No,Category,Issue,Keywords,Platform,Description,Do,Don't,Code Example Good,Code Example Bad,Severity
1,Async Waterfall,Defer Await,async await defer branch,React/Next.js,Move await into branches where actually used to avoid blocking unused code paths,Move await operations into branches where they're needed,Await at top of function blocking all branches,"if (skip) return { skipped: true }; const data = await fetch()","const data = await fetch(); if (skip) return { skipped: true }",Critical
2,Async Waterfall,Promise.all Parallel,promise all parallel concurrent,React/Next.js,Execute independent async operations concurrently using Promise.all(),Use Promise.all() for independent operations,Sequential await for independent operations,"const [user, posts] = await Promise.all([fetchUser(), fetchPosts()])","const user = await fetchUser(); const posts = await fetchPosts()",Critical
3,Async Waterfall,Dependency Parallelization,better-all dependency parallel,React/Next.js,Use better-all for operations with partial dependencies to maximize parallelism,Use better-all to start each task at earliest possible moment,Wait for unrelated data before starting dependent fetch,"await all({ user() {}, config() {}, profile() { return fetch((await this.$.user).id) } })","const [user, config] = await Promise.all([...]); const profile = await fetchProfile(user.id)",Critical
4,Async Waterfall,API Route Optimization,api route waterfall promise,React/Next.js,In API routes start independent operations immediately even if not awaited yet,Start promises early and await late,Sequential awaits in API handlers,"const sessionP = auth(); const configP = fetchConfig(); const session = await sessionP","const session = await auth(); const config = await fetchConfig()",Critical
5,Async Waterfall,Suspense Boundaries,suspense streaming boundary,React/Next.js,Use Suspense to show wrapper UI faster while data loads,Wrap async components in Suspense boundaries,Await data blocking entire page render,"<Suspense fallback={<Skeleton />}><DataDisplay /></Suspense>","const data = await fetchData(); return <DataDisplay data={data} />",High
6,Bundle Size,Barrel Imports,barrel import direct path,React/Next.js,Import directly from source files instead of barrel files to avoid loading unused modules,Import directly from source path,Import from barrel/index files,"import Check from 'lucide-react/dist/esm/icons/check'","import { Check } from 'lucide-react'",Critical
7,Bundle Size,Dynamic Imports,dynamic import lazy next,React/Next.js,Use next/dynamic to lazy-load large components not needed on initial render,Use dynamic() for heavy components,Import heavy components at top level,"const Monaco = dynamic(() => import('./monaco'), { ssr: false })","import { MonacoEditor } from './monaco-editor'",Critical
8,Bundle Size,Defer Third Party,analytics defer third-party,React/Next.js,Load analytics and logging after hydration since they don't block interaction,Load non-critical scripts after hydration,Include analytics in main bundle,"const Analytics = dynamic(() => import('@vercel/analytics'), { ssr: false })","import { Analytics } from '@vercel/analytics/react'",Medium
9,Bundle Size,Conditional Loading,conditional module lazy,React/Next.js,Load large data or modules only when a feature is activated,Dynamic import when feature enabled,Import large modules unconditionally,"useEffect(() => { if (enabled) import('./heavy.js') }, [enabled])","import { heavyData } from './heavy.js'",High
10,Bundle Size,Preload Intent,preload hover focus intent,React/Next.js,Preload heavy bundles on hover/focus before they're needed,Preload on user intent signals,Load only on click,"onMouseEnter={() => import('./editor')}","onClick={() => import('./editor')}",Medium
11,Server,React.cache Dedup,react cache deduplicate request,React/Next.js,Use React.cache() for server-side request deduplication within single request,Wrap data fetchers with cache(),Fetch same data multiple times in tree,"export const getUser = cache(async () => await db.user.find())","export async function getUser() { return await db.user.find() }",Medium
12,Server,LRU Cache Cross-Request,lru cache cross request,React/Next.js,Use LRU cache for data shared across sequential requests,Use LRU for cross-request caching,Refetch same data on every request,"const cache = new LRUCache({ max: 1000, ttl: 5*60*1000 })","Always fetch from database",High
13,Server,Minimize Serialization,serialization rsc boundary,React/Next.js,Only pass fields that client actually uses across RSC boundaries,Pass only needed fields to client components,Pass entire objects to client,"<Profile name={user.name} />","<Profile user={user} /> // 50 fields serialized",High
14,Server,Parallel Fetching,parallel fetch component composition,React/Next.js,Restructure components to parallelize data fetching in RSC,Use component composition for parallel fetches,Sequential fetches in parent component,"<Header /><Sidebar /> // both fetch in parallel","const header = await fetchHeader(); return <><div>{header}</div><Sidebar /></>",Critical
