Agent skill

frontend-dev-guidelines

Frontend development guidelines for React/TypeScript applications. Modern patterns including Suspense, lazy loading, useSWR, file organization with features directory, shadcn/ui components, Tailwind CSS styling, Next.js App Router, performance optimization, and TypeScript best practices. Use when creating components, pages, features, fetching data, styling, routing, or working with frontend code.

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SKILL.md

Frontend Dev Guidelines (Senior Level)

This skill is at senior frontend developer knowledge level. Memorize patterns, recognize edge cases, reject anti-patterns.


1. Senior Developer Mindset

Think performance-first:

  • Ask "is this causing unnecessary renders?" for every component
  • Detect network waterfalls, use parallel fetch
  • Monitor bundle size, lazy load heavy components

Internalize patterns:

  • React.FC<Props> + TypeScript always
  • useSWR with suspense: true is standard
  • SuspenseLoader NEVER early return with spinner

Reject anti-patterns:

  • Convert barrel file imports to direct imports
  • Replace sequential await with Promise.all()
  • Replace hardcoded colors with globals.css variables

2. Non-Negotiables (CRITICAL)

Component Patterns

typescript
// ✅ STANDARD PATTERN
import React from 'react';
import useSWR from 'swr';
import { SuspenseLoader } from '@/shared/components/SuspenseLoader';

const HeavyComponent = React.lazy(() => import('./HeavyComponent'));

interface MyComponentProps {
    id: string;
    className?: string;
}

export const MyComponent: React.FC<MyComponentProps> = ({ id, className }) => {
    const { data } = useSWR(`key-${id}`, () => api.get(id), { suspense: true });
    return <div className={cn("base-class", className)}>{data?.title}</div>;
};

// Usage - with Suspense boundary
<SuspenseLoader>
    <MyComponent id="123" />
</SuspenseLoader>

Performance Rules (Top 15)

Priority Rule Description
CRITICAL Promise.all() Parallelize independent async operations
CRITICAL Direct imports Avoid barrel files, import directly
CRITICAL next/dynamic Dynamic import heavy components
CRITICAL Suspense boundaries Stream content, prevent waterfalls
HIGH Defer await Move await to the branch where it's used
HIGH Minimize RSC serialization Send less data to client
HIGH React.cache() Per-request deduplication
MEDIUM SWR deduplication Auto request deduplication
MEDIUM Functional setState setCount(c => c + 1) stable callback
MEDIUM Lazy state init useState(() => expensiveCalc())
MEDIUM startTransition For non-urgent updates
MEDIUM content-visibility CSS optimize for long lists
LOW Index maps Use Map for repeated lookups
LOW toSorted() For immutable sort
LOW Set/Map lookups O(1) lookup instead of array

Accessibility Rules (Top 10)

Rule Description
aria-label on icon buttons ALWAYS add aria-label to icon-only buttons
button vs a/Link Action = button, Navigation = a/Link
alt on images Add alt to every img (decorative: alt="")
Keyboard handlers Add keyboard support to interactive elements
Heading hierarchy Maintain h1 → h2 → h3 order
autocomplete on inputs Add autocomplete to form inputs
Inline errors Show errors next to field
Submit button enabled Don't disable submit, show loading spinner
Virtualize long lists Virtualize lists with 50+ items
prefers-reduced-motion Check motion preference in animations

3. Browser Compatibility (SAFARI CRITICAL)

Check Safari on every code change! Safari is the most problematic browser.

