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react-best-practices

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SKILL.md

Arcanea React Best Practices

"Leyla guards the Gate of Flow at 285 Hz. Your components must flow, not block. Every waterfall is a Flow violation."

Adapted from Vercel Engineering's 57-rule guide for the Arcanea stack: Next.js 16 App Router + React 19 + TypeScript strict + Supabase + Vercel AI SDK 6.

Rule Categories by Priority

Priority Category Impact Prefix
1 Eliminating Waterfalls CRITICAL async-
2 Bundle Size CRITICAL bundle-
3 Server-Side HIGH server-
4 Client Data Fetching MEDIUM-HIGH client-
5 Re-render Optimization MEDIUM rerender-
6 Rendering Performance MEDIUM rendering-
7 JavaScript Performance LOW-MEDIUM js-
8 Advanced Patterns LOW advanced-

1. Eliminating Waterfalls (CRITICAL)

async-parallel — Use Promise.all for independent fetches

typescript
// BAD — serial waterfall
const user = await getUser(id)
const posts = await getUserPosts(id) // waits for user unnecessarily

// GOOD — parallel
const [user, posts] = await Promise.all([getUser(id), getUserPosts(id)])

async-defer-await — Move await to where it's actually needed

typescript
// BAD
async function handler() {
  const data = await fetchData() // awaited before branch
  if (condition) return data
  return null
}

// GOOD
async function handler() {
  const dataPromise = fetchData() // start early
  if (!condition) return null
  return await dataPromise // await only when needed
}

async-suspense-boundaries — Stream content with Suspense

tsx
// In Arcanea pages — wrap slow data in Suspense for progressive rendering
export default function LorePage() {
  return (
    <Suspense fallback={<GlassCardSkeleton />}>
      <LoreContent /> {/* streams in independently */}
    </Suspense>
  )
}

async-api-routes — Start promises early in route handlers

typescript
// app/api/guardians/route.ts
export async function GET() {
  const guardianPromise = supabase.from('guardians').select() // start immediately
  // ... validation/auth work ...
  const { data } = await guardianPromise // await late
  return Response.json(data)
}

2. Bundle Size (CRITICAL)

bundle-barrel-imports — Import directly, never from barrel files

typescript
// BAD — pulls in entire barrel
import { Button, Card, Modal } from '@/components/ui'

// GOOD — direct imports
import { Button } from '@/components/ui/button'
import { Card } from '@/components/ui/card'

bundle-dynamic-imports — Lazy-load heavy components

typescript
// Heavy Arcanea components: GlobeMap, PromptEditor, LoreCanvas
import dynamic from 'next/dynamic'

const LoreCanvas = dynamic(() => import('@/components/lore/LoreCanvas'), {
  loading: () => <GlassCardSkeleton />,
  ssr: false, // client-only canvas
})

bundle-defer-third-party — Analytics after hydration

typescript
// In Arcanea layout — defer non-critical scripts
import { GoogleAnalytics } from '@next/third-parties/google'
// Renders after hydration automatically via next/third-parties

3. Server-Side Performance (HIGH)

server-cache-react — Deduplicate DB calls per request

typescript
// lib/supabase-cached.ts — use React.cache for per-request dedup
import { cache } from 'react'
import { supabase } from '@/lib/supabase'

export const getGuardian = cache(async (gateName: string) => {
  const { data } = await supabase
    .from('guardians')
    .select()
    .eq('gate', gateName)
    .single()
  return data
})
// Multiple RSCs can call getGuardian('Fire') — only one DB query

server-auth-actions — Always authenticate Server Actions

typescript
// app/actions/prompt.ts
'use server'
import { createServerClient } from '@/lib/supabase-server'

export async function savePrompt(data: PromptData) {
  const supabase = createServerClient()
  const { data: { user } } = await supabase.auth.getUser()
  if (!user) throw new Error('Unauthorized') // NEVER skip this
  // ...
}

server-parallel-fetching — Restructure RSC to parallelize

typescript
// BAD — sequential RSC
async function GuardianPage({ gate }: { gate: string }) {
  const guardian = await getGuardian(gate) // serial
  const godbeast = await getGodbeast(guardian.id) // waits
}

// GOOD — parallel RSC
async function GuardianPage({ gate }: { gate: string }) {
  const [guardian, godbeast] = await Promise.all([
    getGuardian(gate),
    getGodBeastByGate(gate), // use gate directly
  ])
}

4. Client Data Fetching (MEDIUM-HIGH)

client-swr-dedup — SWR/React Query for client fetches

typescript
// Prefer SWR for client-side Arcanea data
import useSWR from 'swr'

function useGuardians() {
  return useSWR('/api/guardians', fetcher, {
    revalidateOnFocus: false, // lore data doesn't change often
  })
}

