refactoring

Refactoring Skill — Safe Code Transformations

This skill provides systematic refactoring guidance ensuring behavior preservation.

Core Principle

Refactoring changes the STRUCTURE without changing the BEHAVIOR. If behavior needs to change, that's a feature — do it separately.

Refactoring Catalog

Extract Method

// BEFORE
function printOwing(invoice) {
  // Print banner
  console.log('*******************');
  console.log('*** Customer Owes ***');
  console.log('*******************');

  // Calculate outstanding
  let outstanding = 0;
  for (const o of invoice.orders) {
    outstanding += o.amount;
  }

  // Print details
  console.log(`name: ${invoice.customer}`);
  console.log(`amount: ${outstanding}`);
}

// AFTER
function printOwing(invoice: Invoice): void {
  printBanner();
  const outstanding = calculateOutstanding(invoice.orders);
  printDetails(invoice.customer, outstanding);
}

Replace Conditional with Polymorphism

// BEFORE
function getDiscount(customerType: string): number {
  if (customerType === 'premium') return 0.2;
  if (customerType === 'gold') return 0.15;
  if (customerType === 'silver') return 0.1;
  return 0;
}

// AFTER
abstract class Customer {
  abstract getDiscount(): number;
}
class PremiumCustomer extends Customer {
  getDiscount(): number {
    return 0.2;
  }
}
class GoldCustomer extends Customer {
  getDiscount(): number {
    return 0.15;
  }
}

Introduce Parameter Object

// BEFORE
function createReport(
  title: string,
  startDate: Date,
  endDate: Date,
  format: string,
  includeCharts: boolean,
  groupBy: string
) { ... }

// AFTER
interface ReportOptions {
  title: string;
  dateRange: { start: Date; end: Date };
  format: 'pdf' | 'html' | 'csv';
  includeCharts: boolean;
  groupBy: 'day' | 'week' | 'month';
}
function createReport(options: ReportOptions) { ... }

Safe Refactoring Protocol

1. GREEN: Ensure all tests pass before starting
2. SMALL: Make one refactoring at a time
3. TEST: Run tests after EACH refactoring step
4. COMMIT: Commit after each successful refactoring
5. REVIEW: Code review the refactoring separately