chore: release v0.6.9

fix: build share urls from auth_url instead of localhost
docker build sets next_public_app_url to localhost; preferring it over runtime auth_url made prod share links point at localhost:3000.
2026-07-18 17:23:06 -05:00 · 2026-07-18 17:22:58 -05:00 · 2026-07-18 17:19:56 -05:00 · 2026-07-18 17:19:41 -05:00 · 2026-07-12 22:28:20 -05:00 · 2026-07-12 22:28:10 -05:00
486 changed files with 39418 additions and 2187 deletions
@@ -0,0 +1,187 @@
 ---
 name: react-best-practices
 description: React best-practices reviewer for TSX files. Triggers after editing multiple TSX components to run a condensed quality checklist covering component structure, hooks usage, accessibility, performance, and TypeScript patterns.
 metadata:
  priority: 4
  docs:
    - "https://react.dev/reference/react"
    - "https://react.dev/learn"
  pathPatterns:
    - "src/components/**/*.tsx"
    - "src/components/**/*.jsx"
    - "app/components/**/*.tsx"
    - "app/components/**/*.jsx"
    - "components/**/*.tsx"
    - "components/**/*.jsx"
    - "src/ui/**/*.tsx"
    - "lib/components/**/*.tsx"
  bashPatterns: []
  importPatterns:
    - "react"
    - "react-dom"
 validate:
  - pattern: 'from\s+[''"](styled-components|@emotion/styled|@emotion/react|@mui/material|@chakra-ui/react)[''"]|styled\.'
    message: "Legacy CSS-in-JS or component library detected. Consider shadcn/ui + Tailwind for modern Vercel-native UI."
    severity: warn
    upgradeToSkill: shadcn
    upgradeWhy: "Migrate from CSS-in-JS/MUI/Chakra to shadcn/ui + Tailwind CSS for better SSR performance and Vercel ecosystem alignment."
    skipIfFileContains: "@/components/ui|shadcn|tailwindcss"
 retrieval:
  aliases:
    - react review
    - component quality
    - tsx linter
    - react patterns
  intents:
    - review react code
    - improve component quality
    - check accessibility
    - optimize react
  entities:
    - hooks
    - accessibility
    - React
    - TSX
    - component
 chainTo:
  - pattern: 'from\s+[''\"](styled-components|@emotion/styled|@emotion/react|@mui/material|@chakra-ui/react)[''"]|styled\.'
    targetSkill: shadcn
    message: "Legacy CSS-in-JS or component library detected — loading shadcn/ui guidance for modern Vercel-native UI."
 ---
 # Vercel React Best Practices
 Comprehensive performance optimization guide for React and Next.js applications, maintained by Vercel. Contains 64 rules across 8 categories, prioritized by impact to guide automated refactoring and code generation.
 ## When to Apply
 Reference these guidelines when:
 - Writing new React components or Next.js pages
 - Implementing data fetching (client or server-side)
 - Reviewing code for performance issues
 - Refactoring existing React/Next.js code
 - Optimizing bundle size or load times
 ## Rule Categories by Priority
 | Priority | Category                  | Impact      | Prefix       |
 | -------- | ------------------------- | ----------- | ------------ |
 | 1        | Eliminating Waterfalls    | CRITICAL    | `async-`     |
 | 2        | Bundle Size Optimization  | CRITICAL    | `bundle-`    |
 | 3        | Server-Side Performance   | HIGH        | `server-`    |
 | 4        | Client-Side 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-`  |
 ## 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-auth-actions` - Authenticate server actions like API routes
 - `server-cache-react` - Use React.cache() for per-request deduplication
 - `server-cache-lru` - Use LRU cache for cross-request caching
 - `server-dedup-props` - Avoid duplicate serialization in RSC props
 - `server-hoist-static-io` - Hoist static I/O (fonts, logos) to module level
 - `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
 - `client-passive-event-listeners` - Use passive listeners for scroll
 - `client-localstorage-schema` - Version and minimize localStorage data
 ### 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-memo-with-default-value` - Hoist default non-primitive props
 - `rerender-dependencies` - Use primitive dependencies in effects
 - `rerender-derived-state` - Subscribe to derived booleans, not raw values
 - `rerender-derived-state-no-effect` - Derive state during render, not effects
 - `rerender-functional-setstate` - Use functional setState for stable callbacks
 - `rerender-lazy-state-init` - Pass function to useState for expensive values
 - `rerender-simple-expression-in-memo` - Avoid memo for simple primitives
 - `rerender-split-combined-hooks` - Split hooks with independent dependencies
 - `rerender-move-effect-to-event` - Put interaction logic in event handlers
 - `rerender-transitions` - Use startTransition for non-urgent updates
 - `rerender-use-deferred-value` - Defer expensive renders to keep input responsive
 - `rerender-use-ref-transient-values` - Use refs for transient frequent values
 - `rerender-no-inline-components` - Don't define components inside components
 ### 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-hydration-suppress-warning` - Suppress expected mismatches
 - `rendering-activity` - Use Activity component for show/hide
 - `rendering-conditional-render` - Use ternary, not && for conditionals
 - `rendering-usetransition-loading` - Prefer useTransition for loading state
 - `rendering-resource-hints` - Use React DOM resource hints for preloading
 - `rendering-script-defer-async` - Use defer or async on script tags
 ### 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
 - `js-flatmap-filter` - Use flatMap to map and filter in one pass
 ### 8. Advanced Patterns (LOW)
 - `advanced-event-handler-refs` - Store event handlers in refs
 - `advanced-init-once` - Initialize app once per app load
 - `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
 ```
 Each rule file contains:
 - Brief explanation of why it matters
 - Incorrect code example with explanation
 - Correct code example with explanation
 - Additional context and references
 ## Full Compiled Document
 For the complete guide with all rules expanded: `AGENTS.md`
@@ -0,0 +1,48 @@
 name: react-best-practices
 description: React best-practices reviewer for TSX files. Triggers after editing multiple TSX components to run a condensed quality checklist covering component structure, hooks usage, accessibility, performance, and TypeScript patterns.
 metadata:
  priority: 4
  docs:
    - "https://react.dev/reference/react"
    - "https://react.dev/learn"
  pathPatterns:
    - "src/components/**/*.tsx"
    - "src/components/**/*.jsx"
    - "app/components/**/*.tsx"
    - "app/components/**/*.jsx"
    - "components/**/*.tsx"
    - "components/**/*.jsx"
    - "src/ui/**/*.tsx"
    - "lib/components/**/*.tsx"
  bashPatterns: []
  importPatterns:
    - "react"
    - "react-dom"
 validate:
  - pattern: 'from\s+[''"](styled-components|@emotion/styled|@emotion/react|@mui/material|@chakra-ui/react)[''"]|styled\.'
    message: "Legacy CSS-in-JS or component library detected. Consider shadcn/ui + Tailwind for modern Vercel-native UI."
    severity: warn
    upgradeToSkill: shadcn
    upgradeWhy: "Migrate from CSS-in-JS/MUI/Chakra to shadcn/ui + Tailwind CSS for better SSR performance and Vercel ecosystem alignment."
    skipIfFileContains: "@/components/ui|shadcn|tailwindcss"
 retrieval:
  aliases:
    - react review
    - component quality
    - tsx linter
    - react patterns
  intents:
    - review react code
    - improve component quality
    - check accessibility
    - optimize react
  entities:
    - hooks
    - accessibility
    - React
    - TSX
    - component
 chainTo:
  - pattern: 'from\s+[''\"](styled-components|@emotion/styled|@emotion/react|@mui/material|@chakra-ui/react)[''"]|styled\.'
    targetSkill: shadcn
    message: "Legacy CSS-in-JS or component library detected — loading shadcn/ui guidance for modern Vercel-native UI."
@@ -0,0 +1,46 @@
 # Sections
 This file defines all sections, their ordering, impact levels, and descriptions.
 The section ID (in parentheses) is the filename prefix used to group rules.
 ---
 ## 1. Eliminating Waterfalls (async)
 **Impact:** CRITICAL  
 **Description:** Waterfalls are the #1 performance killer. Each sequential await adds full network latency. Eliminating them yields the largest gains.
 ## 2. Bundle Size Optimization (bundle)
 **Impact:** CRITICAL  
 **Description:** Reducing initial bundle size improves Time to Interactive and Largest Contentful Paint.
 ## 3. Server-Side Performance (server)
 **Impact:** HIGH  
 **Description:** Optimizing server-side rendering and data fetching eliminates server-side waterfalls and reduces response times.
 ## 4. Client-Side Data Fetching (client)
 **Impact:** MEDIUM-HIGH  
 **Description:** Automatic deduplication and efficient data fetching patterns reduce redundant network requests.
 ## 5. Re-render Optimization (rerender)
 **Impact:** MEDIUM  
 **Description:** Reducing unnecessary re-renders minimizes wasted computation and improves UI responsiveness.
 ## 6. Rendering Performance (rendering)
 **Impact:** MEDIUM  
 **Description:** Optimizing the rendering process reduces the work the browser needs to do.
 ## 7. JavaScript Performance (js)
 **Impact:** LOW-MEDIUM  
 **Description:** Micro-optimizations for hot paths can add up to meaningful improvements.
 ## 8. Advanced Patterns (advanced)
 **Impact:** LOW  
 **Description:** Advanced patterns for specific cases that require careful implementation.
@@ -0,0 +1,28 @@
 ---
 title: Rule Title Here
 impact: MEDIUM
 impactDescription: Optional description of impact (e.g., "20-50% improvement")
 tags: tag1, tag2
 ---
 ## Rule Title Here
 **Impact: MEDIUM (optional impact description)**
 Brief explanation of the rule and why it matters. This should be clear and concise, explaining the performance implications.
 **Incorrect (description of what's wrong):**
 ```typescript
 // Bad code example here
 const bad = example();
 ```
 **Correct (description of what's right):**
 ```typescript
 // Good code example here
 const good = example();
 ```
 Reference: [Link to documentation or resource](https://example.com)
@@ -0,0 +1,55 @@
 ---
 title: Store Event Handlers in Refs
 impact: LOW
 impactDescription: stable subscriptions
 tags: advanced, hooks, refs, event-handlers, optimization
 ---
 ## Store Event Handlers in Refs
 Store callbacks in refs when used in effects that shouldn't re-subscribe on callback changes.
 **Incorrect (re-subscribes on every render):**
 ```tsx
 function useWindowEvent(event: string, handler: (e) => void) {
  useEffect(() => {
    window.addEventListener(event, handler);
    return () => window.removeEventListener(event, handler);
  }, [event, handler]);
 }
 ```
 **Correct (stable subscription):**
 ```tsx
 function useWindowEvent(event: string, handler: (e) => void) {
  const handlerRef = useRef(handler);
  useEffect(() => {
    handlerRef.current = handler;
  }, [handler]);
  useEffect(() => {
    const listener = (e) => handlerRef.current(e);
    window.addEventListener(event, listener);
    return () => window.removeEventListener(event, listener);
  }, [event]);
 }
 ```
 **Alternative: use `useEffectEvent` if you're on latest React:**
 ```tsx
 import { useEffectEvent } from "react";
 function useWindowEvent(event: string, handler: (e) => void) {
  const onEvent = useEffectEvent(handler);
  useEffect(() => {
    window.addEventListener(event, onEvent);
    return () => window.removeEventListener(event, onEvent);
  }, [event]);
 }
 ```
 `useEffectEvent` provides a cleaner API for the same pattern: it creates a stable function reference that always calls the latest version of the handler.
@@ -0,0 +1,42 @@
 ---
 title: Initialize App Once, Not Per Mount
 impact: LOW-MEDIUM
 impactDescription: avoids duplicate init in development
 tags: initialization, useEffect, app-startup, side-effects
 ---
 ## Initialize App Once, Not Per Mount
 Do not put app-wide initialization that must run once per app load inside `useEffect([])` of a component. Components can remount and effects will re-run. Use a module-level guard or top-level init in the entry module instead.
 **Incorrect (runs twice in dev, re-runs on remount):**
 ```tsx
 function Comp() {
  useEffect(() => {
    loadFromStorage();
    checkAuthToken();
  }, []);
  // ...
 }
 ```
 **Correct (once per app load):**
 ```tsx
 let didInit = false;
 function Comp() {
  useEffect(() => {
    if (didInit) return;
    didInit = true;
    loadFromStorage();
    checkAuthToken();
  }, []);
  // ...
 }
 ```
 Reference: [Initializing the application](https://react.dev/learn/you-might-not-need-an-effect#initializing-the-application)
@@ -0,0 +1,39 @@
 ---
 title: useEffectEvent for Stable Callback Refs
 impact: LOW
 impactDescription: prevents effect re-runs
 tags: advanced, hooks, useEffectEvent, refs, optimization
 ---
 ## useEffectEvent for Stable Callback Refs
 Access latest values in callbacks without adding them to dependency arrays. Prevents effect re-runs while avoiding stale closures.
