Refactoring

Refactoring Patterns

Quick Reference

Safe Refactoring Workflow

1. Verify tests pass before touching anything
2. Make one change at a time -- don't combine refactors
3. Run tests after each change to catch regressions immediately
4. Commit frequently -- each refactor step gets its own commit
5. Never mix refactoring with behavior changes in the same commit

Code Smell Detection

Extract Patterns

Extract Function

Before:

def process_order(order):
    # validate
    if not order.items:
        raise ValueError("Empty order")
    if order.total < 0:
        raise ValueError("Negative total")
    # calculate tax
    tax = order.total * 0.08
    if order.state == "CA":
        tax = order.total * 0.0975
    order.tax = tax
    order.final_total = order.total + tax

After:

def process_order(order):
    validate_order(order)
    order.tax = calculate_tax(order.total, order.state)
    order.final_total = order.total + order.tax

def validate_order(order):
    if not order.items:
        raise ValueError("Empty order")
    if order.total < 0:
        raise ValueError("Negative total")

def calculate_tax(total: float, state: str) -> float:
    if state == "CA":
        return total * 0.0975
    return total * 0.08

Replace Nested Conditionals with Guard Clauses

Before:

def get_payment(employee):
    if employee.is_active:
        if employee.is_full_time:
            if employee.years > 5:
                return employee.salary * 1.1
            else:
                return employee.salary
        else:
            return employee.hourly_rate * employee.hours
    else:
        return 0

After:

def get_payment(employee):
    if not employee.is_active:
        return 0
    if not employee.is_full_time:
        return employee.hourly_rate * employee.hours
    if employee.years > 5:
        return employee.salary * 1.1
    return employee.salary

Introduce Parameter Object

Before:

function createUser(name: string, email: string, age: number, role: string, team: string) { ... }

After:

interface CreateUserParams {
  name: string;
  email: string;
  age: number;
  role: string;
  team: string;
}

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

Migration Patterns

JavaScript to TypeScript

1. Rename .js to .ts (one file at a time, start with leaf modules)
2. Add tsconfig.json with strict mode
3. Fix type errors -- add types to function signatures first
4. Use unknown over any when the type is genuinely unclear
5. Run tests after each file migration

CommonJS to ESM

Also update package.json: add "type": "module".

Class to Functional (React)

Safe Rename Workflow

1. Search all references -- grep for the name across the codebase
2. Check for dynamic usage -- string-based lookups, reflection, config files
3. Rename in all locations -- source, tests, docs, configs
4. Update imports/exports -- especially re-exports from index files
5. Run full test suite
6. Commit with descriptive message: refactor: rename X to Y for clarity

When NOT to Refactor

Anti-Patterns