Issue Safari Fix Reference
new Date('2024-01-15 10:30:00') ISO 8601: '2024-01-15T10:30:00' browser-compatibility.md
height: 100vh height: 100dvh or -webkit-fill-available browser-compatibility.md
backdrop-filter Add -webkit-backdrop-filter prefix browser-compatibility.md
Video autoplay playsInline + muted attributes browser-compatibility.md
Input zoom (iOS) font-size: 16px minimum browser-compatibility.md
Touch events { passive: true } listener browser-compatibility.md
Clipboard API Fallback textarea method browser-compatibility.md

Test Checklist (Every PR):

  • Chrome (latest)
  • Safari (macOS)
  • Safari (iOS) - Real device or simulator
  • Firefox (latest)

4. Anti-Patterns (NEVER DO)

Anti-Pattern Correct Pattern Reference
if (isLoading) return <Spinner /> Wrap with <SuspenseLoader> core-patterns.md
Sequential await Use Promise.all() performance-guide.md
Barrel file import Use direct import performance-guide.md
Hardcoded color bg-[#fe4601] CSS variable bg-orange-1 styling-routing.md
<div onClick> <button onClick> accessibility-guide.md
Icon button without label Add aria-label accessibility-guide.md
Server data in useState Use useSWR core-patterns.md
next/router import Use next/navigation styling-routing.md
dangerouslySetInnerHTML DOMPurify sanitization security-error-handling.md
any type unknown + type guard advanced-typescript.md
Type assertion as User Runtime validation (Zod) advanced-typescript.md

5. Decision Trees

useState vs Zustand vs SWR

What's the data source?
├─ Server/API → SWR (suspense: true)
├─ Form input → useState (local)
├─ UI state (modal, sidebar) → useState (local)
└─ Shared across components?
   ├─ Server data → SWR (auto-shares)
   └─ Client state → Zustand (ONLY)

Server vs Client Component

What does the component do?
├─ Static content → Server Component
├─ Data fetch (no interaction) → Server Component
├─ Needs useState/useEffect → Client Component
├─ Has onClick/onChange handler → Client Component
├─ Uses Browser API → Client Component
└─ Third-party client library → Client Component

useMemo vs React.memo vs useCallback

What are you optimizing?
├─ Expensive calculation → useMemo
├─ Object/array reference stability → useMemo
├─ Function reference stability → useCallback
├─ Child component re-render → React.memo (on child)
└─ Event handler in dependency → useCallback

Lazy Loading Decision

Is the component heavy?
├─ DataGrid/Table → lazy load
├─ Chart/Graph → lazy load
├─ Rich text editor → lazy load
├─ Video player → lazy load
├─ Small utility component → don't lazy load
└─ Above-the-fold critical → don't lazy load

6. Quick Reference

For detailed info: styling-routing.md

Topic Source
Import aliases (@/, @/shared, @/features) styling-routing.md
Feature directory structure styling-routing.md
Route groups ((dashboard), (main), etc.) styling-routing.md
Color palettes (globals.css) styling-routing.md

7. Resources (Detailed Info)

Topic Resource
Component, data fetching, React 19 hooks, state core-patterns.md
All performance rules (45 rules) performance-guide.md
Accessibility patterns (WCAG 2.1) accessibility-guide.md
Tailwind, routing, file organization styling-routing.md
Testing patterns, complete examples testing-examples.md
Safari/Chrome/Firefox compatibility, Senior FE skills browser-compatibility.md
XSS/CSRF prevention, error boundaries, resilience security-error-handling.md
Generics, type guards, utility types, inference advanced-typescript.md

Quick Template

typescript
import React from 'react';
import useSWR from 'swr';
import { Card, CardHeader, CardTitle, CardContent } from '@/shared/components/ui/card';
import { cn } from '@/shared/utils/cn';
import { featureApi } from '../api/featureApi';

interface FeatureCardProps {
    id: string;
    className?: string;
}

export const FeatureCard: React.FC<FeatureCardProps> = ({ id, className }) => {
    const { data } = useSWR(`feature-${id}`, () => featureApi.get(id), { suspense: true });

    return (
        <Card className={cn("w-full", className)}>
            <CardHeader>
                <CardTitle>{data?.title}</CardTitle>
            </CardHeader>
            <CardContent>
                <p className="text-muted-foreground">{data?.description}</p>
            </CardContent>
        </Card>
    );
};

Related Skills

  • react-best-practices: Full 45 performance rules (Vercel Engineering)
  • web-design-guidelines: Full UI/UX/a11y rules (Vercel Labs)
  • error-tracking: Sentry integration

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