 **Incorrect (effect re-runs on every callback change):**
 ```tsx
 function SearchInput({ onSearch }: { onSearch: (q: string) => void }) {
  const [query, setQuery] = useState("");
  useEffect(() => {
    const timeout = setTimeout(() => onSearch(query), 300);
    return () => clearTimeout(timeout);
  }, [query, onSearch]);
 }
 ```
 **Correct (using React's useEffectEvent):**
 ```tsx
 import { useEffectEvent } from "react";
 function SearchInput({ onSearch }: { onSearch: (q: string) => void }) {
  const [query, setQuery] = useState("");
  const onSearchEvent = useEffectEvent(onSearch);
  useEffect(() => {
    const timeout = setTimeout(() => onSearchEvent(query), 300);
    return () => clearTimeout(timeout);
  }, [query]);
 }
 ```
@@ -0,0 +1,35 @@
 ---
 title: Prevent Waterfall Chains in API Routes
 impact: CRITICAL
 impactDescription: 2-10× improvement
 tags: api-routes, server-actions, waterfalls, parallelization
 ---
 ## Prevent Waterfall Chains in API Routes
 In API routes and Server Actions, start independent operations immediately, even if you don't await them yet.
 **Incorrect (config waits for auth, data waits for both):**
 ```typescript
 export async function GET(request: Request) {
  const session = await auth();
  const config = await fetchConfig();
  const data = await fetchData(session.user.id);
  return Response.json({ data, config });
 }
 ```
 **Correct (auth and config start immediately):**
 ```typescript
 export async function GET(request: Request) {
  const sessionPromise = auth();
  const configPromise = fetchConfig();
  const session = await sessionPromise;
  const [config, data] = await Promise.all([configPromise, fetchData(session.user.id)]);
  return Response.json({ data, config });
 }
 ```
 For operations with more complex dependency chains, use `better-all` to automatically maximize parallelism (see Dependency-Based Parallelization).
@@ -0,0 +1,80 @@
 ---
 title: Defer Await Until Needed
 impact: HIGH
 impactDescription: avoids blocking unused code paths
 tags: async, await, conditional, optimization
 ---
 ## Defer Await Until Needed
 Move `await` operations into the branches where they're actually used to avoid blocking code paths that don't need them.
 **Incorrect (blocks both branches):**
 ```typescript
 async function handleRequest(userId: string, skipProcessing: boolean) {
  const userData = await fetchUserData(userId);
  if (skipProcessing) {
    // Returns immediately but still waited for userData
    return { skipped: true };
  }
  // Only this branch uses userData
  return processUserData(userData);
 }
 ```
 **Correct (only blocks when needed):**
 ```typescript
 async function handleRequest(userId: string, skipProcessing: boolean) {
  if (skipProcessing) {
    // Returns immediately without waiting
    return { skipped: true };
  }
  // Fetch only when needed
  const userData = await fetchUserData(userId);
  return processUserData(userData);
 }
 ```
 **Another example (early return optimization):**
 ```typescript
 // Incorrect: always fetches permissions
 async function updateResource(resourceId: string, userId: string) {
  const permissions = await fetchPermissions(userId);
  const resource = await getResource(resourceId);
  if (!resource) {
    return { error: "Not found" };
  }
  if (!permissions.canEdit) {
    return { error: "Forbidden" };
  }
  return await updateResourceData(resource, permissions);
 }
 // Correct: fetches only when needed
 async function updateResource(resourceId: string, userId: string) {
  const resource = await getResource(resourceId);
  if (!resource) {
    return { error: "Not found" };
  }
  const permissions = await fetchPermissions(userId);
  if (!permissions.canEdit) {
    return { error: "Forbidden" };
  }
  return await updateResourceData(resource, permissions);
 }
 ```
 This optimization is especially valuable when the skipped branch is frequently taken, or when the deferred operation is expensive.
@@ -0,0 +1,48 @@
 ---
 title: Dependency-Based Parallelization
 impact: CRITICAL
 impactDescription: 2-10× improvement
 tags: async, parallelization, dependencies, better-all
 ---
 ## Dependency-Based Parallelization
 For operations with partial dependencies, use `better-all` to maximize parallelism. It automatically starts each task at the earliest possible moment.
 **Incorrect (profile waits for config unnecessarily):**
 ```typescript
 const [user, config] = await Promise.all([fetchUser(), fetchConfig()]);
 const profile = await fetchProfile(user.id);
 ```
 **Correct (config and profile run in parallel):**
 ```typescript
 import { all } from "better-all";
 const { user, config, profile } = await all({
  async user() {
    return fetchUser();
  },
  async config() {
    return fetchConfig();
  },
  async profile() {
    return fetchProfile((await this.$.user).id);
  },
 });
 ```
 **Alternative without extra dependencies:**
 We can also create all the promises first, and do `Promise.all()` at the end.
 ```typescript
 const userPromise = fetchUser();
 const profilePromise = userPromise.then((user) => fetchProfile(user.id));
 const [user, config, profile] = await Promise.all([userPromise, fetchConfig(), profilePromise]);
 ```
 Reference: [https://github.com/shuding/better-all](https://github.com/shuding/better-all)
@@ -0,0 +1,24 @@
 ---
 title: Promise.all() for Independent Operations
 impact: CRITICAL
 impactDescription: 2-10× improvement
 tags: async, parallelization, promises, waterfalls
 ---
 ## Promise.all() for Independent Operations
 When async operations have no interdependencies, execute them concurrently using `Promise.all()`.
 **Incorrect (sequential execution, 3 round trips):**
 ```typescript
 const user = await fetchUser();
 const posts = await fetchPosts();
 const comments = await fetchComments();
 ```
 **Correct (parallel execution, 1 round trip):**
 ```typescript
 const [user, posts, comments] = await Promise.all([fetchUser(), fetchPosts(), fetchComments()]);
 ```
@@ -0,0 +1,99 @@
 ---
 title: Strategic Suspense Boundaries
 impact: HIGH
 impactDescription: faster initial paint
 tags: async, suspense, streaming, layout-shift
 ---
 ## Strategic Suspense Boundaries
 Instead of awaiting data in async components before returning JSX, use Suspense boundaries to show the wrapper UI faster while data loads.
 **Incorrect (wrapper blocked by data fetching):**
 ```tsx
 async function Page() {
  const data = await fetchData(); // Blocks entire page
  return (
    <div>
      <div>Sidebar</div>
      <div>Header</div>
      <div>
        <DataDisplay data={data} />
      </div>
      <div>Footer</div>
    </div>
  );
 }
 ```
 The entire layout waits for data even though only the middle section needs it.
 **Correct (wrapper shows immediately, data streams in):**
 ```tsx
 function Page() {
  return (
    <div>
      <div>Sidebar</div>
      <div>Header</div>
      <div>
        <Suspense fallback={<Skeleton />}>
          <DataDisplay />
        </Suspense>
      </div>
      <div>Footer</div>
    </div>
  );
 }
 async function DataDisplay() {
  const data = await fetchData(); // Only blocks this component
  return <div>{data.content}</div>;
 }
 ```
 Sidebar, Header, and Footer render immediately. Only DataDisplay waits for data.
 **Alternative (share promise across components):**
 ```tsx
 function Page() {
  // Start fetch immediately, but don't await
  const dataPromise = fetchData();
  return (
    <div>
      <div>Sidebar</div>
      <div>Header</div>
      <Suspense fallback={<Skeleton />}>
        <DataDisplay dataPromise={dataPromise} />
        <DataSummary dataPromise={dataPromise} />
      </Suspense>
      <div>Footer</div>
    </div>
  );
 }
 function DataDisplay({ dataPromise }: { dataPromise: Promise<Data> }) {
  const data = use(dataPromise); // Unwraps the promise
  return <div>{data.content}</div>;
 }
 function DataSummary({ dataPromise }: { dataPromise: Promise<Data> }) {
  const data = use(dataPromise); // Reuses the same promise
  return <div>{data.summary}</div>;
 }
 ```
 Both components share the same promise, so only one fetch occurs. Layout renders immediately while both components wait together.
 **When NOT to use this pattern:**
 - Critical data needed for layout decisions (affects positioning)
 - SEO-critical content above the fold
 - Small, fast queries where suspense overhead isn't worth it
 - When you want to avoid layout shift (loading → content jump)
 **Trade-off:** Faster initial paint vs potential layout shift. Choose based on your UX priorities.
@@ -0,0 +1,59 @@
 ---
 title: Avoid Barrel File Imports
 impact: CRITICAL
 impactDescription: 200-800ms import cost, slow builds
 tags: bundle, imports, tree-shaking, barrel-files, performance
 ---
 ## Avoid Barrel File Imports
 Import directly from source files instead of barrel files to avoid loading thousands of unused modules. **Barrel files** are entry points that re-export multiple modules (e.g., `index.js` that does `export * from './module'`).
 Popular icon and component libraries can have **up to 10,000 re-exports** in their entry file. For many React packages, **it takes 200-800ms just to import them**, affecting both development speed and production cold starts.
 **Why tree-shaking doesn't help:** When a library is marked as external (not bundled), the bundler can't optimize it. If you bundle it to enable tree-shaking, builds become substantially slower analyzing the entire module graph.
 **Incorrect (imports entire library):**
 ```tsx
 import { Check, X, Menu } from "lucide-react";
 // Loads 1,583 modules, takes ~2.8s extra in dev
 // Runtime cost: 200-800ms on every cold start
 import { Button, TextField } from "@mui/material";
 // Loads 2,225 modules, takes ~4.2s extra in dev
 ```
 **Correct (imports only what you need):**
 ```tsx
 import Check from "lucide-react/dist/esm/icons/check";
 import X from "lucide-react/dist/esm/icons/x";
 import Menu from "lucide-react/dist/esm/icons/menu";
 // Loads only 3 modules (~2KB vs ~1MB)
 import Button from "@mui/material/Button";
 import TextField from "@mui/material/TextField";
 // Loads only what you use
 ```
 **Alternative (Next.js 13.5+):**
 ```js
 // next.config.js - use optimizePackageImports
 module.exports = {
  experimental: {
    optimizePackageImports: ["lucide-react", "@mui/material"],
  },
 };
 // Then you can keep the ergonomic barrel imports:
 import { Check, X, Menu } from "lucide-react";
 // Automatically transformed to direct imports at build time
 ```
 Direct imports provide 15-70% faster dev boot, 28% faster builds, 40% faster cold starts, and significantly faster HMR.
 Libraries commonly affected: `lucide-react`, `@mui/material`, `@mui/icons-material`, `@tabler/icons-react`, `react-icons`, `@headlessui/react`, `@radix-ui/react-*`, `lodash`, `ramda`, `date-fns`, `rxjs`, `react-use`.
 Reference: [How we optimized package imports in Next.js](https://vercel.com/blog/how-we-optimized-package-imports-in-next-js)
@@ -0,0 +1,37 @@
 ---
 title: Conditional Module Loading
 impact: HIGH
 impactDescription: loads large data only when needed
 tags: bundle, conditional-loading, lazy-loading
 ---
 ## Conditional Module Loading
 Load large data or modules only when a feature is activated.
 **Example (lazy-load animation frames):**
 ```tsx
 function AnimationPlayer({
  enabled,
  setEnabled,
 }: {
  enabled: boolean;
  setEnabled: React.Dispatch<React.SetStateAction<boolean>>;
 }) {
  const [frames, setFrames] = useState<Frame[] | null>(null);
  useEffect(() => {
    if (enabled && !frames && typeof window !== "undefined") {
      import("./animation-frames.js")
        .then((mod) => setFrames(mod.frames))
        .catch(() => setEnabled(false));
    }
  }, [enabled, frames, setEnabled]);
  if (!frames) return <Skeleton />;
  return <Canvas frames={frames} />;
 }
 ```
 The `typeof window !== 'undefined'` check prevents bundling this module for SSR, optimizing server bundle size and build speed.
@@ -0,0 +1,48 @@
 ---
 title: Defer Non-Critical Third-Party Libraries
 impact: MEDIUM
 impactDescription: loads after hydration
 tags: bundle, third-party, analytics, defer
 ---
 ## Defer Non-Critical Third-Party Libraries
 Analytics, logging, and error tracking don't block user interaction. Load them after hydration.
 **Incorrect (blocks initial bundle):**
 ```tsx
 import { Analytics } from "@vercel/analytics/react";
 export default function RootLayout({ children }) {
  return (
    <html>
      <body>
        {children}
        <Analytics />
      </body>
    </html>
  );
 }
 ```
 **Correct (loads after hydration):**
 ```tsx
 import dynamic from "next/dynamic";
 const Analytics = dynamic(() => import("@vercel/analytics/react").then((m) => m.Analytics), {
  ssr: false,
 });
 export default function RootLayout({ children }) {
  return (
    <html>
      <body>
        {children}
        <Analytics />
      </body>
    </html>
  );
 }
 ```
@@ -0,0 +1,34 @@
 ---
 title: Dynamic Imports for Heavy Components
 impact: CRITICAL
 impactDescription: directly affects TTI and LCP
 tags: bundle, dynamic-import, code-splitting, next-dynamic
 ---
 ## Dynamic Imports for Heavy Components
 Use `next/dynamic` to lazy-load large components not needed on initial render.
