Design Principles

Design rules that complement fn(args, deps), Result types, and validation boundaries.

Critical Rules

Fail-Fast Over Fallbacks

Never use nullish coalescing chains:

// WRONG - Hides bugs, debugging nightmare
const name = user?.profile?.name ?? settings?.defaultName ?? 'Unknown';

// CORRECT - Fail immediately with context
if (!user) {
  throw new Error(`Expected user, got null. Context: userId=${userId}`);
}
if (!user.profile) {
  throw new Error(`User ${user.id} has no profile`);
}
return user.profile.name;

Error message format: Expected [X]. Got [Y]. Context: [debugging info]

No Type Escape Hatches

Forbidden without explicit user approval:

// FORBIDDEN
const x: any = something;
const y = something as SomeType;
const z = something as unknown as OtherType;
// @ts-ignore
// @ts-expect-error

There is always a type-safe alternative:

// Instead of `as`, use type guards
function isUser(x: unknown): x is User {
  return typeof x === 'object' && x !== null && 'id' in x;
}

if (isUser(data)) {
  // data is User here
}

// Instead of `any`, use unknown + validation
const data: unknown = JSON.parse(input);
const user = UserSchema.parse(data);  // Zod validates and types

Make Illegal States Unrepresentable

Use discriminated unions, not optional fields:

// WRONG - Illegal states possible
type Order = {
  status: string;
  shippedDate?: Date;      // Can be set when status !== 'shipped'
  cancelReason?: string;   // Can be set when status !== 'cancelled'
};

// CORRECT - Type system prevents illegal states
type Order =
  | { status: 'pending'; items: Item[] }
  | { status: 'shipped'; items: Item[]; shippedDate: Date; trackingNumber: string }
  | { status: 'cancelled'; items: Item[]; cancelReason: string };

If a state combination shouldn't exist, make the type forbid it.

Domain Names Only

Forbidden generic names:

• data, info, item
• utils, helpers, common, shared
• manager, handler, processor, service (when vague)

Use domain language:

// WRONG
class DataProcessor {
  processData(data: any) { }
}
function handleItem(item: Item) { }
const utils = { formatThing };

// CORRECT
class OrderTotalCalculator {
  calculate(order: Order): Money { }
}
function shipOrder(order: Order) { }
const priceFormatter = { formatCurrency };

No Code Comments

Comments indicate failure to express intent in code:

// WRONG - Comment explains unclear code
// Check if user is admin and not suspended
if (user.role === 'admin' && !user.suspendedAt) { }

// CORRECT - Code is self-documenting
const isActiveAdmin = user.role === 'admin' && !user.suspendedAt;
if (isActiveAdmin) { }

// Or extract to function
function isActiveAdmin(user: User): boolean {
  return user.role === 'admin' && !user.suspendedAt;
}

Acceptable comments:

• // TODO: with ticket reference
• Legal/license headers
• Complex regex explanations (but prefer named patterns)

Immutability by Default

Return new values, don't mutate inputs:

// WRONG - Mutates input
function addItem(order: Order, item: Item): void {
  order.items.push(item);  // Caller's object changed!
}

// CORRECT - Returns new value
function addItem(order: Order, item: Item): Order {
  return {
    ...order,
    items: [...order.items, item],
  };
}

Prefer:

• const over let
• Spread (...) over mutation
• map/filter/reduce over forEach with mutation

When mutation IS acceptable:

• Building arrays in loops (push is faster than spread for large arrays)
• Performance-critical hot paths (measure first)
• Local scope only (never mutate inputs, only local variables)

// OK: Mutation in local scope for performance
function processLargeDataset(items: Item[]): ProcessedItem[] {
  const results: ProcessedItem[] = [];  // Local mutable array
  for (const item of items) {
    results.push(transform(item));  // Much faster than spread
  }
  return results;  // Return immutable result
}

Feature Envy Detection

When a function uses another object's data more than its own, move the logic:

// FEATURE ENVY - obsessed with Order's internals
function calculateInvoiceTotal(order: Order): Money {
  return order.items
    .map(i => i.price * i.quantity)
    .reduce((a, b) => a + b, 0)
    + order.taxRate * subtotal
    + order.shippingCost;
}

// CORRECT - Logic belongs on Order
class Order {
  calculateTotal(): Money {
    // Uses this.items, this.taxRate, this.shippingCost
  }
}

function createInvoice(order: Order): Invoice {
  return new Invoice(order.calculateTotal());
}

Detection: Count references to this vs external objects. More external? Feature envy.

YAGNI - You Aren't Gonna Need It

Don't build for hypothetical future needs:

// WRONG - Speculative generalization
interface PaymentProcessor {
  process(payment: Payment): Result<Receipt, PaymentError>;
  refund(payment: Payment): Result<Receipt, PaymentError>;
  partialRefund(payment: Payment, amount: Money): Result<Receipt, PaymentError>;
  schedulePayment(payment: Payment, date: Date): Result<Receipt, PaymentError>;
  recurringPayment(payment: Payment, schedule: Schedule): Result<Receipt, PaymentError>;
  // ... 10 more methods "we might need"
}

// CORRECT - Build what you need now
interface PaymentProcessor {
  process(payment: Payment): Result<Receipt, PaymentError>;
}
// Add refund() when requirements actually demand it

"But we might need it" is not a requirement.

Object Calisthenics (Adapted)

No ELSE keyword

// WRONG
function getStatus(user: User): string {
  if (user.isAdmin) {
    return 'admin';
  } else {
    return 'user';
  }
}

// CORRECT - Early return
function getStatus(user: User): string {
  if (user.isAdmin) return 'admin';
  return 'user';
}

Keep entities small

• Functions: < 20 lines
• Files: < 200 lines
• If larger, split

One level of indentation (prefer max 2)

// WRONG - 4 levels deep
function process(orders: Order[]) {
  for (const order of orders) {
    for (const item of order.items) {
      if (item.inStock) {
        if (item.price > 0) {
          // deeply nested
        }
      }
    }
  }
}

// CORRECT - Extract and flatten
function process(orders: Order[]) {
  const items = orders.flatMap(o => o.items);
  const validItems = items.filter(isValidItem);
  validItems.forEach(processItem);
}

Integration with Other Skills

When Tempted to Cut Corners