davila7/claude-code-templates/kaizen

// Iteration 1: Make it work
const calculateTotal = (items: Item[]) => {
  let total = 0;
  for (let i = 0; i < items.length; i++) {
    total += items[i].price * items[i].quantity;
  }
  return total;
};
// Iteration 2: Make it clear (refactor)
const calculateTotal = (items: Item[]): number => {
return items.reduce((total, item) => {
return total + (item.price \* item.quantity);
}, 0);
};
// Iteration 3: Make it robust (add validation)
const calculateTotal = (items: Item[]): number => {
if (!items?.length) return 0;
return items.reduce((total, item) => {
if (item.price < 0 || item.quantity < 0) {
throw new Error('Price and quantity must be non-negative');
}
return total + (item.price \* item.quantity);
}, 0);
};

// Trying to do everything at once
const calculateTotal = (items: Item[]): number => {
  // Validate, optimize, add features, handle edge cases all together
  if (!items?.length) return 0;
  const validItems = items.filter(item => {
    if (item.price < 0) throw new Error('Negative price');
    if (item.quantity < 0) throw new Error('Negative quantity');
    return item.quantity > 0; // Also filtering zero quantities
  });
  // Plus caching, plus logging, plus currency conversion...
  return validItems.reduce(...); // Too many concerns at once
};

// Error: string status can be any value
type OrderBad = {
  status: string; // Can be "pending", "PENDING", "pnding", anything!
  total: number;
};
// Good: Only valid states possible
type OrderStatus = 'pending' | 'processing' | 'shipped' | 'delivered';
type Order = {
status: OrderStatus;
total: number;
};
// Better: States with associated data
type Order =
| { status: 'pending'; createdAt: Date }
| { status: 'processing'; startedAt: Date; estimatedCompletion: Date }
| { status: 'shipped'; trackingNumber: string; shippedAt: Date }
| { status: 'delivered'; deliveredAt: Date; signature: string };
// Now impossible to have shipped without trackingNumber

// Make invalid states unrepresentable
type NonEmptyArray<T> = [T, ...T[]];
const firstItem = <T>(items: NonEmptyArray<T>): T => {
  return items[0]; // Always safe, never undefined!
};
// Caller must prove array is non-empty
const items: number[] = [1, 2, 3];
if (items.length > 0) {
  firstItem(items as NonEmptyArray<number>); // Safe
}

// Error: Validation after use
const processPayment = (amount: number) => {
  const fee = amount * 0.03; // Used before validation!
  if (amount <= 0) throw new Error('Invalid amount');
  // ...
};
// Good: Validate immediately
const processPayment = (amount: number) => {
if (amount <= 0) {
throw new Error('Payment amount must be positive');
}
if (amount > 10000) {
throw new Error('Payment exceeds maximum allowed');
}
const fee = amount \* 0.03;
// ... now safe to use
};
// Better: Validation at boundary with branded type
type PositiveNumber = number & { readonly \_\_brand: 'PositiveNumber' };
const validatePositive = (n: number): PositiveNumber => {
if (n <= 0) throw new Error('Must be positive');
return n as PositiveNumber;
};
const processPayment = (amount: PositiveNumber) => {
// amount is guaranteed positive, no need to check
const fee = amount \* 0.03;
};
// Validate at system boundary
const handlePaymentRequest = (req: Request) => {
const amount = validatePositive(req.body.amount); // Validate once
processPayment(amount); // Use everywhere safely
};

// Early returns prevent deeply nested code
const processUser = (user: User | null) => {
  if (!user) {
    logger.error('User not found');
    return;
  }
  if (!user.email) {
    logger.error('User email missing');
    return;
  }
  if (!user.isActive) {
    logger.info('User inactive, skipping');
    return;
  }
  // Main logic here, guaranteed user is valid and active
  sendEmail(user.email, 'Welcome!');
};

// Error: Optional config with unsafe defaults
type ConfigBad = {
  apiKey?: string;
  timeout?: number;
};
const client = new APIClient({ timeout: 5000 }); // apiKey missing!
// Good: Required config, fails early
type Config = {
apiKey: string;
timeout: number;
};
const loadConfig = (): Config => {
const apiKey = process.env.API_KEY;
if (!apiKey) {
throw new Error('API_KEY environment variable required');
}
return {
apiKey,
timeout: 5000,
};
};
// App fails at startup if config invalid, not during request
const config = loadConfig();
const client = new APIClient(config);

// Existing codebase pattern for API clients
class UserAPIClient {
  async getUser(id: string): Promise<User> {
    return this.fetch(`/users/${id}`);
  }
}
// New code follows the same pattern
class OrderAPIClient {
  async getOrder(id: string): Promise<Order> {
    return this.fetch(`/orders/${id}`);
  }
}

// Existing pattern uses classes
class UserAPIClient { /* ... */ }
// New code introduces different pattern without discussion
const getOrder = async (id: string): Promise<Order> => {
// Breaking consistency "because I prefer functions"
};

// Project standard: Result type for recoverable errors
type Result<T, E> = { ok: true; value: T } | { ok: false; error: E };
// All services follow this pattern
const fetchUser = async (id: string): Promise<Result<User, Error>> => {
  try {
    const user = await db.users.findById(id);
    if (!user) {
      return { ok: false, error: new Error('User not found') };
    }
    return { ok: true, value: user };
  } catch (err) {
    return { ok: false, error: err as Error };
  }
};
// Callers use consistent pattern
const result = await fetchUser('123');
if (!result.ok) {
  logger.error('Failed to fetch user', result.error);
  return;
}
const user = result.value; // Type-safe!

/**
 * Retries an async operation with exponential backoff.
 *
 * Why: Network requests fail temporarily; retrying improves reliability
 * When to use: External API calls, database operations
 * When not to use: User input validation, internal function calls
 *
 * @example
 * const result = await retry(
 *   () => fetch('https://api.example.com/data'),
 *   { maxAttempts: 3, baseDelay: 1000 }
 * );
 */
const retry = async <T>(
  operation: () => Promise<T>,
  options: RetryOptions
): Promise<T> => {
  // Implementation...
};