 **Incorrect (Monaco bundles with main chunk ~300KB):**
 ```tsx
 import { MonacoEditor } from "./monaco-editor";
 function CodePanel({ code }: { code: string }) {
  return <MonacoEditor value={code} />;
 }
 ```
 **Correct (Monaco loads on demand):**
 ```tsx
 import dynamic from "next/dynamic";
 const MonacoEditor = dynamic(() => import("./monaco-editor").then((m) => m.MonacoEditor), {
  ssr: false,
 });
 function CodePanel({ code }: { code: string }) {
  return <MonacoEditor value={code} />;
 }
 ```
@@ -0,0 +1,44 @@
 ---
 title: Preload Based on User Intent
 impact: MEDIUM
 impactDescription: reduces perceived latency
 tags: bundle, preload, user-intent, hover
 ---
 ## Preload Based on User Intent
 Preload heavy bundles before they're needed to reduce perceived latency.
 **Example (preload on hover/focus):**
 ```tsx
 function EditorButton({ onClick }: { onClick: () => void }) {
  const preload = () => {
    if (typeof window !== "undefined") {
      void import("./monaco-editor");
    }
  };
  return (
    <button onMouseEnter={preload} onFocus={preload} onClick={onClick}>
      Open Editor
    </button>
  );
 }
 ```
 **Example (preload when feature flag is enabled):**
 ```tsx
 function FlagsProvider({ children, flags }: Props) {
  useEffect(() => {
    if (flags.editorEnabled && typeof window !== "undefined") {
      void import("./monaco-editor").then((mod) => mod.init());
    }
  }, [flags.editorEnabled]);
  return <FlagsContext.Provider value={flags}>{children}</FlagsContext.Provider>;
 }
 ```
 The `typeof window !== 'undefined'` check prevents bundling preloaded modules for SSR, optimizing server bundle size and build speed.
@@ -0,0 +1,78 @@
 ---
 title: Deduplicate Global Event Listeners
 impact: LOW
 impactDescription: single listener for N components
 tags: client, swr, event-listeners, subscription
 ---
 ## Deduplicate Global Event Listeners
 Use `useSWRSubscription()` to share global event listeners across component instances.
 **Incorrect (N instances = N listeners):**
 ```tsx
 function useKeyboardShortcut(key: string, callback: () => void) {
  useEffect(() => {
    const handler = (e: KeyboardEvent) => {
      if (e.metaKey && e.key === key) {
        callback();
      }
    };
    window.addEventListener("keydown", handler);
    return () => window.removeEventListener("keydown", handler);
  }, [key, callback]);
 }
 ```
 When using the `useKeyboardShortcut` hook multiple times, each instance will register a new listener.
 **Correct (N instances = 1 listener):**
 ```tsx
 import useSWRSubscription from "swr/subscription";
 // Module-level Map to track callbacks per key
 const keyCallbacks = new Map<string, Set<() => void>>();
 function useKeyboardShortcut(key: string, callback: () => void) {
  // Register this callback in the Map
  useEffect(() => {
    if (!keyCallbacks.has(key)) {
      keyCallbacks.set(key, new Set());
    }
    keyCallbacks.get(key)!.add(callback);
    return () => {
      const set = keyCallbacks.get(key);
      if (set) {
        set.delete(callback);
        if (set.size === 0) {
          keyCallbacks.delete(key);
        }
      }
    };
  }, [key, callback]);
  useSWRSubscription("global-keydown", () => {
    const handler = (e: KeyboardEvent) => {
      if (e.metaKey && keyCallbacks.has(e.key)) {
        keyCallbacks.get(e.key)!.forEach((cb) => cb());
      }
    };
    window.addEventListener("keydown", handler);
    return () => window.removeEventListener("keydown", handler);
  });
 }
 function Profile() {
  // Multiple shortcuts will share the same listener
  useKeyboardShortcut("p", () => {
    /* ... */
  });
  useKeyboardShortcut("k", () => {
    /* ... */
  });
  // ...
 }
 ```
@@ -0,0 +1,74 @@
 ---
 title: Version and Minimize localStorage Data
 impact: MEDIUM
 impactDescription: prevents schema conflicts, reduces storage size
 tags: client, localStorage, storage, versioning, data-minimization
 ---
 ## Version and Minimize localStorage Data
 Add version prefix to keys and store only needed fields. Prevents schema conflicts and accidental storage of sensitive data.
 **Incorrect:**
 ```typescript
 // No version, stores everything, no error handling
 localStorage.setItem("userConfig", JSON.stringify(fullUserObject));
 const data = localStorage.getItem("userConfig");
 ```
 **Correct:**
 ```typescript
 const VERSION = "v2";
 function saveConfig(config: { theme: string; language: string }) {
  try {
    localStorage.setItem(`userConfig:${VERSION}`, JSON.stringify(config));
  } catch {
    // Throws in incognito/private browsing, quota exceeded, or disabled
  }
 }
 function loadConfig() {
  try {
    const data = localStorage.getItem(`userConfig:${VERSION}`);
    return data ? JSON.parse(data) : null;
  } catch {
    return null;
  }
 }
 // Migration from v1 to v2
 function migrate() {
  try {
    const v1 = localStorage.getItem("userConfig:v1");
    if (v1) {
      const old = JSON.parse(v1);
      saveConfig({ theme: old.darkMode ? "dark" : "light", language: old.lang });
      localStorage.removeItem("userConfig:v1");
    }
  } catch {}
 }
 ```
 **Store minimal fields from server responses:**
 ```typescript
 // User object has 20+ fields, only store what UI needs
 function cachePrefs(user: FullUser) {
  try {
    localStorage.setItem(
      "prefs:v1",
      JSON.stringify({
        theme: user.preferences.theme,
        notifications: user.preferences.notifications,
      }),
    );
  } catch {}
 }
 ```
 **Always wrap in try-catch:** `getItem()` and `setItem()` throw in incognito/private browsing (Safari, Firefox), when quota exceeded, or when disabled.
 **Benefits:** Schema evolution via versioning, reduced storage size, prevents storing tokens/PII/internal flags.
@@ -0,0 +1,48 @@
 ---
 title: Use Passive Event Listeners for Scrolling Performance
 impact: MEDIUM
 impactDescription: eliminates scroll delay caused by event listeners
 tags: client, event-listeners, scrolling, performance, touch, wheel
 ---
 ## Use Passive Event Listeners for Scrolling Performance
 Add `{ passive: true }` to touch and wheel event listeners to enable immediate scrolling. Browsers normally wait for listeners to finish to check if `preventDefault()` is called, causing scroll delay.
 **Incorrect:**
 ```typescript
 useEffect(() => {
  const handleTouch = (e: TouchEvent) => console.log(e.touches[0].clientX);
  const handleWheel = (e: WheelEvent) => console.log(e.deltaY);
  document.addEventListener("touchstart", handleTouch);
  document.addEventListener("wheel", handleWheel);
  return () => {
    document.removeEventListener("touchstart", handleTouch);
    document.removeEventListener("wheel", handleWheel);
  };
 }, []);
 ```
 **Correct:**
 ```typescript
 useEffect(() => {
  const handleTouch = (e: TouchEvent) => console.log(e.touches[0].clientX);
  const handleWheel = (e: WheelEvent) => console.log(e.deltaY);
  document.addEventListener("touchstart", handleTouch, { passive: true });
  document.addEventListener("wheel", handleWheel, { passive: true });
  return () => {
    document.removeEventListener("touchstart", handleTouch);
    document.removeEventListener("wheel", handleWheel);
  };
 }, []);
 ```
 **Use passive when:** tracking/analytics, logging, any listener that doesn't call `preventDefault()`.
 **Don't use passive when:** implementing custom swipe gestures, custom zoom controls, or any listener that needs `preventDefault()`.
@@ -0,0 +1,56 @@
 ---
 title: Use SWR for Automatic Deduplication
 impact: MEDIUM-HIGH
 impactDescription: automatic deduplication
 tags: client, swr, deduplication, data-fetching
 ---
 ## Use SWR for Automatic Deduplication
 SWR enables request deduplication, caching, and revalidation across component instances.
 **Incorrect (no deduplication, each instance fetches):**
 ```tsx
 function UserList() {
  const [users, setUsers] = useState([]);
  useEffect(() => {
    fetch("/api/users")
      .then((r) => r.json())
      .then(setUsers);
  }, []);
 }
 ```
 **Correct (multiple instances share one request):**
 ```tsx
 import useSWR from "swr";
 function UserList() {
  const { data: users } = useSWR("/api/users", fetcher);
 }
 ```
 **For immutable data:**
 ```tsx
 import { useImmutableSWR } from "@/lib/swr";
 function StaticContent() {
  const { data } = useImmutableSWR("/api/config", fetcher);
 }
 ```
 **For mutations:**
 ```tsx
 import { useSWRMutation } from "swr/mutation";
 function UpdateButton() {
  const { trigger } = useSWRMutation("/api/user", updateUser);
  return <button onClick={() => trigger()}>Update</button>;
 }
 ```
 Reference: [https://swr.vercel.app](https://swr.vercel.app)
@@ -0,0 +1,110 @@
 ---
 title: Avoid Layout Thrashing
 impact: MEDIUM
 impactDescription: prevents forced synchronous layouts and reduces performance bottlenecks
 tags: javascript, dom, css, performance, reflow, layout-thrashing
 ---
 ## Avoid Layout Thrashing
 Avoid interleaving style writes with layout reads. When you read a layout property (like `offsetWidth`, `getBoundingClientRect()`, or `getComputedStyle()`) between style changes, the browser is forced to trigger a synchronous reflow.
 **This is OK (browser batches style changes):**
 ```typescript
 function updateElementStyles(element: HTMLElement) {
  // Each line invalidates style, but browser batches the recalculation
  element.style.width = "100px";
  element.style.height = "200px";
  element.style.backgroundColor = "blue";
  element.style.border = "1px solid black";
 }
 ```
 **Incorrect (interleaved reads and writes force reflows):**
 ```typescript
 function layoutThrashing(element: HTMLElement) {
  element.style.width = "100px";
  const width = element.offsetWidth; // Forces reflow
  element.style.height = "200px";
  const height = element.offsetHeight; // Forces another reflow
 }
 ```
 **Correct (batch writes, then read once):**
 ```typescript
 function updateElementStyles(element: HTMLElement) {
  // Batch all writes together
  element.style.width = "100px";
  element.style.height = "200px";
  element.style.backgroundColor = "blue";
  element.style.border = "1px solid black";
  // Read after all writes are done (single reflow)
  const { width, height } = element.getBoundingClientRect();
 }
 ```
 **Correct (batch reads, then writes):**
 ```typescript
 function avoidThrashing(element: HTMLElement) {
  // Read phase - all layout queries first
  const rect1 = element.getBoundingClientRect();
  const offsetWidth = element.offsetWidth;
  const offsetHeight = element.offsetHeight;
  // Write phase - all style changes after
  element.style.width = "100px";
  element.style.height = "200px";
 }
 ```
 **Better: use CSS classes**
 ```css
 .highlighted-box {
  width: 100px;
  height: 200px;
  background-color: blue;
  border: 1px solid black;
 }
 ```
 ```typescript
 function updateElementStyles(element: HTMLElement) {
  element.classList.add("highlighted-box");
  const { width, height } = element.getBoundingClientRect();
 }
 ```
 **React example:**
 ```tsx
 // Incorrect: interleaving style changes with layout queries
 function Box({ isHighlighted }: { isHighlighted: boolean }) {
  const ref = useRef<HTMLDivElement>(null);
  useEffect(() => {
    if (ref.current && isHighlighted) {
      ref.current.style.width = "100px";
      const width = ref.current.offsetWidth; // Forces layout
      ref.current.style.height = "200px";
    }
  }, [isHighlighted]);
  return <div ref={ref}>Content</div>;
 }
 // Correct: toggle class
 function Box({ isHighlighted }: { isHighlighted: boolean }) {
  return <div className={isHighlighted ? "highlighted-box" : ""}>Content</div>;
 }
 ```
 Prefer CSS classes over inline styles when possible. CSS files are cached by the browser, and classes provide better separation of concerns and are easier to maintain.
 See [this gist](https://gist.github.com/paulirish/5d52fb081b3570c81e3a) and [CSS Triggers](https://csstriggers.com/) for more information on layout-forcing operations.
@@ -0,0 +1,80 @@
 ---
 title: Cache Repeated Function Calls
 impact: MEDIUM
 impactDescription: avoid redundant computation
 tags: javascript, cache, memoization, performance
 ---
 ## Cache Repeated Function Calls
 Use a module-level Map to cache function results when the same function is called repeatedly with the same inputs during render.
 **Incorrect (redundant computation):**
 ```typescript
 function ProjectList({ projects }: { projects: Project[] }) {
  return (
    <div>
      {projects.map(project => {
        // slugify() called 100+ times for same project names
        const slug = slugify(project.name)
        return <ProjectCard key={project.id} slug={slug} />
      })}
    </div>
  )
 }
 ```
 **Correct (cached results):**
 ```typescript
 // Module-level cache
 const slugifyCache = new Map<string, string>()
 function cachedSlugify(text: string): string {
  if (slugifyCache.has(text)) {
    return slugifyCache.get(text)!