5. Re-render Optimization (MEDIUM)

rerender-memo — Memoize expensive Arcanea components

typescript
// Heavy visual components: GuardianCard, LoreGrid, GodBeastProfile
import { memo } from 'react'

const GuardianCard = memo(function GuardianCard({ guardian }: Props) {
  return <div className="glass-card">...</div>
})

rerender-derived-state-no-effect — Derive during render

typescript
// BAD — unnecessary effect
const [filteredGuardians, setFiltered] = useState([])
useEffect(() => {
  setFiltered(guardians.filter(g => g.element === selectedElement))
}, [guardians, selectedElement])

// GOOD — derive during render
const filteredGuardians = guardians.filter(g => g.element === selectedElement)

rerender-lazy-state-init — Lazy init for expensive state

typescript
// BAD — recalculates every render
const [state, setState] = useState(computeExpensiveDefault())

// GOOD — only runs once
const [state, setState] = useState(() => computeExpensiveDefault())

rerender-transitions — Non-urgent updates via startTransition

typescript
import { useTransition } from 'react'

function GateFilter() {
  const [isPending, startTransition] = useTransition()
  
  return (
    <select onChange={e => startTransition(() => setGate(e.target.value))}>
      {/* Gate filter won't block urgent UI updates */}
    </select>
  )
}

6. Rendering Performance (MEDIUM)

rendering-hoist-jsx — Extract static JSX outside components

typescript
// BAD — recreated every render
function GuardianList() {
  const header = <h2 className="text-gradient-aurora">The Ten Gates</h2>
  return <div>{header}{items}</div>
}

// GOOD — hoisted static
const HEADER = <h2 className="text-gradient-aurora">The Ten Gates</h2>
function GuardianList() {
  return <div>{HEADER}{items}</div>
}

rendering-conditional-render — Ternary, not &&

typescript
// BAD — renders "0" if count is 0
{count && <Badge>{count}</Badge>}

// GOOD
{count > 0 ? <Badge>{count}</Badge> : null}

7. JavaScript Performance (LOW-MEDIUM)

js-index-maps — Map for repeated lookups

typescript
// BAD — O(n) per lookup
function getGuardian(gate: string) {
  return guardians.find(g => g.gate === gate)
}

// GOOD — O(1) lookup
const guardianMap = new Map(guardians.map(g => [g.gate, g]))
const getGuardian = (gate: string) => guardianMap.get(gate)

js-early-exit — Return early from functions

typescript
// GOOD Arcanea pattern
function validateGateUnlock(user: User, gate: Gate): ValidationResult {
  if (!user.authenticated) return { valid: false, reason: 'unauthenticated' }
  if (user.gatesOpen < gate.requiredRank) return { valid: false, reason: 'rank' }
  if (gate.locked) return { valid: false, reason: 'sealed' }
  return { valid: true }
}

8. Arcanea-Specific Patterns

AI SDK 6 — Vercel AI SDK streaming

typescript
// app/api/guardian-chat/route.ts — AI SDK 6 pattern
import { streamText } from 'ai'
import { google } from '@ai-sdk/google'

export async function POST(req: Request) {
  const { messages } = await req.json()
  const result = streamText({
    model: google('gemini-2.5-flash'),
    messages,
    maxOutputTokens: 2048, // NOT maxTokens (SDK 6 change)
  })
  return result.toUIMessageStreamResponse() // NOT toDataStreamResponse (SDK 6)
}

Supabase Auth in Server Components

typescript
// Always use server-side auth for SSR
import { createServerClient } from '@supabase/ssr'
import { cookies } from 'next/headers'

export async function getServerUser() {
  const cookieStore = cookies()
  const supabase = createServerClient(
    process.env.NEXT_PUBLIC_SUPABASE_URL!,
    process.env.NEXT_PUBLIC_SUPABASE_ANON_KEY!,
    { cookies: { get: (name) => cookieStore.get(name)?.value } }
  )
  const { data: { user } } = await supabase.auth.getUser()
  return user
}

Glass Components — Design System compliance

typescript
// Always use Arcanean Design System classes
// See: .arcanea/design/DESIGN_BIBLE.md

// Glass tiers: glass-subtle (8px) | glass (16px) | glass-strong (24px) | liquid-glass (40px)
// Text: text-gradient-aurora | text-gradient-gold | text-gradient-violet
// Animations: float | pulse-glow | shimmer | cosmic-drift
// FONTS: Inter everywhere (NO Cinzel in new code — MEMORY.md override)

function GuardianCard({ guardian }: Props) {
  return (
    <div className="glass hover-lift glow-card">
      <h3 className="text-gradient-aurora">{guardian.name}</h3>
    </div>
  )
}

Quick Checklist

Before any React/Next.js PR in Arcanea:

  • No sequential awaits where Promise.all applies
  • Heavy components use next/dynamic
  • No barrel imports — direct imports only
  • Server Actions authenticate with Supabase before any mutation
  • React.cache() wraps repeated DB calls in same RSC tree
  • State derived during render, not in useEffect
  • Vercel AI SDK 6: maxOutputTokens and toUIMessageStreamResponse
  • Glass components use Design Bible classes
  • Fonts: Inter only (no Cinzel in new code)

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