}")

After:

def send_email(user, subject, template, **kwargs):
    body = render_template(template, user=user, **kwargs)
    smtp.send(user.email, subject, body)
    log.info(f"Sent {template} email to {user.email}")

def send_welcome_email(user):
    send_email(user, f"Welcome {user.name}!", "welcome.html")

def send_reset_email(user, token):
    send_email(user, "Password Reset Request", "reset.html", token=token)

Benefits: DRY principle, single place to update email logic

6. Improve Naming

Problem: Unclear or misleading names

Solution: Descriptive, intention-revealing names

Before:

function calc(a, b) {
    const tmp = a * b;
    const res = tmp * 0.08;
    return tmp + res;
}

After:

function calculateOrderTotal(subtotal, taxRate) {
    const tax = subtotal * taxRate;
    return subtotal + tax;
}

Benefits: Self-documenting, no need for comments

7. Split Large Classes

Problem: Class doing too many things (God Object)

Solution: Extract responsibilities into focused classes

Before:

class User:
    def __init__(self, email, password):
        self.email = email
        self.password = password

    def hash_password(self):
        # Password hashing logic
        pass

    def send_email(self, subject, body):
        # Email sending logic
        pass

    def save_to_db(self):
        # Database logic
        pass

    def generate_report(self):
        # Reporting logic
        pass

After:

class User:
    def __init__(self, email, password_hash):
        self.email = email
        self.password_hash = password_hash

class PasswordHasher:
    @staticmethod
    def hash(password):
        # Password hashing logic
        pass

class EmailService:
    @staticmethod
    def send(recipient, subject, body):
        # Email sending logic
        pass

class UserRepository:
    @staticmethod
    def save(user):
        # Database logic
        pass

class UserReportGenerator:
    def generate(self, user):
        # Reporting logic
        pass

Benefits: Single Responsibility Principle, easier to test and maintain

Refactoring Workflow

Step 1: Ensure Tests Exist

Before refactoring:

# Run existing tests
npm test  # JavaScript
pytest    # Python
cargo test  # Rust

# If tests don't exist or are insufficient:
# STOP and write tests first

Rule: Never refactor without tests. Tests are your safety net.

Step 2: Make One Small Change

Keep refactorings atomic:

✅ Good: Extract one function ✅ Good: Rename one variable ✅ Good: Replace one magic number

❌ Bad: Extract function + rename variables + restructure class ❌ Bad: Refactor multiple files at once

Step 3: Run Tests After Each Change

After every refactoring step:

# Run tests immediately
npm test
# Tests should still pass

# If tests fail:
# 1. You changed behavior (not just structure)
# 2. Revert and try again
git checkout -- .

Step 4: Commit Frequently

Commit after each successful refactoring:

git add .
git commit -m "refactor: extract validate_order function

Extracted order validation logic into separate function for better
testability and readability. No behavior change."

Commit message pattern:

refactor: <what you changed>

<why you changed it>. No behavior change.

Step 5: Review Before Pushing

Before pushing refactored code:

• All tests pass
• No behavior changed (tests prove this)
• Code is simpler/clearer
• No dead code left behind
• Documentation updated if needed

Code Smells (When to Refactor)

Smell: Long Function

Signs:

• Function > 50 lines
• Multiple levels of indentation
• Hard to name because it does too many things

Refactoring: Extract Function, Extract Class

Smell: Duplicated Code

Signs:

• Same code block in multiple places
• Similar logic with minor variations
• Copy-paste programming

Refactoring: Extract Function, Extract Superclass, Use Template Method

Smell: Large Class

Signs:

• Class > 500 lines
• Too many methods
• Multiple reasons to change (violates SRP)

Refactoring: Extract Class, Split Responsibilities

Smell: Long Parameter List

Signs:

• Function takes > 3 parameters
• Parameters are related (should be object)
• Adding parameters frequently

Refactoring: Introduce Parameter Object, Replace with Builder

Smell: Primitive Obsession

Signs:

• Using primitives instead of small objects
• Type codes (using int constants for types)
• String parsing throughout codebase

Refactoring: Replace with Value Object, Extract Class

Smell: Feature Envy

Signs:

• Method uses data from another class more than its own
• Method would be better in another class

Refactoring: Move Method, Extract Method and Move

Safety Checklist

Before starting refactoring:

• Tests exist and pass
• Tests have good coverage (>80% for code being refactored)
• You understand the code behavior
• No deadline pressure
• Git working directory is clean

During refactoring:

• Making one small change at a time
• Running tests after each change
• Committing after each successful refactoring
• No behavior changes (tests prove it)

After refactoring:

• All tests pass
• Code is simpler
• No dead code remains
• Documentation updated
• Ready for code review

Integration with Other Skills

With Code Review

• Refactor before requesting review
• Self-review using code-review skill patterns
• Smaller refactorings = easier reviews

With Testing Strategy

• Write tests before refactoring (if missing)
• Use test-driven refactoring
• Tests prove behavior preservation

With Git Hygiene

• Commit refactorings separately from features
• Use "refactor:" prefix in commit messages
• Keep refactoring commits small and focused

Common Pitfalls

❌ Refactoring without tests — Can't prove behavior unchanged ❌ Big bang refactoring — Too risky, hard to review ❌ Refactoring while adding features — Mix of concerns ❌ Refactoring unfamiliar code — Understand first ❌ Refactoring for perfection — Good enough is good enough ❌ Breaking backward compatibility — If public API, be careful

✅ Small, tested, incremental refactorings ✅ Clear purpose and benefit ✅ Preserve behavior ✅ Frequent commits

Remember: Refactoring is about making code better without changing what it does. Keep changes small, test frequently, commit often. If tests fail after refactoring, you changed behavior—revert and try again.