  }
  const result = slugify(text)
  slugifyCache.set(text, result)
  return result
 }
 function ProjectList({ projects }: { projects: Project[] }) {
  return (
    <div>
      {projects.map(project => {
        // Computed only once per unique project name
        const slug = cachedSlugify(project.name)
        return <ProjectCard key={project.id} slug={slug} />
      })}
    </div>
  )
 }
 ```
 **Simpler pattern for single-value functions:**
 ```typescript
 let isLoggedInCache: boolean | null = null;
 function isLoggedIn(): boolean {
  if (isLoggedInCache !== null) {
    return isLoggedInCache;
  }
  isLoggedInCache = document.cookie.includes("auth=");
  return isLoggedInCache;
 }
 // Clear cache when auth changes
 function onAuthChange() {
  isLoggedInCache = null;
 }
 ```
 Use a Map (not a hook) so it works everywhere: utilities, event handlers, not just React components.
 Reference: [How we made the Vercel Dashboard twice as fast](https://vercel.com/blog/how-we-made-the-vercel-dashboard-twice-as-fast)
@@ -0,0 +1,28 @@
 ---
 title: Cache Property Access in Loops
 impact: LOW-MEDIUM
 impactDescription: reduces lookups
 tags: javascript, loops, optimization, caching
 ---
 ## Cache Property Access in Loops
 Cache object property lookups in hot paths.
 **Incorrect (3 lookups × N iterations):**
 ```typescript
 for (let i = 0; i < arr.length; i++) {
  process(obj.config.settings.value);
 }
 ```
 **Correct (1 lookup total):**
 ```typescript
 const value = obj.config.settings.value;
 const len = arr.length;
 for (let i = 0; i < len; i++) {
  process(value);
 }
 ```
@@ -0,0 +1,68 @@
 ---
 title: Cache Storage API Calls
 impact: LOW-MEDIUM
 impactDescription: reduces expensive I/O
 tags: javascript, localStorage, storage, caching, performance
 ---
 ## Cache Storage API Calls
 `localStorage`, `sessionStorage`, and `document.cookie` are synchronous and expensive. Cache reads in memory.
 **Incorrect (reads storage on every call):**
 ```typescript
 function getTheme() {
  return localStorage.getItem("theme") ?? "light";
 }
 // Called 10 times = 10 storage reads
 ```
 **Correct (Map cache):**
 ```typescript
 const storageCache = new Map<string, string | null>();
 function getLocalStorage(key: string) {
  if (!storageCache.has(key)) {
    storageCache.set(key, localStorage.getItem(key));
  }
  return storageCache.get(key);
 }
 function setLocalStorage(key: string, value: string) {
  localStorage.setItem(key, value);
  storageCache.set(key, value); // keep cache in sync
 }
 ```
 Use a Map (not a hook) so it works everywhere: utilities, event handlers, not just React components.
 **Cookie caching:**
 ```typescript
 let cookieCache: Record<string, string> | null = null;
 function getCookie(name: string) {
  if (!cookieCache) {
    cookieCache = Object.fromEntries(document.cookie.split("; ").map((c) => c.split("=")));
  }
  return cookieCache[name];
 }
 ```
 **Important (invalidate on external changes):**
 If storage can change externally (another tab, server-set cookies), invalidate cache:
 ```typescript
 window.addEventListener("storage", (e) => {
  if (e.key) storageCache.delete(e.key);
 });
 document.addEventListener("visibilitychange", () => {
  if (document.visibilityState === "visible") {
    storageCache.clear();
  }
 });
 ```
@@ -0,0 +1,32 @@
 ---
 title: Combine Multiple Array Iterations
 impact: LOW-MEDIUM
 impactDescription: reduces iterations
 tags: javascript, arrays, loops, performance
 ---
 ## Combine Multiple Array Iterations
 Multiple `.filter()` or `.map()` calls iterate the array multiple times. Combine into one loop.
 **Incorrect (3 iterations):**
 ```typescript
 const admins = users.filter((u) => u.isAdmin);
 const testers = users.filter((u) => u.isTester);
 const inactive = users.filter((u) => !u.isActive);
 ```
 **Correct (1 iteration):**
 ```typescript
 const admins: User[] = [];
 const testers: User[] = [];
 const inactive: User[] = [];
 for (const user of users) {
  if (user.isAdmin) admins.push(user);
  if (user.isTester) testers.push(user);
  if (!user.isActive) inactive.push(user);
 }
 ```
@@ -0,0 +1,50 @@
 ---
 title: Early Return from Functions
 impact: LOW-MEDIUM
 impactDescription: avoids unnecessary computation
 tags: javascript, functions, optimization, early-return
 ---
 ## Early Return from Functions
 Return early when result is determined to skip unnecessary processing.
 **Incorrect (processes all items even after finding answer):**
 ```typescript
 function validateUsers(users: User[]) {
  let hasError = false;
  let errorMessage = "";
  for (const user of users) {
    if (!user.email) {
      hasError = true;
      errorMessage = "Email required";
    }
    if (!user.name) {
      hasError = true;
      errorMessage = "Name required";
    }
    // Continues checking all users even after error found
  }
  return hasError ? { valid: false, error: errorMessage } : { valid: true };
 }
 ```
 **Correct (returns immediately on first error):**
 ```typescript
 function validateUsers(users: User[]) {
  for (const user of users) {
    if (!user.email) {
      return { valid: false, error: "Email required" };
    }
    if (!user.name) {
      return { valid: false, error: "Name required" };
    }
  }
  return { valid: true };
 }
 ```
@@ -0,0 +1,51 @@
 ---
 title: Use flatMap to Map and Filter in One Pass
 impact: LOW-MEDIUM
 impactDescription: eliminates intermediate array
 tags: javascript, arrays, flatMap, filter, performance
 ---
 ## Use flatMap to Map and Filter in One Pass
 **Impact: LOW-MEDIUM (eliminates intermediate array)**
 Chaining `.map().filter(Boolean)` creates an intermediate array and iterates twice. Use `.flatMap()` to transform and filter in a single pass.
 **Incorrect (2 iterations, intermediate array):**
 ```typescript
 const userNames = users.map((user) => (user.isActive ? user.name : null)).filter(Boolean);
 ```
 **Correct (1 iteration, no intermediate array):**
 ```typescript
 const userNames = users.flatMap((user) => (user.isActive ? [user.name] : []));
 ```
 **More examples:**
 ```typescript
 // Extract valid emails from responses
 // Before
 const emails = responses.map((r) => (r.success ? r.data.email : null)).filter(Boolean);
 // After
 const emails = responses.flatMap((r) => (r.success ? [r.data.email] : []));
 // Parse and filter valid numbers
 // Before
 const numbers = strings.map((s) => parseInt(s, 10)).filter((n) => !isNaN(n));
 // After
 const numbers = strings.flatMap((s) => {
  const n = parseInt(s, 10);
  return isNaN(n) ? [] : [n];
 });
 ```
 **When to use:**
 - Transforming items while filtering some out
 - Conditional mapping where some inputs produce no output
 - Parsing/validating where invalid inputs should be skipped
@@ -0,0 +1,45 @@
 ---
 title: Hoist RegExp Creation
 impact: LOW-MEDIUM
 impactDescription: avoids recreation
 tags: javascript, regexp, optimization, memoization
 ---
 ## Hoist RegExp Creation
 Don't create RegExp inside render. Hoist to module scope or memoize with `useMemo()`.
 **Incorrect (new RegExp every render):**
 ```tsx
 function Highlighter({ text, query }: Props) {
  const regex = new RegExp(`(${query})`, 'gi')
  const parts = text.split(regex)
  return <>{parts.map((part, i) => ...)}</>
 }
 ```
 **Correct (memoize or hoist):**
 ```tsx
 const EMAIL_REGEX = /^[^\s@]+@[^\s@]+\.[^\s@]+$/
 function Highlighter({ text, query }: Props) {
  const regex = useMemo(
    () => new RegExp(`(${escapeRegex(query)})`, 'gi'),
    [query]
  )
  const parts = text.split(regex)
  return <>{parts.map((part, i) => ...)}</>
 }
 ```
 **Warning (global regex has mutable state):**
 Global regex (`/g`) has mutable `lastIndex` state:
 ```typescript
 const regex = /foo/g;
 regex.test("foo"); // true, lastIndex = 3
 regex.test("foo"); // false, lastIndex = 0
 ```
@@ -0,0 +1,37 @@
 ---
 title: Build Index Maps for Repeated Lookups
 impact: LOW-MEDIUM
 impactDescription: 1M ops to 2K ops
 tags: javascript, map, indexing, optimization, performance
 ---
 ## Build Index Maps for Repeated Lookups
 Multiple `.find()` calls by the same key should use a Map.
 **Incorrect (O(n) per lookup):**
 ```typescript
 function processOrders(orders: Order[], users: User[]) {
  return orders.map((order) => ({
    ...order,
    user: users.find((u) => u.id === order.userId),
  }));
 }
 ```
 **Correct (O(1) per lookup):**
 ```typescript
 function processOrders(orders: Order[], users: User[]) {
  const userById = new Map(users.map((u) => [u.id, u]));
  return orders.map((order) => ({
    ...order,
    user: userById.get(order.userId),
  }));
 }
 ```
 Build map once (O(n)), then all lookups are O(1).
 For 1000 orders × 1000 users: 1M ops → 2K ops.
@@ -0,0 +1,50 @@
 ---
 title: Early Length Check for Array Comparisons
 impact: MEDIUM-HIGH
 impactDescription: avoids expensive operations when lengths differ
 tags: javascript, arrays, performance, optimization, comparison
 ---
 ## Early Length Check for Array Comparisons
 When comparing arrays with expensive operations (sorting, deep equality, serialization), check lengths first. If lengths differ, the arrays cannot be equal.
 In real-world applications, this optimization is especially valuable when the comparison runs in hot paths (event handlers, render loops).
 **Incorrect (always runs expensive comparison):**
 ```typescript
 function hasChanges(current: string[], original: string[]) {
  // Always sorts and joins, even when lengths differ
  return current.sort().join() !== original.sort().join();
 }
 ```
 Two O(n log n) sorts run even when `current.length` is 5 and `original.length` is 100. There is also overhead of joining the arrays and comparing the strings.
 **Correct (O(1) length check first):**
 ```typescript
 function hasChanges(current: string[], original: string[]) {
  // Early return if lengths differ
  if (current.length !== original.length) {
    return true;
  }
  // Only sort when lengths match
  const currentSorted = current.toSorted();
  const originalSorted = original.toSorted();
  for (let i = 0; i < currentSorted.length; i++) {
    if (currentSorted[i] !== originalSorted[i]) {
      return true;
    }
  }
  return false;
 }
 ```
 This new approach is more efficient because:
 - It avoids the overhead of sorting and joining the arrays when lengths differ
 - It avoids consuming memory for the joined strings (especially important for large arrays)
 - It avoids mutating the original arrays
 - It returns early when a difference is found
@@ -0,0 +1,82 @@
 ---
 title: Use Loop for Min/Max Instead of Sort
 impact: LOW
 impactDescription: O(n) instead of O(n log n)
 tags: javascript, arrays, performance, sorting, algorithms
 ---
 ## Use Loop for Min/Max Instead of Sort
 Finding the smallest or largest element only requires a single pass through the array. Sorting is wasteful and slower.
 **Incorrect (O(n log n) - sort to find latest):**
 ```typescript
 interface Project {
  id: string;
  name: string;
  updatedAt: number;
 }
 function getLatestProject(projects: Project[]) {
  const sorted = [...projects].sort((a, b) => b.updatedAt - a.updatedAt);
  return sorted[0];
 }
 ```
 Sorts the entire array just to find the maximum value.
 **Incorrect (O(n log n) - sort for oldest and newest):**
 ```typescript
 function getOldestAndNewest(projects: Project[]) {
  const sorted = [...projects].sort((a, b) => a.updatedAt - b.updatedAt);
  return { oldest: sorted[0], newest: sorted[sorted.length - 1] };
 }
 ```
 Still sorts unnecessarily when only min/max are needed.
 **Correct (O(n) - single loop):**
 ```typescript
 function getLatestProject(projects: Project[]) {
  if (projects.length === 0) return null;
  let latest = projects[0];
  for (let i = 1; i < projects.length; i++) {
    if (projects[i].updatedAt > latest.updatedAt) {
      latest = projects[i];
    }
  }
  return latest;
 }
 function getOldestAndNewest(projects: Project[]) {
  if (projects.length === 0) return { oldest: null, newest: null };
  let oldest = projects[0];
  let newest = projects[0];
  for (let i = 1; i < projects.length; i++) {
    if (projects[i].updatedAt < oldest.updatedAt) oldest = projects[i];
    if (projects[i].updatedAt > newest.updatedAt) newest = projects[i];
  }
  return { oldest, newest };
 }
 ```
 Single pass through the array, no copying, no sorting.
 **Alternative (Math.min/Math.max for small arrays):**
 ```typescript
 const numbers = [5, 2, 8, 1, 9];
 const min = Math.min(...numbers);
 const max = Math.max(...numbers);
 ```
 This works for small arrays, but can be slower or just throw an error for very large arrays due to spread operator limitations. Maximal array length is approximately 124000 in Chrome 143 and 638000 in Safari 18; exact numbers may vary - see [the fiddle](https://jsfiddle.net/qw1jabsx/4/). Use the loop approach for reliability.
