# Coding Standards & Best Practices
Baseline coding conventions applicable across projects.
This skill is the shared floor, not the detailed framework playbook.
- Use `frontend-patterns` for React, state, forms, rendering, and UI architecture.
- Use `backend-patterns` or `api-design` for repository/service layers, endpoint design, validation, and server-specific concerns.
- Use `rules/common/coding-style.md` when you need the shortest reusable rule layer instead of a full skill walkthrough.
When to Activate
- Starting a new project or module
- Reviewing code for quality and maintainability
- Refactoring existing code to follow conventions
- Enforcing naming, formatting, or structural consistency
- Setting up linting, formatting, or type-checking rules
- Onboarding new contributors to coding conventions
Scope Boundaries
Activate this skill for: - descriptive naming - immutability defaults - readability, KISS, DRY, and YAGNI enforcement - error-handling expectations and code-smell review
Do not use this skill as the primary source for: - React composition, hooks, or rendering patterns - backend architecture, API design, or database layering - domain-specific framework guidance when a narrower ECC skill already exists
Code Quality Principles
### 1. Readability First - Code is read more than written - Clear variable and function names - Self-documenting code preferred over comments - Consistent formatting
### 2. KISS (Keep It Simple, Stupid) - Simplest solution that works - Avoid over-engineering - No premature optimization - Easy to understand > clever code
### 3. DRY (Don't Repeat Yourself) - Extract common logic into functions - Create reusable components - Share utilities across modules - Avoid copy-paste programming
### 4. YAGNI (You Aren't Gonna Need It) - Don't build features before they're needed - Avoid speculative generality - Add complexity only when required - Start simple, refactor when needed
TypeScript/JavaScript Standards
Variable Naming
```typescript // PASS: GOOD: Descriptive names const marketSearchQuery = 'election' const isUserAuthenticated = true const totalRevenue = 1000
// FAIL: BAD: Unclear names const q = 'election' const flag = true const x = 1000 ```
Function Naming
```typescript // PASS: GOOD: Verb-noun pattern async function fetchMarketData(marketId: string) { } function calculateSimilarity(a: number[], b: number[]) { } function isValidEmail(email: string): boolean { }
// FAIL: BAD: Unclear or noun-only async function market(id: string) { } function similarity(a, b) { } function email(e) { } ```
Immutability Pattern (CRITICAL)
```typescript // PASS: ALWAYS use spread operator const updatedUser = { ...user, name: 'New Name' }
const updatedArray = [...items, newItem]
// FAIL: NEVER mutate directly user.name = 'New Name' // BAD items.push(newItem) // BAD ```
Error Handling
```typescript // PASS: GOOD: Comprehensive error handling async function fetchData(url: string) { try { const response = await fetch(url)
if (!response.ok) { throw new Error(`HTTP ${response.status}: ${response.statusText}`) }
return await response.json() } catch (error) { console.error('Fetch failed:', error) throw new Error('Failed to fetch data') } }
// FAIL: BAD: No error handling async function fetchData(url) { const response = await fetch(url) return response.json() } ```
Async/Await Best Practices
```typescript // PASS: GOOD: Parallel execution when possible const [users, markets, stats] = await Promise.all([ fetchUsers(), fetchMarkets(), fetchStats() ])
// FAIL: BAD: Sequential when unnecessary const users = await fetchUsers() const markets = await fetchMarkets() const stats = await fetchStats() ```
Type Safety
```typescript // PASS: GOOD: Proper types interface Market { id: string name: string status: 'active' | 'resolved' | 'closed' created_at: Date }
function getMarket(id: string): Promise<Market> { // Implementation }
// FAIL: BAD: Using 'any' function getMarket(id: any): Promise<any> { // Implementation } ```
React Best Practices
Component Structure
```typescript // PASS: GOOD: Functional component with types interface ButtonProps { children: React.ReactNode onClick: () => void disabled?: boolean variant?: 'primary' | 'secondary' }
export function Button({ children, onClick, disabled = false, variant = 'primary' }: ButtonProps) { return ( <button onClick={onClick} disabled={disabled} className={`btn btn-${variant}`} > {children} </button> ) }
// FAIL: BAD: No types, unclear structure export function Button(props) { return <button onClick={props.onClick}>{props.children}</button> } ```
Custom Hooks
```typescript // PASS: GOOD: 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 }
// Usage const debouncedQuery = useDebounce(searchQuery, 500) ```
State Management
```typescript // PASS: GOOD: Proper state updates const [count, setCount] = useState(0)
// Functional update for state based on previous state setCount(prev => prev + 1)
// FAIL: BAD: Direct state reference setCount(count + 1) // Can be stale in async scenarios ```
Conditional Rendering
```typescript // PASS: GOOD: Clear conditional rendering {isLoading && <Spinner />} {error && <ErrorMessage error={error} />} {data && <DataDisplay data={data} />}
// FAIL: BAD: Ternary hell {isLoading ? <Spinner /> : error ? <ErrorMessage error={error} /> : data ? <DataDisplay data={data} /> : null} ```
API Design Standards
REST API Conventions
