Refactoring Legacy Code Without Breaking Everything

// ❌ Writing tests based on what you think the code should do
test("calculateDiscount returns 10% for premium users", () => {
  expect(calculateDiscount("premium", 100)).toBe(90);
});
// This test might fail because the actual code has a bug
// that gives premium users 15%. The bug might be a feature
// that sales promised to customers.

// ✅ Characterization tests: capture what the code actually does
function characterize(
  fn: (...args: unknown[]) => unknown,
  inputs: unknown[][]
): void {
  for (const args of inputs) {
    const result = fn(...args);
    console.log(
      `${fn.name}(${args.map(a => JSON.stringify(a)).join(", ")}) => ${JSON.stringify(result)}`
    );
    // Copy these outputs into test assertions
  }
}
 
// Step 1: Run with various inputs and record actual outputs
characterize(calculateDiscount, [
  ["premium", 100],
  ["premium", 0],
  ["standard", 100],
  ["standard", 50],
  ["", 100],
  [null, 100],
  ["premium", -10],
]);
 
// Step 2: Turn recorded outputs into tests
describe("calculateDiscount (characterization)", () => {
  test("premium 100 → 85", () => {
    expect(calculateDiscount("premium", 100)).toBe(85);
  });
  test("premium 0 → 0", () => {
    expect(calculateDiscount("premium", 0)).toBe(0);
  });
  test("standard 100 → 95", () => {
    expect(calculateDiscount("standard", 100)).toBe(95);
  });
  test("null tier → 100 (no discount)", () => {
    expect(calculateDiscount(null, 100)).toBe(100);
  });
  test("negative amount → -10 (no guard)", () => {
    expect(calculateDiscount("premium", -10)).toBe(-10);
  });
});
// These tests document reality, not intent

// Legacy function with embedded dependencies
function processOrder(orderId: string): void {
  // Direct database call — hard to test
  const order = db.query(
    `SELECT * FROM orders WHERE id = '${orderId}'`
  );
 
  // Business logic buried in the middle
  let total = 0;
  for (const item of order.items) {
    let price = item.price;
    if (item.category === "electronics" && order.memberTier === "gold") {
      price = price * 0.9;
    }
    if (item.quantity > 10) {
      price = price * 0.95;
    }
    total += price * item.quantity;
  }
 
  // Direct email service call
  emailService.send(order.email, `Your total is $${total}`);
 
  // Direct database update
  db.query(
    `UPDATE orders SET total = ${total} WHERE id = '${orderId}'`
  );
}

// Step 1: Extract parameters to create seams
function processOrder(
  order: Order,
  notify: (email: string, message: string) => void,
  save: (orderId: string, total: number) => void
): number {
  let total = 0;
  for (const item of order.items) {
    let price = item.price;
    if (item.category === "electronics" && order.memberTier === "gold") {
      price = price * 0.9;
    }
    if (item.quantity > 10) {
      price = price * 0.95;
    }
    total += price * item.quantity;
  }
 
  notify(order.email, `Your total is $${total}`);
  save(order.id, total);
  return total;
}
 
// Step 2: Now you can test the business logic
test("gold member gets 10% off electronics", () => {
  const order: Order = {
    id: "1",
    email: "test@example.com",
    memberTier: "gold",
    items: [
      { category: "electronics", price: 100, quantity: 1 },
    ],
  };
 
  const total = processOrder(
    order,
    () => {}, // stub notification
    () => {}  // stub persistence
  );
 
  expect(total).toBe(90);
});

// Step 1: Extract the pricing logic into a clean module
interface PricingRule {
  applies: (item: OrderItem, order: Order) => boolean;
  calculate: (price: number) => number;
}
 
const pricingRules: PricingRule[] = [
  {
    applies: (item, order) =>
      item.category === "electronics" && order.memberTier === "gold",
    calculate: (price) => price * 0.9,
  },
  {
    applies: (item) => item.quantity > 10,
    calculate: (price) => price * 0.95,
  },
];
 
function calculateOrderTotal(
  order: Order,
  rules: PricingRule[]
): number {
  let total = 0;
 
  for (const item of order.items) {
    let price = item.price;
 
    for (const rule of rules) {
      if (rule.applies(item, order)) {
        price = rule.calculate(price);
      }
    }
 
    total += price * item.quantity;
  }
 
  return total;
}

// Step 2: Verify new module matches old behavior
function verifyEquivalence(testCases: Order[]): void {
  for (const order of testCases) {
    const oldResult = legacyCalculateTotal(order);
    const newResult = calculateOrderTotal(order, pricingRules);
 
    if (oldResult !== newResult) {
      console.error(
        `Mismatch for order ${order.id}: ` +
        `legacy=${oldResult}, new=${newResult}`
      );
    }
  }
}
 
// Step 3: Deploy behind a feature flag
function getOrderTotal(order: Order): number {
  if (featureFlags.isEnabled("new-pricing-engine")) {
    return calculateOrderTotal(order, pricingRules);
  }
  return legacyCalculateTotal(order);
}

interface MigrationTracker {
  module: string;
  totalEndpoints: number;
  migratedEndpoints: number;
  legacyEndpoints: string[];
  migratedOn: Map<string, Date>;
}
 
class StranglerRouter {
  private migrated: Set<string> = new Set();
  private tracker: MigrationTracker;
 
  constructor(module: string, totalEndpoints: number) {
    this.tracker = {
      module,
      totalEndpoints,
      migratedEndpoints: 0,
      legacyEndpoints: [],
      migratedOn: new Map(),
    };
  }
 
  markMigrated(endpoint: string): void {
    this.migrated.add(endpoint);
    this.tracker.migratedEndpoints++;
    this.tracker.migratedOn.set(endpoint, new Date());
  }
 
  route(
    endpoint: string,
    legacyHandler: () => unknown,
    newHandler: () => unknown
  ): unknown {
    if (this.migrated.has(endpoint)) {
      return newHandler();
    }
    return legacyHandler();
  }
 
  getProgress(): { percentage: number; remaining: string[] } {
    return {
      percentage:
        (this.tracker.migratedEndpoints / this.tracker.totalEndpoints) * 100,
      remaining: this.tracker.legacyEndpoints.filter(
        e => !this.migrated.has(e)
      ),
    };
  }
}

interface RefactoringStep {
  step: string;
  verification: string;
  rollbackPlan: string;
}
 
const safeRefactoringProcess: RefactoringStep[] = [
  {
    step: "Write characterization tests for existing behavior",
    verification: "All tests pass against current code",
    rollbackPlan: "N/A — no code changes yet",
  },
  {
    step: "Extract testable interfaces (seams)",
    verification: "Characterization tests still pass",
    rollbackPlan: "Revert extraction commit",
  },
  {
    step: "Write unit tests for extracted logic",
    verification: "Unit tests match characterization test behavior",
    rollbackPlan: "Delete new tests, keep old code",
  },
  {
    step: "Implement new module alongside legacy",
    verification: "Run both, compare outputs for N days",
    rollbackPlan: "Feature flag to legacy path",
  },
  {
    step: "Route traffic to new module",
    verification: "Monitor error rates, latencies, business metrics",
    rollbackPlan: "Feature flag back to legacy",
  },
  {
    step: "Remove legacy code",
    verification: "All tests pass, monitoring stable for 2 weeks",
    rollbackPlan: "Git revert — legacy code still in history",
  },
];