@@ -0,0 +1,24 @@
 ---
 title: Use Set/Map for O(1) Lookups
 impact: LOW-MEDIUM
 impactDescription: O(n) to O(1)
 tags: javascript, set, map, data-structures, performance
 ---
 ## Use Set/Map for O(1) Lookups
 Convert arrays to Set/Map for repeated membership checks.
 **Incorrect (O(n) per check):**
 ```typescript
 const allowedIds = ['a', 'b', 'c', ...]
 items.filter(item => allowedIds.includes(item.id))
 ```
 **Correct (O(1) per check):**
 ```typescript
 const allowedIds = new Set(['a', 'b', 'c', ...])
 items.filter(item => allowedIds.has(item.id))
 ```
@@ -0,0 +1,57 @@
 ---
 title: Use toSorted() Instead of sort() for Immutability
 impact: MEDIUM-HIGH
 impactDescription: prevents mutation bugs in React state
 tags: javascript, arrays, immutability, react, state, mutation
 ---
 ## Use toSorted() Instead of sort() for Immutability
 `.sort()` mutates the array in place, which can cause bugs with React state and props. Use `.toSorted()` to create a new sorted array without mutation.
 **Incorrect (mutates original array):**
 ```typescript
 function UserList({ users }: { users: User[] }) {
  // Mutates the users prop array!
  const sorted = useMemo(
    () => users.sort((a, b) => a.name.localeCompare(b.name)),
    [users]
  )
  return <div>{sorted.map(renderUser)}</div>
 }
 ```
 **Correct (creates new array):**
 ```typescript
 function UserList({ users }: { users: User[] }) {
  // Creates new sorted array, original unchanged
  const sorted = useMemo(
    () => users.toSorted((a, b) => a.name.localeCompare(b.name)),
    [users]
  )
  return <div>{sorted.map(renderUser)}</div>
 }
 ```
 **Why this matters in React:**
 1. Props/state mutations break React's immutability model - React expects props and state to be treated as read-only
 2. Causes stale closure bugs - Mutating arrays inside closures (callbacks, effects) can lead to unexpected behavior
 **Browser support (fallback for older browsers):**
 `.toSorted()` is available in all modern browsers (Chrome 110+, Safari 16+, Firefox 115+, Node.js 20+). For older environments, use spread operator:
 ```typescript
 // Fallback for older browsers
 const sorted = [...items].sort((a, b) => a.value - b.value);
 ```
 **Other immutable array methods:**
 - `.toSorted()` - immutable sort
 - `.toReversed()` - immutable reverse
 - `.toSpliced()` - immutable splice
 - `.with()` - immutable element replacement
@@ -0,0 +1,26 @@
 ---
 title: Use Activity Component for Show/Hide
 impact: MEDIUM
 impactDescription: preserves state/DOM
 tags: rendering, activity, visibility, state-preservation
 ---
 ## Use Activity Component for Show/Hide
 Use React's `<Activity>` to preserve state/DOM for expensive components that frequently toggle visibility.
 **Usage:**
 ```tsx
 import { Activity } from "react";
 function Dropdown({ isOpen }: Props) {
  return (
    <Activity mode={isOpen ? "visible" : "hidden"}>
      <ExpensiveMenu />
    </Activity>
  );
 }
 ```
 Avoids expensive re-renders and state loss.
@@ -0,0 +1,38 @@
 ---
 title: Animate SVG Wrapper Instead of SVG Element
 impact: LOW
 impactDescription: enables hardware acceleration
 tags: rendering, svg, css, animation, performance
 ---
 ## Animate SVG Wrapper Instead of SVG Element
 Many browsers don't have hardware acceleration for CSS3 animations on SVG elements. Wrap SVG in a `<div>` and animate the wrapper instead.
 **Incorrect (animating SVG directly - no hardware acceleration):**
 ```tsx
 function LoadingSpinner() {
  return (
    <svg className="animate-spin" width="24" height="24" viewBox="0 0 24 24">
      <circle cx="12" cy="12" r="10" stroke="currentColor" />
    </svg>
  );
 }
 ```
 **Correct (animating wrapper div - hardware accelerated):**
 ```tsx
 function LoadingSpinner() {
  return (
    <div className="animate-spin">
      <svg width="24" height="24" viewBox="0 0 24 24">
        <circle cx="12" cy="12" r="10" stroke="currentColor" />
      </svg>
    </div>
  );
 }
 ```
 This applies to all CSS transforms and transitions (`transform`, `opacity`, `translate`, `scale`, `rotate`). The wrapper div allows browsers to use GPU acceleration for smoother animations.
@@ -0,0 +1,32 @@
 ---
 title: Use Explicit Conditional Rendering
 impact: LOW
 impactDescription: prevents rendering 0 or NaN
 tags: rendering, conditional, jsx, falsy-values
 ---
 ## Use Explicit Conditional Rendering
 Use explicit ternary operators (`? :`) instead of `&&` for conditional rendering when the condition can be `0`, `NaN`, or other falsy values that render.
 **Incorrect (renders "0" when count is 0):**
 ```tsx
 function Badge({ count }: { count: number }) {
  return <div>{count && <span className="badge">{count}</span>}</div>;
 }
 // When count = 0, renders: <div>0</div>
 // When count = 5, renders: <div><span class="badge">5</span></div>
 ```
 **Correct (renders nothing when count is 0):**
 ```tsx
 function Badge({ count }: { count: number }) {
  return <div>{count > 0 ? <span className="badge">{count}</span> : null}</div>;
 }
 // When count = 0, renders: <div></div>
 // When count = 5, renders: <div><span class="badge">5</span></div>
 ```
@@ -0,0 +1,38 @@
 ---
 title: CSS content-visibility for Long Lists
 impact: HIGH
 impactDescription: faster initial render
 tags: rendering, css, content-visibility, long-lists
 ---
 ## CSS content-visibility for Long Lists
 Apply `content-visibility: auto` to defer off-screen rendering.
 **CSS:**
 ```css
 .message-item {
  content-visibility: auto;
  contain-intrinsic-size: 0 80px;
 }
 ```
 **Example:**
 ```tsx
 function MessageList({ messages }: { messages: Message[] }) {
  return (
    <div className="overflow-y-auto h-screen">
      {messages.map((msg) => (
        <div key={msg.id} className="message-item">
          <Avatar user={msg.author} />
          <div>{msg.content}</div>
        </div>
      ))}
    </div>
  );
 }
 ```
 For 1000 messages, browser skips layout/paint for ~990 off-screen items (10× faster initial render).
@@ -0,0 +1,36 @@
 ---
 title: Hoist Static JSX Elements
 impact: LOW
 impactDescription: avoids re-creation
 tags: rendering, jsx, static, optimization
 ---
 ## Hoist Static JSX Elements
 Extract static JSX outside components to avoid re-creation.
 **Incorrect (recreates element every render):**
 ```tsx
 function LoadingSkeleton() {
  return <div className="animate-pulse h-20 bg-gray-200" />;
 }
 function Container() {
  return <div>{loading && <LoadingSkeleton />}</div>;
 }
 ```
 **Correct (reuses same element):**
 ```tsx
 const loadingSkeleton = <div className="animate-pulse h-20 bg-gray-200" />;
 function Container() {
  return <div>{loading && loadingSkeleton}</div>;
 }
 ```
 This is especially helpful for large and static SVG nodes, which can be expensive to recreate on every render.
 **Note:** If your project has [React Compiler](https://react.dev/learn/react-compiler) enabled, the compiler automatically hoists static JSX elements and optimizes component re-renders, making manual hoisting unnecessary.
@@ -0,0 +1,72 @@
 ---
 title: Prevent Hydration Mismatch Without Flickering
 impact: MEDIUM
 impactDescription: avoids visual flicker and hydration errors
 tags: rendering, ssr, hydration, localStorage, flicker
 ---
 ## Prevent Hydration Mismatch Without Flickering
 When rendering content that depends on client-side storage (localStorage, cookies), avoid both SSR breakage and post-hydration flickering by injecting a synchronous script that updates the DOM before React hydrates.
 **Incorrect (breaks SSR):**
 ```tsx
 function ThemeWrapper({ children }: { children: ReactNode }) {
  // localStorage is not available on server - throws error
  const theme = localStorage.getItem("theme") || "light";
  return <div className={theme}>{children}</div>;
 }
 ```
 Server-side rendering will fail because `localStorage` is undefined.
 **Incorrect (visual flickering):**
 ```tsx
 function ThemeWrapper({ children }: { children: ReactNode }) {
  const [theme, setTheme] = useState("light");
  useEffect(() => {
    // Runs after hydration - causes visible flash
    const stored = localStorage.getItem("theme");
    if (stored) {
      setTheme(stored);
    }
  }, []);
  return <div className={theme}>{children}</div>;
 }
 ```
 Component first renders with default value (`light`), then updates after hydration, causing a visible flash of incorrect content.
 **Correct (no flicker, no hydration mismatch):**
 ```tsx
 function ThemeWrapper({ children }: { children: ReactNode }) {
  return (
    <>
      <div id="theme-wrapper">{children}</div>
      <script
        dangerouslySetInnerHTML={{
          __html: `
            (function() {
              try {
                var theme = localStorage.getItem('theme') || 'light';
                var el = document.getElementById('theme-wrapper');
                if (el) el.className = theme;
              } catch (e) {}
            })();
          `,
        }}
      />
    </>
  );
 }
 ```
 The inline script executes synchronously before showing the element, ensuring the DOM already has the correct value. No flickering, no hydration mismatch.
 This pattern is especially useful for theme toggles, user preferences, authentication states, and any client-only data that should render immediately without flashing default values.
@@ -0,0 +1,26 @@
 ---
 title: Suppress Expected Hydration Mismatches
 impact: LOW-MEDIUM
 impactDescription: avoids noisy hydration warnings for known differences
 tags: rendering, hydration, ssr, nextjs
 ---
 ## Suppress Expected Hydration Mismatches
 In SSR frameworks (e.g., Next.js), some values are intentionally different on server vs client (random IDs, dates, locale/timezone formatting). For these _expected_ mismatches, wrap the dynamic text in an element with `suppressHydrationWarning` to prevent noisy warnings. Do not use this to hide real bugs. Don’t overuse it.
 **Incorrect (known mismatch warnings):**
 ```tsx
 function Timestamp() {
  return <span>{new Date().toLocaleString()}</span>;
 }
 ```
 **Correct (suppress expected mismatch only):**
 ```tsx
 function Timestamp() {
  return <span suppressHydrationWarning>{new Date().toLocaleString()}</span>;
 }
 ```
@@ -0,0 +1,85 @@
 ---
 title: Use React DOM Resource Hints
 impact: HIGH
 impactDescription: reduces load time for critical resources
 tags: rendering, preload, preconnect, prefetch, resource-hints
 ---
 ## Use React DOM Resource Hints
 **Impact: HIGH (reduces load time for critical resources)**
 React DOM provides APIs to hint the browser about resources it will need. These are especially useful in server components to start loading resources before the client even receives the HTML.
 - **`prefetchDNS(href)`**: Resolve DNS for a domain you expect to connect to
 - **`preconnect(href)`**: Establish connection (DNS + TCP + TLS) to a server
 - **`preload(href, options)`**: Fetch a resource (stylesheet, font, script, image) you'll use soon
 - **`preloadModule(href)`**: Fetch an ES module you'll use soon
 - **`preinit(href, options)`**: Fetch and evaluate a stylesheet or script
 - **`preinitModule(href)`**: Fetch and evaluate an ES module
 **Example (preconnect to third-party APIs):**
 ```tsx
 import { preconnect, prefetchDNS } from "react-dom";
 export default function App() {
  prefetchDNS("https://analytics.example.com");
  preconnect("https://api.example.com");
  return <main>{/* content */}</main>;
 }
 ```
 **Example (preload critical fonts and styles):**
 ```tsx
 import { preload, preinit } from "react-dom";
 export default function RootLayout({ children }) {
  // Preload font file
  preload("/fonts/inter.woff2", { as: "font", type: "font/woff2", crossOrigin: "anonymous" });
  // Fetch and apply critical stylesheet immediately
  preinit("/styles/critical.css", { as: "style" });
  return (
    <html>
      <body>{children}</body>
    </html>
  );
 }
 ```
 **Example (preload modules for code-split routes):**
 ```tsx
 import { preloadModule, preinitModule } from "react-dom";
 function Navigation() {
  const preloadDashboard = () => {
    preloadModule("/dashboard.js", { as: "script" });
  };
  return (
    <nav>
      <a href="/dashboard" onMouseEnter={preloadDashboard}>
        Dashboard
      </a>
    </nav>
  );
 }
 ```
 **When to use each:**
 | API             | Use case                                    |
 | --------------- | ------------------------------------------- |
 | `prefetchDNS`   | Third-party domains you'll connect to later |
 | `preconnect`    | APIs or CDNs you'll fetch from immediately  |
 | `preload`       | Critical resources needed for current page  |
 | `preloadModule` | JS modules for likely next navigation       |
 | `preinit`       | Stylesheets/scripts that must execute early |
 | `preinitModule` | ES modules that must execute early          |
 Reference: [React DOM Resource Preloading APIs](https://react.dev/reference/react-dom#resource-preloading-apis)
@@ -0,0 +1,68 @@
 ---
 title: Use defer or async on Script Tags
 impact: HIGH
 impactDescription: eliminates render-blocking
 tags: rendering, script, defer, async, performance
 ---
 ## Use defer or async on Script Tags
 **Impact: HIGH (eliminates render-blocking)**
 Script tags without `defer` or `async` block HTML parsing while the script downloads and executes. This delays First Contentful Paint and Time to Interactive.