15,Server,After Non-blocking,after non-blocking logging,React/Next.js,Use Next.js after() to schedule work after response is sent,Use after() for logging/analytics,Block response for non-critical operations,"after(async () => { await logAction() }); return Response.json(data)","await logAction(); return Response.json(data)",Medium
16,Client,SWR Deduplication,swr dedup cache revalidate,React/Next.js,Use SWR for automatic request deduplication and caching,Use useSWR for client data fetching,Manual fetch in useEffect,"const { data } = useSWR('/api/users', fetcher)","useEffect(() => { fetch('/api/users').then(setUsers) }, [])",Medium-High
17,Client,Event Listener Dedup,event listener deduplicate global,React/Next.js,Share global event listeners across component instances,Use useSWRSubscription for shared listeners,Register listener per component instance,"useSWRSubscription('global-keydown', () => { window.addEventListener... })","useEffect(() => { window.addEventListener('keydown', handler) }, [])",Low
18,Rerender,Defer State Reads,state read callback subscription,React/Next.js,Don't subscribe to state only used in callbacks,Read state on-demand in callbacks,Subscribe to state used only in handlers,"const handleClick = () => { const params = new URLSearchParams(location.search) }","const params = useSearchParams(); const handleClick = () => { params.get('ref') }",Medium
19,Rerender,Memoized Components,memo extract expensive,React/Next.js,Extract expensive work into memoized components for early returns,Extract to memo() components,Compute expensive values before early return,"const UserAvatar = memo(({ user }) => ...); if (loading) return <Skeleton />","const avatar = useMemo(() => compute(user)); if (loading) return <Skeleton />",Medium
20,Rerender,Narrow Dependencies,effect dependency primitive,React/Next.js,Specify primitive dependencies instead of objects in effects,Use primitive values in dependency arrays,Use object references as dependencies,"useEffect(() => { console.log(user.id) }, [user.id])","useEffect(() => { console.log(user.id) }, [user])",Low
21,Rerender,Derived State,derived boolean subscription,React/Next.js,Subscribe to derived booleans instead of continuous values,Use derived boolean state,Subscribe to continuous values,"const isMobile = useMediaQuery('(max-width: 767px)')","const width = useWindowWidth(); const isMobile = width < 768",Medium
22,Rerender,Functional setState,functional setstate callback,React/Next.js,Use functional setState updates for stable callbacks and no stale closures,Use functional form: setState(curr => ...),Reference state directly in setState,"setItems(curr => [...curr, newItem])","setItems([...items, newItem]) // items in deps",Medium
23,Rerender,Lazy State Init,usestate lazy initialization,React/Next.js,Pass function to useState for expensive initial values,Use function form for expensive init,Compute expensive value directly,"useState(() => buildSearchIndex(items))","useState(buildSearchIndex(items)) // runs every render",Medium
24,Rerender,Transitions,starttransition non-urgent,React/Next.js,Mark frequent non-urgent state updates as transitions,Use startTransition for non-urgent updates,Block UI on every state change,"startTransition(() => setScrollY(window.scrollY))","setScrollY(window.scrollY) // blocks on every scroll",Medium
25,Rendering,SVG Animation Wrapper,svg animation wrapper div,React/Next.js,Wrap SVG in div and animate wrapper for hardware acceleration,Animate div wrapper around SVG,Animate SVG element directly,"<div class='animate-spin'><svg>...</svg></div>","<svg class='animate-spin'>...</svg>",Low
26,Rendering,Content Visibility,content-visibility auto,React/Next.js,Apply content-visibility: auto to defer off-screen rendering,Use content-visibility for long lists,Render all list items immediately,".item { content-visibility: auto; contain-intrinsic-size: 0 80px }","Render 1000 items without optimization",High
27,Rendering,Hoist Static JSX,hoist static jsx element,React/Next.js,Extract static JSX outside components to avoid re-creation,Hoist static elements to module scope,Create static elements inside components,"const skeleton = <div class='animate-pulse' />; function C() { return skeleton }","function C() { return <div class='animate-pulse' /> }",Low
28,Rendering,Hydration No Flicker,hydration mismatch flicker,React/Next.js,Use inline script to set client-only data before hydration,Inject sync script for client-only values,Use useEffect causing flash,"<script dangerouslySetInnerHTML={{ __html: 'el.className = localStorage.theme' }} />","useEffect(() => setTheme(localStorage.theme), []) // flickers",Medium
29,Rendering,Conditional Render,conditional render ternary,React/Next.js,Use ternary instead of && when condition can be 0 or NaN,Use explicit ternary for conditionals,Use && with potentially falsy numbers,"{count > 0 ? <Badge>{count}</Badge> : null}","{count && <Badge>{count}</Badge>} // renders '0'",Low
