Frontend React Development

Merged Skill: Combines project-specific frontend patterns with Vercel's React best practices.

Part 1: Project Patterns

SSR vs CSR Handling

Mounted State Pattern

Handle hydration mismatches when using client-side state (localStorage, window):

const [mounted, setMounted] = useState(false);
useEffect(() => setMounted(true), []);

if (!mounted) return null; // or loading skeleton

Applied In: ThemeToggle.tsx, Sidebar.tsx

Theme System

Use CSS variables to avoid FOUC and support system preferences:

/* Good */
bg-[var(--background-card)]
text-[var(--muted-foreground)]

/* Avoid */
bg-white
text-gray-500

Structure:

• •app/globals.css: Defines :root and .dark variables
• •lib/theme.ts: Theme state management
• •ThemeToggle.tsx: Theme switcher component

API Integration

All API calls must use apiFetch or apiUpload from lib/api.ts:

• •NO Direct fetch(): Never use native fetch for /api/* calls
• •X-User-Id: Auto-injected from local storage
• •Absolute URLs: Use APP_URL constant

Security

• •AuthGuard: Protects all (main) routes
• •Public Routes: Only /login and /ping-pong exempt
• •Injection Protection: Via apiFetch wrapper

Shared Types

Define in src/lib/types.ts:

• •Account, JournalEntry, JournalLine
• •BankStatement, BankStatementTransaction
• •ReportLine, BalanceSheetResponse, IncomeStatementResponse

Part 2: Vercel React Best Practices

Comprehensive performance optimization guide for React and Next.js applications.

Rule Categories by Priority

Quick Reference

1. Eliminating Waterfalls (CRITICAL)

• •async-defer-await - Move await into branches where actually used
• •async-parallel - Use Promise.all() for independent operations
• •async-dependencies - Use better-all for partial dependencies
• •async-api-routes - Start promises early, await late in API routes
• •async-suspense-boundaries - Use Suspense to stream content

2. Bundle Size Optimization (CRITICAL)

• •bundle-barrel-imports - Import directly, avoid barrel files
• •bundle-dynamic-imports - Use next/dynamic for heavy components
• •bundle-defer-third-party - Load analytics/logging after hydration
• •bundle-conditional - Load modules only when feature is activated
• •bundle-preload - Preload on hover/focus for perceived speed

3. Server-Side Performance (HIGH)

• •server-cache-react - Use React.cache() for per-request deduplication
• •server-cache-lru - Use LRU cache for cross-request caching
• •server-serialization - Minimize data passed to client components
• •server-parallel-fetching - Restructure components to parallelize fetches
• •server-after-nonblocking - Use after() for non-blocking operations

4. Client-Side Data Fetching (MEDIUM-HIGH)

• •client-swr-dedup - Use SWR for automatic request deduplication
• •client-event-listeners - Deduplicate global event listeners

5. Re-render Optimization (MEDIUM)

• •rerender-defer-reads - Don't subscribe to state only used in callbacks
• •rerender-memo - Extract expensive work into memoized components
• •rerender-dependencies - Use primitive dependencies in effects
• •rerender-derived-state - Subscribe to derived booleans, not raw values
• •rerender-functional-setstate - Use functional setState for stable callbacks
• •rerender-lazy-state-init - Pass function to useState for expensive values
• •rerender-transitions - Use startTransition for non-urgent updates

6. Rendering Performance (MEDIUM)

• •rendering-animate-svg-wrapper - Animate div wrapper, not SVG element
• •rendering-content-visibility - Use content-visibility for long lists
• •rendering-hoist-jsx - Extract static JSX outside components
• •rendering-svg-precision - Reduce SVG coordinate precision
• •rendering-hydration-no-flicker - Use inline script for client-only data
• •rendering-activity - Use Activity component for show/hide
• •rendering-conditional-render - Use ternary, not && for conditionals

7. JavaScript Performance (LOW-MEDIUM)

• •js-batch-dom-css - Group CSS changes via classes or cssText
• •js-index-maps - Build Map for repeated lookups
• •js-cache-property-access - Cache object properties in loops
• •js-cache-function-results - Cache function results in module-level Map
• •js-cache-storage - Cache localStorage/sessionStorage reads
• •js-combine-iterations - Combine multiple filter/map into one loop
• •js-length-check-first - Check array length before expensive comparison
• •js-early-exit - Return early from functions
• •js-hoist-regexp - Hoist RegExp creation outside loops
• •js-min-max-loop - Use loop for min/max instead of sort
• •js-set-map-lookups - Use Set/Map for O(1) lookups
• •js-tosorted-immutable - Use toSorted() for immutability

8. Advanced Patterns (LOW)

• •advanced-event-handler-refs - Store event handlers in refs
• •advanced-use-latest - useLatest for stable callback refs

How to Use

Read individual rule files for detailed explanations and code examples:

rules/async-parallel.md
rules/bundle-barrel-imports.md

For the complete guide with all rules expanded: AGENTS.md