 - **`defer`**: Downloads in parallel, executes after HTML parsing completes, maintains execution order
 - **`async`**: Downloads in parallel, executes immediately when ready, no guaranteed order
 Use `defer` for scripts that depend on DOM or other scripts. Use `async` for independent scripts like analytics.
 **Incorrect (blocks rendering):**
 ```tsx
 export default function Document() {
  return (
    <html>
      <head>
        <script src="https://example.com/analytics.js" />
        <script src="/scripts/utils.js" />
      </head>
      <body>{/* content */}</body>
    </html>
  );
 }
 ```
 **Correct (non-blocking):**
 ```tsx
 export default function Document() {
  return (
    <html>
      <head>
        {/* Independent script - use async */}
        <script src="https://example.com/analytics.js" async />
        {/* DOM-dependent script - use defer */}
        <script src="/scripts/utils.js" defer />
      </head>
      <body>{/* content */}</body>
    </html>
  );
 }
 ```
 **Note:** In Next.js, prefer the `next/script` component with `strategy` prop instead of raw script tags:
 ```tsx
 import Script from "next/script";
 export default function Page() {
  return (
    <>
      <Script src="https://example.com/analytics.js" strategy="afterInteractive" />
      <Script src="/scripts/utils.js" strategy="beforeInteractive" />
    </>
  );
 }
 ```
 Reference: [MDN - Script element](https://developer.mozilla.org/en-US/docs/Web/HTML/Element/script#defer)
@@ -0,0 +1,28 @@
 ---
 title: Optimize SVG Precision
 impact: LOW
 impactDescription: reduces file size
 tags: rendering, svg, optimization, svgo
 ---
 ## Optimize SVG Precision
 Reduce SVG coordinate precision to decrease file size. The optimal precision depends on the viewBox size, but in general reducing precision should be considered.
 **Incorrect (excessive precision):**
 ```svg
 <path d="M 10.293847 20.847362 L 30.938472 40.192837" />
 ```
 **Correct (1 decimal place):**
 ```svg
 <path d="M 10.3 20.8 L 30.9 40.2" />
 ```
 **Automate with SVGO:**
 ```bash
 npx svgo --precision=1 --multipass icon.svg
 ```
@@ -0,0 +1,75 @@
 ---
 title: Use useTransition Over Manual Loading States
 impact: LOW
 impactDescription: reduces re-renders and improves code clarity
 tags: rendering, transitions, useTransition, loading, state
 ---
 ## Use useTransition Over Manual Loading States
 Use `useTransition` instead of manual `useState` for loading states. This provides built-in `isPending` state and automatically manages transitions.
 **Incorrect (manual loading state):**
 ```tsx
 function SearchResults() {
  const [query, setQuery] = useState("");
  const [results, setResults] = useState([]);
  const [isLoading, setIsLoading] = useState(false);
  const handleSearch = async (value: string) => {
    setIsLoading(true);
    setQuery(value);
    const data = await fetchResults(value);
    setResults(data);
    setIsLoading(false);
  };
  return (
    <>
      <input onChange={(e) => handleSearch(e.target.value)} />
      {isLoading && <Spinner />}
      <ResultsList results={results} />
    </>
  );
 }
 ```
 **Correct (useTransition with built-in pending state):**
 ```tsx
 import { useTransition, useState } from "react";
 function SearchResults() {
  const [query, setQuery] = useState("");
  const [results, setResults] = useState([]);
  const [isPending, startTransition] = useTransition();
  const handleSearch = (value: string) => {
    setQuery(value); // Update input immediately
    startTransition(async () => {
      // Fetch and update results
      const data = await fetchResults(value);
      setResults(data);
    });
  };
  return (
    <>
      <input onChange={(e) => handleSearch(e.target.value)} />
      {isPending && <Spinner />}
      <ResultsList results={results} />
    </>
  );
 }
 ```
 **Benefits:**
 - **Automatic pending state**: No need to manually manage `setIsLoading(true/false)`
 - **Error resilience**: Pending state correctly resets even if the transition throws
 - **Better responsiveness**: Keeps the UI responsive during updates
 - **Interrupt handling**: New transitions automatically cancel pending ones
 Reference: [useTransition](https://react.dev/reference/react/useTransition)
@@ -0,0 +1,39 @@
 ---
 title: Defer State Reads to Usage Point
 impact: MEDIUM
 impactDescription: avoids unnecessary subscriptions
 tags: rerender, searchParams, localStorage, optimization
 ---
 ## Defer State Reads to Usage Point
 Don't subscribe to dynamic state (searchParams, localStorage) if you only read it inside callbacks.
 **Incorrect (subscribes to all searchParams changes):**
 ```tsx
 function ShareButton({ chatId }: { chatId: string }) {
  const searchParams = useSearchParams();
  const handleShare = () => {
    const ref = searchParams.get("ref");
    shareChat(chatId, { ref });
  };
  return <button onClick={handleShare}>Share</button>;
 }
 ```
 **Correct (reads on demand, no subscription):**
 ```tsx
 function ShareButton({ chatId }: { chatId: string }) {
  const handleShare = () => {
    const params = new URLSearchParams(window.location.search);
    const ref = params.get("ref");
    shareChat(chatId, { ref });
  };
  return <button onClick={handleShare}>Share</button>;
 }
 ```
@@ -0,0 +1,45 @@
 ---
 title: Narrow Effect Dependencies
 impact: LOW
 impactDescription: minimizes effect re-runs
 tags: rerender, useEffect, dependencies, optimization
 ---
 ## Narrow Effect Dependencies
 Specify primitive dependencies instead of objects to minimize effect re-runs.
 **Incorrect (re-runs on any user field change):**
 ```tsx
 useEffect(() => {
  console.log(user.id);
 }, [user]);
 ```
 **Correct (re-runs only when id changes):**
 ```tsx
 useEffect(() => {
  console.log(user.id);
 }, [user.id]);
 ```
 **For derived state, compute outside effect:**
 ```tsx
 // Incorrect: runs on width=767, 766, 765...
 useEffect(() => {
  if (width < 768) {
    enableMobileMode();
  }
 }, [width]);
 // Correct: runs only on boolean transition
 const isMobile = width < 768;
 useEffect(() => {
  if (isMobile) {
    enableMobileMode();
  }
 }, [isMobile]);
 ```
@@ -0,0 +1,40 @@
 ---
 title: Calculate Derived State During Rendering
 impact: MEDIUM
 impactDescription: avoids redundant renders and state drift
 tags: rerender, derived-state, useEffect, state
 ---
 ## Calculate Derived State During Rendering
 If a value can be computed from current props/state, do not store it in state or update it in an effect. Derive it during render to avoid extra renders and state drift. Do not set state in effects solely in response to prop changes; prefer derived values or keyed resets instead.
 **Incorrect (redundant state and effect):**
 ```tsx
 function Form() {
  const [firstName, setFirstName] = useState("First");
  const [lastName, setLastName] = useState("Last");
  const [fullName, setFullName] = useState("");
  useEffect(() => {
    setFullName(firstName + " " + lastName);
  }, [firstName, lastName]);
  return <p>{fullName}</p>;
 }
 ```
 **Correct (derive during render):**
 ```tsx
 function Form() {
  const [firstName, setFirstName] = useState("First");
  const [lastName, setLastName] = useState("Last");
  const fullName = firstName + " " + lastName;
  return <p>{fullName}</p>;
 }
 ```
 References: [You Might Not Need an Effect](https://react.dev/learn/you-might-not-need-an-effect)
@@ -0,0 +1,29 @@
 ---
 title: Subscribe to Derived State
 impact: MEDIUM
 impactDescription: reduces re-render frequency
 tags: rerender, derived-state, media-query, optimization
 ---
 ## Subscribe to Derived State
 Subscribe to derived boolean state instead of continuous values to reduce re-render frequency.
 **Incorrect (re-renders on every pixel change):**
 ```tsx
 function Sidebar() {
  const width = useWindowWidth(); // updates continuously
  const isMobile = width < 768;
  return <nav className={isMobile ? "mobile" : "desktop"} />;
 }
 ```
 **Correct (re-renders only when boolean changes):**
 ```tsx
 function Sidebar() {
  const isMobile = useMediaQuery("(max-width: 767px)");
  return <nav className={isMobile ? "mobile" : "desktop"} />;
 }
 ```
@@ -0,0 +1,77 @@
 ---
 title: Use Functional setState Updates
 impact: MEDIUM
 impactDescription: prevents stale closures and unnecessary callback recreations
 tags: react, hooks, useState, useCallback, callbacks, closures
 ---
 ## Use Functional setState Updates
 When updating state based on the current state value, use the functional update form of setState instead of directly referencing the state variable. This prevents stale closures, eliminates unnecessary dependencies, and creates stable callback references.
 **Incorrect (requires state as dependency):**
 ```tsx
 function TodoList() {
  const [items, setItems] = useState(initialItems);
  // Callback must depend on items, recreated on every items change
  const addItems = useCallback(
    (newItems: Item[]) => {
      setItems([...items, ...newItems]);
    },
    [items],
  ); // ❌ items dependency causes recreations
  // Risk of stale closure if dependency is forgotten
  const removeItem = useCallback((id: string) => {
    setItems(items.filter((item) => item.id !== id));
  }, []); // ❌ Missing items dependency - will use stale items!
  return <ItemsEditor items={items} onAdd={addItems} onRemove={removeItem} />;
 }
 ```
 The first callback is recreated every time `items` changes, which can cause child components to re-render unnecessarily. The second callback has a stale closure bug—it will always reference the initial `items` value.
 **Correct (stable callbacks, no stale closures):**
 ```tsx
 function TodoList() {
  const [items, setItems] = useState(initialItems);
  // Stable callback, never recreated
  const addItems = useCallback((newItems: Item[]) => {
    setItems((curr) => [...curr, ...newItems]);
  }, []); // ✅ No dependencies needed
  // Always uses latest state, no stale closure risk
  const removeItem = useCallback((id: string) => {
    setItems((curr) => curr.filter((item) => item.id !== id));
  }, []); // ✅ Safe and stable
  return <ItemsEditor items={items} onAdd={addItems} onRemove={removeItem} />;
 }
 ```
 **Benefits:**
 1. **Stable callback references** - Callbacks don't need to be recreated when state changes
 2. **No stale closures** - Always operates on the latest state value
 3. **Fewer dependencies** - Simplifies dependency arrays and reduces memory leaks
 4. **Prevents bugs** - Eliminates the most common source of React closure bugs
 **When to use functional updates:**
 - Any setState that depends on the current state value
 - Inside useCallback/useMemo when state is needed
 - Event handlers that reference state
 - Async operations that update state
 **When direct updates are fine:**
 - Setting state to a static value: `setCount(0)`
 - Setting state from props/arguments only: `setName(newName)`
 - State doesn't depend on previous value
 **Note:** If your project has [React Compiler](https://react.dev/learn/react-compiler) enabled, the compiler can automatically optimize some cases, but functional updates are still recommended for correctness and to prevent stale closure bugs.
@@ -0,0 +1,56 @@
 ---
 title: Use Lazy State Initialization
 impact: MEDIUM
 impactDescription: wasted computation on every render
 tags: react, hooks, useState, performance, initialization
 ---
 ## Use Lazy State Initialization
 Pass a function to `useState` for expensive initial values. Without the function form, the initializer runs on every render even though the value is only used once.
 **Incorrect (runs on every render):**
 ```tsx
 function FilteredList({ items }: { items: Item[] }) {
  // buildSearchIndex() runs on EVERY render, even after initialization
  const [searchIndex, setSearchIndex] = useState(buildSearchIndex(items));
  const [query, setQuery] = useState("");
  // When query changes, buildSearchIndex runs again unnecessarily
  return <SearchResults index={searchIndex} query={query} />;
 }
 function UserProfile() {
  // JSON.parse runs on every render
  const [settings, setSettings] = useState(JSON.parse(localStorage.getItem("settings") || "{}"));
  return <SettingsForm settings={settings} onChange={setSettings} />;
 }
 ```
 **Correct (runs only once):**
 ```tsx
 function FilteredList({ items }: { items: Item[] }) {
  // buildSearchIndex() runs ONLY on initial render
  const [searchIndex, setSearchIndex] = useState(() => buildSearchIndex(items));
  const [query, setQuery] = useState("");
  return <SearchResults index={searchIndex} query={query} />;
 }
 function UserProfile() {
  // JSON.parse runs only on initial render
  const [settings, setSettings] = useState(() => {
    const stored = localStorage.getItem("settings");
    return stored ? JSON.parse(stored) : {};
  });
  return <SettingsForm settings={settings} onChange={setSettings} />;
 }
 ```
 Use lazy initialization when computing initial values from localStorage/sessionStorage, building data structures (indexes, maps), reading from the DOM, or performing heavy transformations.