``` GET /api/markets # List all markets GET /api/markets/:id # Get specific market POST /api/markets # Create new market PUT /api/markets/:id # Update market (full) PATCH /api/markets/:id # Update market (partial) DELETE /api/markets/:id # Delete market
# Query parameters for filtering GET /api/markets?status=active&limit=10&offset=0 ```
Response Format
```typescript // PASS: GOOD: Consistent response structure interface ApiResponse<T> { success: boolean data?: T error?: string meta?: { total: number page: number limit: number } }
// Success response return NextResponse.json({ success: true, data: markets, meta: { total: 100, page: 1, limit: 10 } })
// Error response return NextResponse.json({ success: false, error: 'Invalid request' }, { status: 400 }) ```
Input Validation
```typescript import { z } from 'zod'
// PASS: GOOD: Schema validation const CreateMarketSchema = z.object({ name: z.string().min(1).max(200), description: z.string().min(1).max(2000), endDate: z.string().datetime(), categories: z.array(z.string()).min(1) })
export async function POST(request: Request) { const body = await request.json()
try { const validated = CreateMarketSchema.parse(body) // Proceed with validated data } catch (error) { if (error instanceof z.ZodError) { return NextResponse.json({ success: false, error: 'Validation failed', details: error.errors }, { status: 400 }) } } } ```
File Organization
Project Structure
``` src/ ├── app/ # Next.js App Router │ ├── api/ # API routes │ ├── markets/ # Market pages │ └── (auth)/ # Auth pages (route groups) ├── components/ # React components │ ├── ui/ # Generic UI components │ ├── forms/ # Form components │ └── layouts/ # Layout components ├── hooks/ # Custom React hooks ├── lib/ # Utilities and configs │ ├── api/ # API clients │ ├── utils/ # Helper functions │ └── constants/ # Constants ├── types/ # TypeScript types └── styles/ # Global styles ```
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 ```
Comments & Documentation
When to Comment
```typescript // PASS: GOOD: Explain WHY, not WHAT // Use exponential backoff to avoid overwhelming the API during outages const delay = Math.min(1000 * Math.pow(2, retryCount), 30000)
// Deliberately using mutation here for performance with large arrays items.push(newItem)
// FAIL: BAD: Stating the obvious // Increment counter by 1 count++
// Set name to user's name name = user.name ```
JSDoc for Public APIs
```typescript /** * Searches markets using semantic similarity. * * @param query - Natural language search query * @param limit - Maximum number of results (default: 10) * @returns Array of markets sorted by similarity score * @throws {Error} If OpenAI API fails or Redis unavailable * * @example * ```typescript * const results = await searchMarkets('election', 5) * console.log(results[0].name) // "Trump vs Biden" * ``` */ export async function searchMarkets( query: string, limit: number = 10 ): Promise<Market[]> { // Implementation } ```
Performance Best Practices
Memoization
```typescript import { useMemo, useCallback } from 'react'
// PASS: GOOD: Memoize expensive computations const sortedMarkets = useMemo(() => { return markets.sort((a, b) => b.volume - a.volume) }, [markets])
// PASS: GOOD: Memoize callbacks const handleSearch = useCallback((query: string) => { setSearchQuery(query) }, []) ```
Lazy Loading
```typescript import { lazy, Suspense } from 'react'
// PASS: GOOD: Lazy load heavy components const HeavyChart = lazy(() => import('./HeavyChart'))
export function Dashboard() { return ( <Suspense fallback={<Spinner />}> <HeavyChart /> </Suspense> ) } ```
Database Queries
```typescript // PASS: GOOD: Select only needed columns const { data } = await supabase .from('markets') .select('id, name, status') .limit(10)
// FAIL: BAD: Select everything const { data } = await supabase .from('markets') .select('*') ```
Testing Standards
Test Structure (AAA Pattern)
```typescript test('calculates similarity correctly', () => { // Arrange const vector1 = [1, 0, 0] const vector2 = [0, 1, 0]
// Act const similarity = calculateCosineSimilarity(vector1, vector2)
// Assert expect(similarity).toBe(0) }) ```
Test Naming
```typescript // PASS: GOOD: Descriptive test names test('returns empty array when no markets match query', () => { }) test('throws error when OpenAI API key is missing', () => { }) test('falls back to substring search when Redis unavailable', () => { })
// FAIL: BAD: Vague test names test('works', () => { }) test('test search', () => { }) ```
Code Smell Detection
Watch for these anti-patterns:
### 1. Long Functions ```typescript // FAIL: BAD: Function > 50 lines function processMarketData() { // 100 lines of code }
// PASS: GOOD: Split into smaller functions function processMarketData() { const validated = validateData() const transformed = transformData(validated) return saveData(transformed) } ```
### 2. Deep Nesting ```typescript // FAIL: BAD: 5+ levels of nesting if (user) { if (user.isAdmin) { if (market) { if (market.isActive) { if (hasPermission) { // Do something } } } } }
// PASS: GOOD: Early returns if (!user) return if (!user.isAdmin) return if (!market) return if (!market.isActive) return if (!hasPermission) return
// Do something ```
### 3. Magic Numbers ```typescript // FAIL: BAD: Unexplained numbers if (retryCount > 3) { } setTimeout(callback, 500)
// PASS: GOOD: Named constants const MAX_RETRIES = 3 const DEBOUNCE_DELAY_MS = 500
if (retryCount > MAX_RETRIES) { } setTimeout(callback, DEBOUNCE_DELAY_MS) ```
**Remember**: Code quality is not negotiable. Clear, maintainable code enables rapid development and confident refactoring.