30,Rendering,Activity Component,activity show hide preserve,React/Next.js,Use Activity component to preserve state/DOM for toggled components,Use Activity for expensive toggle components,Unmount/remount on visibility toggle,"<Activity mode={isOpen ? 'visible' : 'hidden'}><Menu /></Activity>","{isOpen && <Menu />} // loses state",Medium
31,JS Perf,Batch DOM CSS,batch dom css reflow,React/Next.js,Group CSS changes via classes or cssText to minimize reflows,Use class toggle or cssText,Change styles one property at a time,"element.classList.add('highlighted')","el.style.width='100px'; el.style.height='200px'",Medium
32,JS Perf,Index Map Lookup,map index lookup find,React/Next.js,Build Map for repeated lookups instead of multiple .find() calls,Build index Map for O(1) lookups,Use .find() in loops,"const byId = new Map(users.map(u => [u.id, u])); byId.get(id)","users.find(u => u.id === order.userId) // O(n) each time",Low-Medium
33,JS Perf,Cache Property Access,cache property loop,React/Next.js,Cache object property lookups in hot paths,Cache values before loops,Access nested properties in loops,"const val = obj.config.settings.value; for (...) process(val)","for (...) process(obj.config.settings.value)",Low-Medium
34,JS Perf,Cache Function Results,memoize cache function,React/Next.js,Use module-level Map to cache repeated function results,Use Map cache for repeated calls,Recompute same values repeatedly,"const cache = new Map(); if (cache.has(x)) return cache.get(x)","slugify(name) // called 100 times same input",Medium
35,JS Perf,Cache Storage API,localstorage cache read,React/Next.js,Cache localStorage/sessionStorage reads in memory,Cache storage reads in Map,Read storage on every call,"if (!cache.has(key)) cache.set(key, localStorage.getItem(key))","localStorage.getItem('theme') // every call",Low-Medium
36,JS Perf,Combine Iterations,combine filter map loop,React/Next.js,Combine multiple filter/map into single loop,Single loop for multiple categorizations,Chain multiple filter() calls,"for (u of users) { if (u.isAdmin) admins.push(u); if (u.isTester) testers.push(u) }","users.filter(admin); users.filter(tester); users.filter(inactive)",Low-Medium
37,JS Perf,Length Check First,length check array compare,React/Next.js,Check array lengths before expensive comparisons,Early return if lengths differ,Always run expensive comparison,"if (a.length !== b.length) return true; // then compare","a.sort().join() !== b.sort().join() // even when lengths differ",Medium-High
38,JS Perf,Early Return,early return exit function,React/Next.js,Return early when result is determined to skip processing,Return immediately on first error,Process all items then check errors,"for (u of users) { if (!u.email) return { error: 'Email required' } }","let hasError; for (...) { if (!email) hasError=true }; if (hasError)...",Low-Medium
39,JS Perf,Hoist RegExp,regexp hoist module,React/Next.js,Don't create RegExp inside render - hoist or memoize,Hoist RegExp to module scope,Create RegExp every render,"const EMAIL_RE = /^[^@]+@[^@]+$/; function validate() { EMAIL_RE.test(x) }","function C() { const re = new RegExp(pattern); re.test(x) }",Low-Medium
40,JS Perf,Loop Min Max,loop min max sort,React/Next.js,Use loop for min/max instead of sort - O(n) vs O(n log n),Single pass loop for min/max,Sort array to find min/max,"let max = arr[0]; for (x of arr) if (x > max) max = x","arr.sort((a,b) => b-a)[0] // O(n log n)",Low
41,JS Perf,Set Map Lookups,set map includes has,React/Next.js,Use Set/Map for O(1) lookups instead of array.includes(),Convert to Set for membership checks,Use .includes() for repeated checks,"const allowed = new Set(['a','b']); allowed.has(id)","const allowed = ['a','b']; allowed.includes(id)",Low-Medium
42,JS Perf,toSorted Immutable,tosorted sort immutable,React/Next.js,Use toSorted() instead of sort() to avoid mutating arrays,Use toSorted() for immutability,Mutate arrays with sort(),"users.toSorted((a,b) => a.name.localeCompare(b.name))","users.sort((a,b) => a.name.localeCompare(b.name)) // mutates",Medium-High
43,Advanced,Event Handler Refs,useeffectevent ref handler,React/Next.js,Store callbacks in refs for stable effect subscriptions,Use useEffectEvent for stable handlers,Re-subscribe on every callback change,"const onEvent = useEffectEvent(handler); useEffect(() => { listen(onEvent) }, [])","useEffect(() => { listen(handler) }, [handler]) // re-subscribes",Low
44,Advanced,useLatest Hook,uselatest ref callback,React/Next.js,Access latest values in callbacks without adding to dependency arrays,Use useLatest for fresh values in stable callbacks,Add callback to effect dependencies,"const cbRef = useLatest(cb); useEffect(() => { setTimeout(() => cbRef.current()) }, [])","useEffect(() => { setTimeout(() => cb()) }, [cb]) // re-runs",Low