 For simple primitives (`useState(0)`), direct references (`useState(props.value)`), or cheap literals (`useState({})`), the function form is unnecessary.
@@ -0,0 +1,36 @@
 ---
 title: Extract Default Non-primitive Parameter Value from Memoized Component to Constant
 impact: MEDIUM
 impactDescription: restores memoization by using a constant for default value
 tags: rerender, memo, optimization
 ---
 ## Extract Default Non-primitive Parameter Value from Memoized Component to Constant
 When memoized component has a default value for some non-primitive optional parameter, such as an array, function, or object, calling the component without that parameter results in broken memoization. This is because new value instances are created on every rerender, and they do not pass strict equality comparison in `memo()`.
 To address this issue, extract the default value into a constant.
 **Incorrect (`onClick` has different values on every rerender):**
 ```tsx
 const UserAvatar = memo(function UserAvatar({ onClick = () => {} }: { onClick?: () => void }) {
  // ...
 })
 // Used without optional onClick
 <UserAvatar />
 ```
 **Correct (stable default value):**
 ```tsx
 const NOOP = () => {};
 const UserAvatar = memo(function UserAvatar({ onClick = NOOP }: { onClick?: () => void }) {
  // ...
 })
 // Used without optional onClick
 <UserAvatar />
 ```
@@ -0,0 +1,44 @@
 ---
 title: Extract to Memoized Components
 impact: MEDIUM
 impactDescription: enables early returns
 tags: rerender, memo, useMemo, optimization
 ---
 ## Extract to Memoized Components
 Extract expensive work into memoized components to enable early returns before computation.
 **Incorrect (computes avatar even when loading):**
 ```tsx
 function Profile({ user, loading }: Props) {
  const avatar = useMemo(() => {
    const id = computeAvatarId(user);
    return <Avatar id={id} />;
  }, [user]);
  if (loading) return <Skeleton />;
  return <div>{avatar}</div>;
 }
 ```
 **Correct (skips computation when loading):**
 ```tsx
 const UserAvatar = memo(function UserAvatar({ user }: { user: User }) {
  const id = useMemo(() => computeAvatarId(user), [user]);
  return <Avatar id={id} />;
 });
 function Profile({ user, loading }: Props) {
  if (loading) return <Skeleton />;
  return (
    <div>
      <UserAvatar user={user} />
    </div>
  );
 }
 ```
 **Note:** If your project has [React Compiler](https://react.dev/learn/react-compiler) enabled, manual memoization with `memo()` and `useMemo()` is not necessary. The compiler automatically optimizes re-renders.
@@ -0,0 +1,45 @@
 ---
 title: Put Interaction Logic in Event Handlers
 impact: MEDIUM
 impactDescription: avoids effect re-runs and duplicate side effects
 tags: rerender, useEffect, events, side-effects, dependencies
 ---
 ## Put Interaction Logic in Event Handlers
 If a side effect is triggered by a specific user action (submit, click, drag), run it in that event handler. Do not model the action as state + effect; it makes effects re-run on unrelated changes and can duplicate the action.
 **Incorrect (event modeled as state + effect):**
 ```tsx
 function Form() {
  const [submitted, setSubmitted] = useState(false);
  const theme = useContext(ThemeContext);
  useEffect(() => {
    if (submitted) {
      post("/api/register");
      showToast("Registered", theme);
    }
  }, [submitted, theme]);
  return <button onClick={() => setSubmitted(true)}>Submit</button>;
 }
 ```
 **Correct (do it in the handler):**
 ```tsx
 function Form() {
  const theme = useContext(ThemeContext);
  function handleSubmit() {
    post("/api/register");
    showToast("Registered", theme);
  }
  return <button onClick={handleSubmit}>Submit</button>;
 }
 ```
 Reference: [Should this code move to an event handler?](https://react.dev/learn/removing-effect-dependencies#should-this-code-move-to-an-event-handler)
@@ -0,0 +1,75 @@
 ---
 title: Don't Define Components Inside Components
 impact: HIGH
 impactDescription: prevents remount on every render
 tags: rerender, components, remount, performance
 ---
 ## Don't Define Components Inside Components
 **Impact: HIGH (prevents remount on every render)**
 Defining a component inside another component creates a new component type on every render. React sees a different component each time and fully remounts it, destroying all state and DOM.
 A common reason developers do this is to access parent variables without passing props. Always pass props instead.
 **Incorrect (remounts on every render):**
 ```tsx
 function UserProfile({ user, theme }) {
  // Defined inside to access `theme` - BAD
  const Avatar = () => (
    <img src={user.avatarUrl} className={theme === "dark" ? "avatar-dark" : "avatar-light"} />
  );
  // Defined inside to access `user` - BAD
  const Stats = () => (
    <div>
      <span>{user.followers} followers</span>
      <span>{user.posts} posts</span>
    </div>
  );
  return (
    <div>
      <Avatar />
      <Stats />
    </div>
  );
 }
 ```
 Every time `UserProfile` renders, `Avatar` and `Stats` are new component types. React unmounts the old instances and mounts new ones, losing any internal state, running effects again, and recreating DOM nodes.
 **Correct (pass props instead):**
 ```tsx
 function Avatar({ src, theme }: { src: string; theme: string }) {
  return <img src={src} className={theme === "dark" ? "avatar-dark" : "avatar-light"} />;
 }
 function Stats({ followers, posts }: { followers: number; posts: number }) {
  return (
    <div>
      <span>{followers} followers</span>
      <span>{posts} posts</span>
    </div>
  );
 }
 function UserProfile({ user, theme }) {
  return (
    <div>
      <Avatar src={user.avatarUrl} theme={theme} />
      <Stats followers={user.followers} posts={user.posts} />
    </div>
  );
 }
 ```
 **Symptoms of this bug:**
 - Input fields lose focus on every keystroke
 - Animations restart unexpectedly
 - `useEffect` cleanup/setup runs on every parent render
 - Scroll position resets inside the component
@@ -0,0 +1,35 @@
 ---
 title: Do not wrap a simple expression with a primitive result type in useMemo
 impact: LOW-MEDIUM
 impactDescription: wasted computation on every render
 tags: rerender, useMemo, optimization
 ---
 ## Do not wrap a simple expression with a primitive result type in useMemo
 When an expression is simple (few logical or arithmetical operators) and has a primitive result type (boolean, number, string), do not wrap it in `useMemo`.
 Calling `useMemo` and comparing hook dependencies may consume more resources than the expression itself.
 **Incorrect:**
 ```tsx
 function Header({ user, notifications }: Props) {
  const isLoading = useMemo(() => {
    return user.isLoading || notifications.isLoading;
  }, [user.isLoading, notifications.isLoading]);
  if (isLoading) return <Skeleton />;
  // return some markup
 }
 ```
 **Correct:**
 ```tsx
 function Header({ user, notifications }: Props) {
  const isLoading = user.isLoading || notifications.isLoading;
  if (isLoading) return <Skeleton />;
  // return some markup
 }
 ```
@@ -0,0 +1,64 @@
 ---
 title: Split Combined Hook Computations
 impact: MEDIUM
 impactDescription: avoids recomputing independent steps
 tags: rerender, useMemo, useEffect, dependencies, optimization
 ---
 ## Split Combined Hook Computations
 When a hook contains multiple independent tasks with different dependencies, split them into separate hooks. A combined hook reruns all tasks when any dependency changes, even if some tasks don't use the changed value.
 **Incorrect (changing `sortOrder` recomputes filtering):**
 ```tsx
 const sortedProducts = useMemo(() => {
  const filtered = products.filter((p) => p.category === category);
  const sorted = filtered.toSorted((a, b) =>
    sortOrder === "asc" ? a.price - b.price : b.price - a.price,
  );
  return sorted;
 }, [products, category, sortOrder]);
 ```
 **Correct (filtering only recomputes when products or category change):**
 ```tsx
 const filteredProducts = useMemo(
  () => products.filter((p) => p.category === category),
  [products, category],
 );
 const sortedProducts = useMemo(
  () =>
    filteredProducts.toSorted((a, b) =>
      sortOrder === "asc" ? a.price - b.price : b.price - a.price,
    ),
  [filteredProducts, sortOrder],
 );
 ```
 This pattern also applies to `useEffect` when combining unrelated side effects:
 **Incorrect (both effects run when either dependency changes):**
 ```tsx
 useEffect(() => {
  analytics.trackPageView(pathname);
  document.title = `${pageTitle} | My App`;
 }, [pathname, pageTitle]);
 ```
 **Correct (effects run independently):**
 ```tsx
 useEffect(() => {
  analytics.trackPageView(pathname);
 }, [pathname]);
 useEffect(() => {
  document.title = `${pageTitle} | My App`;
 }, [pageTitle]);
 ```
 **Note:** If your project has [React Compiler](https://react.dev/learn/react-compiler) enabled, it automatically optimizes dependency tracking and may handle some of these cases for you.
@@ -0,0 +1,40 @@
 ---
 title: Use Transitions for Non-Urgent Updates
 impact: MEDIUM
 impactDescription: maintains UI responsiveness
 tags: rerender, transitions, startTransition, performance
 ---
 ## Use Transitions for Non-Urgent Updates
 Mark frequent, non-urgent state updates as transitions to maintain UI responsiveness.
 **Incorrect (blocks UI on every scroll):**
 ```tsx
 function ScrollTracker() {
  const [scrollY, setScrollY] = useState(0);
  useEffect(() => {
    const handler = () => setScrollY(window.scrollY);
    window.addEventListener("scroll", handler, { passive: true });
    return () => window.removeEventListener("scroll", handler);
  }, []);
 }
 ```
 **Correct (non-blocking updates):**
 ```tsx
 import { startTransition } from "react";
 function ScrollTracker() {
  const [scrollY, setScrollY] = useState(0);
  useEffect(() => {
    const handler = () => {
      startTransition(() => setScrollY(window.scrollY));
    };
    window.addEventListener("scroll", handler, { passive: true });
    return () => window.removeEventListener("scroll", handler);
  }, []);
 }
 ```
@@ -0,0 +1,59 @@
 ---
 title: Use useDeferredValue for Expensive Derived Renders
 impact: MEDIUM
 impactDescription: keeps input responsive during heavy computation
 tags: rerender, useDeferredValue, optimization, concurrent
 ---
 ## Use useDeferredValue for Expensive Derived Renders
 When user input triggers expensive computations or renders, use `useDeferredValue` to keep the input responsive. The deferred value lags behind, allowing React to prioritize the input update and render the expensive result when idle.
 **Incorrect (input feels laggy while filtering):**
 ```tsx
 function Search({ items }: { items: Item[] }) {
  const [query, setQuery] = useState("");
  const filtered = items.filter((item) => fuzzyMatch(item, query));
  return (
    <>
      <input value={query} onChange={(e) => setQuery(e.target.value)} />
      <ResultsList results={filtered} />
    </>
  );
 }
 ```
 **Correct (input stays snappy, results render when ready):**
 ```tsx
 function Search({ items }: { items: Item[] }) {
  const [query, setQuery] = useState("");
  const deferredQuery = useDeferredValue(query);
  const filtered = useMemo(
    () => items.filter((item) => fuzzyMatch(item, deferredQuery)),
    [items, deferredQuery],
  );
  const isStale = query !== deferredQuery;
  return (
    <>
      <input value={query} onChange={(e) => setQuery(e.target.value)} />
      <div style={{ opacity: isStale ? 0.7 : 1 }}>
        <ResultsList results={filtered} />
      </div>
    </>
  );
 }
 ```
 **When to use:**
 - Filtering/searching large lists
 - Expensive visualizations (charts, graphs) reacting to input
 - Any derived state that causes noticeable render delays
 **Note:** Wrap the expensive computation in `useMemo` with the deferred value as a dependency, otherwise it still runs on every render.
 Reference: [React useDeferredValue](https://react.dev/reference/react/useDeferredValue)
@@ -0,0 +1,73 @@
 ---
 title: Use useRef for Transient Values
 impact: MEDIUM
 impactDescription: avoids unnecessary re-renders on frequent updates
 tags: rerender, useref, state, performance
 ---
 ## Use useRef for Transient Values
 When a value changes frequently and you don't want a re-render on every update (e.g., mouse trackers, intervals, transient flags), store it in `useRef` instead of `useState`. Keep component state for UI; use refs for temporary DOM-adjacent values. Updating a ref does not trigger a re-render.
