Coding Standards

---
name: coding-standards
description: |
  Universal coding standards, best practices, and patterns for TypeScript, JavaScript, React, and Node.js development.
  Use when writing any code to ensure consistent quality.
allowed-tools: Read, Write, Edit, Grep, Glob
---

# Coding Standards & Best Practices

## Code Quality Principles

### 1. Readability First
- Code is read more than written
- Clear variable and function names
- Self-documenting code preferred over comments

### 2. KISS (Keep It Simple, Stupid)
- Simplest solution that works
- Avoid over-engineering
- Easy to understand > clever code

### 3. DRY (Don't Repeat Yourself)
- Extract common logic into functions
- Create reusable components

### 4. YAGNI (You Aren't Gonna Need It)
- Don't build features before they're needed
- Start simple, refactor when needed

## TypeScript/JavaScript Standards

### Variable Naming

```typescript
// GOOD: Descriptive names
const marketSearchQuery = 'election'
const isUserAuthenticated = true
const totalRevenue = 1000

// BAD: Unclear names
const q = 'election'
const flag = true
const x = 1000
```

### Function Naming

```typescript
// GOOD: Verb-noun pattern
async function fetchMarketData(marketId: string) { }
function calculateSimilarity(a: number[], b: number[]) { }
function isValidEmail(email: string): boolean { }

// BAD: Unclear or noun-only
async function market(id: string) { }
function similarity(a, b) { }
```

### Immutability Pattern (CRITICAL)

```typescript
// ALWAYS use spread operator
const updatedUser = {
  ...user,
  name: 'New Name'
}

const updatedArray = [...items, newItem]

// NEVER mutate directly
user.name = 'New Name'  // BAD
items.push(newItem)     // BAD
```

### Error Handling

```typescript
// GOOD: Comprehensive error handling
async function fetchData(url: string) {
  try {
    const response = await fetch(url)

    if (!response.ok) {
      throw new Error(`HTTP ${response.status}: ${response.statusText}`)
    }

    return await response.json()
  } catch (error) {
    console.error('Fetch failed:', error)
    throw new Error('Failed to fetch data')
  }
}

// BAD: No error handling
async function fetchData(url) {
  const response = await fetch(url)
  return response.json()
}
```

### Async/Await Best Practices

```typescript
// GOOD: Parallel execution when possible
const [users, markets, stats] = await Promise.all([
  fetchUsers(),
  fetchMarkets(),
  fetchStats()
])

// BAD: Sequential when unnecessary
const users = await fetchUsers()
const markets = await fetchMarkets()
const stats = await fetchStats()
```

### Type Safety

```typescript
// GOOD: Proper types
interface Market {
  id: string
  name: string
  status: 'active' | 'resolved' | 'closed'
}

function getMarket(id: string): Promise<Market> {
  // Implementation
}

// BAD: Using 'any'
function getMarket(id: any): Promise<any> {
  // Implementation
}
```

## React Best Practices

### Component Structure

```typescript
// GOOD: Functional component with types
interface ButtonProps {
  children: React.ReactNode
  onClick: () => void
  disabled?: boolean
  variant?: 'primary' | 'secondary'
}

export function Button({
  children,
  onClick,
  disabled = false,
  variant = 'primary'
}: ButtonProps) {
  return (
    <button
      onClick={onClick}
      disabled={disabled}
      className={`btn btn-${variant}`}
    >
      {children}
    </button>
  )
}
```

### Custom Hooks

```typescript
// Reusable custom hook
export function useDebounce<T>(value: T, delay: number): T {
  const [debouncedValue, setDebouncedValue] = useState<T>(value)

  useEffect(() => {
    const handler = setTimeout(() => {
      setDebouncedValue(value)
    }, delay)

    return () => clearTimeout(handler)
  }, [value, delay])

  return debouncedValue
}
```

### State Management

```typescript
// GOOD: Functional update for state based on previous state
setCount(prev => prev + 1)

// BAD: Direct state reference (can be stale)
setCount(count + 1)
```

### Conditional Rendering

```typescript
// GOOD: Clear conditional rendering
{isLoading && <Spinner />}
{error && <ErrorMessage error={error} />}
{data && <DataDisplay data={data} />}

// BAD: Ternary hell
{isLoading ? <Spinner /> : error ? <ErrorMessage /> : data ? <DataDisplay /> : null}
```

## API Design Standards

### Response Format

```typescript
interface ApiResponse<T> {
  success: boolean
  data?: T
  error?: string
  meta?: {
    total: number
    page: number
    limit: number
  }
}

// Success response
return { success: true, data: markets, meta: { total: 100, page: 1, limit: 10 } }

// Error response
return { success: false, error: 'Invalid request' }
```

### Input Validation

```typescript
import { z } from 'zod'

const CreateMarketSchema = z.object({
  name: z.string().min(1).max(200),
  description: z.string().min(1).max(2000),
  endDate: z.string().datetime()
})

const validated = CreateMarketSchema.parse(body)
```

## File Organization

### Many Small Files > Few Large Files
- 200-400 lines typical
- 800 lines maximum
- High cohesion, low coupling
- Single responsibility per file

### File Naming
```
components/Button.tsx          # PascalCase for components
hooks/useAuth.ts              # camelCase with 'use' prefix
lib/formatDate.ts             # camelCase for utilities
types/market.types.ts         # camelCase with .types suffix
```

## Code Smell Detection

### Long Functions (>50 lines)
```typescript
// BAD: Split into smaller functions
function processMarketData() {
  // 100 lines of code
}

// GOOD
function processMarketData() {
  const validated = validateData()
  const transformed = transformData(validated)
  return saveData(transformed)
}
```

### Deep Nesting (>4 levels)
```typescript
// BAD: 5+ levels of nesting
if (user) {
  if (user.isAdmin) {
    if (market) {
      // ...
    }
  }
}

// GOOD: Early returns
if (!user) return
if (!user.isAdmin) return
if (!market) return
// Do something
```

### Magic Numbers
```typescript
// BAD
if (retryCount > 3) { }

// GOOD
const MAX_RETRIES = 3
if (retryCount > MAX_RETRIES) { }
```

## Checklist

- [ ] Code is readable and well-named
- [ ] Functions are small (<50 lines)
- [ ] Files are focused (<800 lines)
- [ ] No deep nesting (>4 levels)
- [ ] Proper error handling
- [ ] No console.log statements
- [ ] No hardcoded values
- [ ] No mutation (immutable patterns used)