Each step is complete, tested, and working
</Good>

<Bad>
```typescript
// 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
};

Type system prevents entire classes of errors
</Good>

<Good>
```typescript
// 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
}

Validate once at boundary, safe everywhere else
</Good>

Guards make assumptions explicit and enforced
</Good>

Fail at startup, not in production
</Good>

Consistency makes codebase predictable
</Good>

<Bad>
```typescript
// 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"
};

Standard pattern across codebase
</Good>

Building for imaginary future requirements
</Bad>

**When to add complexity:**
- Current requirement demands it
- Pain points identified through use
- Measured performance issues
- Multiple use cases emerged

<Good>
```typescript
// Start simple
const formatCurrency = (amount: number): string => {
  return `$${amount.toFixed(2)}`;
};

// Requirement evolves: support multiple currencies
const formatCurrency = (amount: number, currency: string): string => {
  const symbols = { USD: '$', EUR: '€', GBP: '£' };
  return `${symbols[currency]}${amount.toFixed(2)}`;
};

// Requirement evolves: support localization
const formatCurrency = (amount: number, locale: string): string => {
  return new Intl.NumberFormat(locale, {
    style: 'currency',
    currency: locale === 'en-US' ? 'USD' : 'EUR',
  }).format(amount);
};

Massive abstraction for uncertain future
</Bad>

<Good>
```typescript
// Simple functions for current needs
const getUsers = async (): Promise<User[]> => {
  return db.query('SELECT * FROM users');
};

const getUserById = async (id: string): Promise<User | null> => {
  return db.query('SELECT * FROM users WHERE id = $1', [id]);
};

// When pattern emerges across multiple entities, then abstract

Optimize based on measurement, not assumptions
</Good>

<Bad>
```typescript
// Premature optimization
const filterActiveUsers = (users: User[]): User[] => {
  // "This might be slow, so let's cache and index"
  const cache = new WeakMap();
  const indexed = buildBTreeIndex(users, 'isActive');
  // 100 lines of optimization code
  // Adds complexity, harder to maintain
  // No evidence it was needed
};