 **Incorrect (renders every update):**
 ```tsx
 function Tracker() {
  const [lastX, setLastX] = useState(0);
  useEffect(() => {
    const onMove = (e: MouseEvent) => setLastX(e.clientX);
    window.addEventListener("mousemove", onMove);
    return () => window.removeEventListener("mousemove", onMove);
  }, []);
  return (
    <div
      style={{
        position: "fixed",
        top: 0,
        left: lastX,
        width: 8,
        height: 8,
        background: "black",
      }}
    />
  );
 }
 ```
 **Correct (no re-render for tracking):**
 ```tsx
 function Tracker() {
  const lastXRef = useRef(0);
  const dotRef = useRef<HTMLDivElement>(null);
  useEffect(() => {
    const onMove = (e: MouseEvent) => {
      lastXRef.current = e.clientX;
      const node = dotRef.current;
      if (node) {
        node.style.transform = `translateX(${e.clientX}px)`;
      }
    };
    window.addEventListener("mousemove", onMove);
    return () => window.removeEventListener("mousemove", onMove);
  }, []);
  return (
    <div
      ref={dotRef}
      style={{
        position: "fixed",
        top: 0,
        left: 0,
        width: 8,
        height: 8,
        background: "black",
        transform: "translateX(0px)",
      }}
    />
  );
 }
 ```
@@ -0,0 +1,73 @@
 ---
 title: Use after() for Non-Blocking Operations
 impact: MEDIUM
 impactDescription: faster response times
 tags: server, async, logging, analytics, side-effects
 ---
 ## Use after() for Non-Blocking Operations
 Use Next.js's `after()` to schedule work that should execute after a response is sent. This prevents logging, analytics, and other side effects from blocking the response.
 **Incorrect (blocks response):**
 ```tsx
 import { logUserAction } from "@/app/utils";
 export async function POST(request: Request) {
  // Perform mutation
  await updateDatabase(request);
  // Logging blocks the response
  const userAgent = request.headers.get("user-agent") || "unknown";
  await logUserAction({ userAgent });
  return new Response(JSON.stringify({ status: "success" }), {
    status: 200,
    headers: { "Content-Type": "application/json" },
  });
 }
 ```
 **Correct (non-blocking):**
 ```tsx
 import { after } from "next/server";
 import { headers, cookies } from "next/headers";
 import { logUserAction } from "@/app/utils";
 export async function POST(request: Request) {
  // Perform mutation
  await updateDatabase(request);
  // Log after response is sent
  after(async () => {
    const userAgent = (await headers()).get("user-agent") || "unknown";
    const sessionCookie = (await cookies()).get("session-id")?.value || "anonymous";
    logUserAction({ sessionCookie, userAgent });
  });
  return new Response(JSON.stringify({ status: "success" }), {
    status: 200,
    headers: { "Content-Type": "application/json" },
  });
 }
 ```
 The response is sent immediately while logging happens in the background.
 **Common use cases:**
 - Analytics tracking
 - Audit logging
 - Sending notifications
 - Cache invalidation
 - Cleanup tasks
 **Important notes:**
 - `after()` runs even if the response fails or redirects
 - Works in Server Actions, Route Handlers, and Server Components
 Reference: [https://nextjs.org/docs/app/api-reference/functions/after](https://nextjs.org/docs/app/api-reference/functions/after)
@@ -0,0 +1,96 @@
 ---
 title: Authenticate Server Actions Like API Routes
 impact: CRITICAL
 impactDescription: prevents unauthorized access to server mutations
 tags: server, server-actions, authentication, security, authorization
 ---
 ## Authenticate Server Actions Like API Routes
 **Impact: CRITICAL (prevents unauthorized access to server mutations)**
 Server Actions (functions with `"use server"`) are exposed as public endpoints, just like API routes. Always verify authentication and authorization **inside** each Server Action—do not rely solely on middleware, layout guards, or page-level checks, as Server Actions can be invoked directly.
 Next.js documentation explicitly states: "Treat Server Actions with the same security considerations as public-facing API endpoints, and verify if the user is allowed to perform a mutation."
 **Incorrect (no authentication check):**
 ```typescript
 "use server";
 export async function deleteUser(userId: string) {
  // Anyone can call this! No auth check
  await db.user.delete({ where: { id: userId } });
  return { success: true };
 }
 ```
 **Correct (authentication inside the action):**
 ```typescript
 "use server";
 import { verifySession } from "@/lib/auth";
 import { unauthorized } from "@/lib/errors";
 export async function deleteUser(userId: string) {
  // Always check auth inside the action
  const session = await verifySession();
  if (!session) {
    throw unauthorized("Must be logged in");
  }
  // Check authorization too
  if (session.user.role !== "admin" && session.user.id !== userId) {
    throw unauthorized("Cannot delete other users");
  }
  await db.user.delete({ where: { id: userId } });
  return { success: true };
 }
 ```
 **With input validation:**
 ```typescript
 "use server";
 import { verifySession } from "@/lib/auth";
 import { z } from "zod";
 const updateProfileSchema = z.object({
  userId: z.string().uuid(),
  name: z.string().min(1).max(100),
  email: z.string().email(),
 });
 export async function updateProfile(data: unknown) {
  // Validate input first
  const validated = updateProfileSchema.parse(data);
  // Then authenticate
  const session = await verifySession();
  if (!session) {
    throw new Error("Unauthorized");
  }
  // Then authorize
  if (session.user.id !== validated.userId) {
    throw new Error("Can only update own profile");
  }
  // Finally perform the mutation
  await db.user.update({
    where: { id: validated.userId },
    data: {
      name: validated.name,
      email: validated.email,
    },
  });
  return { success: true };
 }
 ```
 Reference: [https://nextjs.org/docs/app/guides/authentication](https://nextjs.org/docs/app/guides/authentication)
@@ -0,0 +1,41 @@
 ---
 title: Cross-Request LRU Caching
 impact: HIGH
 impactDescription: caches across requests
 tags: server, cache, lru, cross-request
 ---
 ## Cross-Request LRU Caching
 `React.cache()` only works within one request. For data shared across sequential requests (user clicks button A then button B), use an LRU cache.
 **Implementation:**
 ```typescript
 import { LRUCache } from "lru-cache";
 const cache = new LRUCache<string, any>({
  max: 1000,
  ttl: 5 * 60 * 1000, // 5 minutes
 });
 export async function getUser(id: string) {
  const cached = cache.get(id);
  if (cached) return cached;
  const user = await db.user.findUnique({ where: { id } });
  cache.set(id, user);
  return user;
 }
 // Request 1: DB query, result cached
 // Request 2: cache hit, no DB query
 ```
 Use when sequential user actions hit multiple endpoints needing the same data within seconds.
 **With Vercel's [Fluid Compute](https://vercel.com/docs/fluid-compute):** LRU caching is especially effective because multiple concurrent requests can share the same function instance and cache. This means the cache persists across requests without needing external storage like Redis.
 **In traditional serverless:** Each invocation runs in isolation, so consider Redis for cross-process caching.
 Reference: [https://github.com/isaacs/node-lru-cache](https://github.com/isaacs/node-lru-cache)
@@ -0,0 +1,76 @@
 ---
 title: Per-Request Deduplication with React.cache()
 impact: MEDIUM
 impactDescription: deduplicates within request
 tags: server, cache, react-cache, deduplication
 ---
 ## Per-Request Deduplication with React.cache()
 Use `React.cache()` for server-side request deduplication. Authentication and database queries benefit most.
 **Usage:**
 ```typescript
 import { cache } from "react";
 export const getCurrentUser = cache(async () => {
  const session = await auth();
  if (!session?.user?.id) return null;
  return await db.user.findUnique({
    where: { id: session.user.id },
  });
 });
 ```
 Within a single request, multiple calls to `getCurrentUser()` execute the query only once.
 **Avoid inline objects as arguments:**
 `React.cache()` uses shallow equality (`Object.is`) to determine cache hits. Inline objects create new references each call, preventing cache hits.
 **Incorrect (always cache miss):**
 ```typescript
 const getUser = cache(async (params: { uid: number }) => {
  return await db.user.findUnique({ where: { id: params.uid } });
 });
 // Each call creates new object, never hits cache
 getUser({ uid: 1 });
 getUser({ uid: 1 }); // Cache miss, runs query again
 ```
 **Correct (cache hit):**
 ```typescript
 const getUser = cache(async (uid: number) => {
  return await db.user.findUnique({ where: { id: uid } });
 });
 // Primitive args use value equality
 getUser(1);
 getUser(1); // Cache hit, returns cached result
 ```
 If you must pass objects, pass the same reference:
 ```typescript
 const params = { uid: 1 };
 getUser(params); // Query runs
 getUser(params); // Cache hit (same reference)
 ```
 **Next.js-Specific Note:**
 In Next.js, the `fetch` API is automatically extended with request memoization. Requests with the same URL and options are automatically deduplicated within a single request, so you don't need `React.cache()` for `fetch` calls. However, `React.cache()` is still essential for other async tasks:
 - Database queries (Prisma, Drizzle, etc.)
 - Heavy computations
 - Authentication checks
 - File system operations
 - Any non-fetch async work
 Use `React.cache()` to deduplicate these operations across your component tree.
 Reference: [React.cache documentation](https://react.dev/reference/react/cache)
@@ -0,0 +1,65 @@
 ---
 title: Avoid Duplicate Serialization in RSC Props
 impact: LOW
 impactDescription: reduces network payload by avoiding duplicate serialization
 tags: server, rsc, serialization, props, client-components
 ---
 ## Avoid Duplicate Serialization in RSC Props
 **Impact: LOW (reduces network payload by avoiding duplicate serialization)**
 RSC→client serialization deduplicates by object reference, not value. Same reference = serialized once; new reference = serialized again. Do transformations (`.toSorted()`, `.filter()`, `.map()`) in client, not server.
 **Incorrect (duplicates array):**
 ```tsx
 // RSC: sends 6 strings (2 arrays × 3 items)
 <ClientList usernames={usernames} usernamesOrdered={usernames.toSorted()} />
 ```
 **Correct (sends 3 strings):**
 ```tsx
 // RSC: send once
 <ClientList usernames={usernames} />;
 // Client: transform there
 ("use client");
 const sorted = useMemo(() => [...usernames].sort(), [usernames]);
 ```
 **Nested deduplication behavior:**
 Deduplication works recursively. Impact varies by data type:
 - `string[]`, `number[]`, `boolean[]`: **HIGH impact** - array + all primitives fully duplicated
 - `object[]`: **LOW impact** - array duplicated, but nested objects deduplicated by reference
 ```tsx
 // string[] - duplicates everything
 usernames={['a','b']} sorted={usernames.toSorted()} // sends 4 strings
 // object[] - duplicates array structure only
 users={[{id:1},{id:2}]} sorted={users.toSorted()} // sends 2 arrays + 2 unique objects (not 4)
 ```
 **Operations breaking deduplication (create new references):**
 - Arrays: `.toSorted()`, `.filter()`, `.map()`, `.slice()`, `[...arr]`
 - Objects: `{...obj}`, `Object.assign()`, `structuredClone()`, `JSON.parse(JSON.stringify())`
 **More examples:**
 ```tsx
 // ❌ Bad
 <C users={users} active={users.filter(u => u.active)} />
 <C product={product} productName={product.name} />
 // ✅ Good
 <C users={users} />
 <C product={product} />
 // Do filtering/destructuring in client
 ```
 **Exception:** Pass derived data when transformation is expensive or client doesn't need original.
@@ -0,0 +1,136 @@
 ---
 title: Hoist Static I/O to Module Level
 impact: HIGH
 impactDescription: avoids repeated file/network I/O per request
 tags: server, io, performance, next.js, route-handlers, og-image
 ---
 ## Hoist Static I/O to Module Level
 **Impact: HIGH (avoids repeated file/network I/O per request)**
 When loading static assets (fonts, logos, images, config files) in route handlers or server functions, hoist the I/O operation to module level. Module-level code runs once when the module is first imported, not on every request. This eliminates redundant file system reads or network fetches that would otherwise run on every invocation.
 **Incorrect: reads font file on every request**
 ```typescript
 // app/api/og/route.tsx
 import { ImageResponse } from 'next/og'
 export async function GET(request: Request) {
  // Runs on EVERY request - expensive!
  const fontData = await fetch(
    new URL('./fonts/Inter.ttf', import.meta.url)
  ).then(res => res.arrayBuffer())
  const logoData = await fetch(
    new URL('./images/logo.png', import.meta.url)
  ).then(res => res.arrayBuffer())
  return new ImageResponse(
    <div style={{ fontFamily: 'Inter' }}>
      <img src={logoData} />
      Hello World
    </div>,
    { fonts: [{ name: 'Inter', data: fontData }] }
  )
 }
 ```
 **Correct: loads once at module initialization**
 ```typescript
 // app/api/og/route.tsx
 import { ImageResponse } from 'next/og'
 // Module-level: runs ONCE when module is first imported
 const fontData = fetch(
  new URL('./fonts/Inter.ttf', import.meta.url)
 ).then(res => res.arrayBuffer())
 const logoData = fetch(
  new URL('./images/logo.png', import.meta.url)
 ).then(res => res.arrayBuffer())
 export async function GET(request: Request) {
  // Await the already-started promises
  const [font, logo] = await Promise.all([fontData, logoData])
  return new ImageResponse(
    <div style={{ fontFamily: 'Inter' }}>
      <img src={logo} />
      Hello World
    </div>,
    { fonts: [{ name: 'Inter', data: font }] }
  )
 }
 ```
 **Alternative: synchronous file reads with Node.js fs**
 ```typescript
 // app/api/og/route.tsx
 import { ImageResponse } from 'next/og'
 import { readFileSync } from 'fs'
 import { join } from 'path'
 // Synchronous read at module level - blocks only during module init
 const fontData = readFileSync(
  join(process.cwd(), 'public/fonts/Inter.ttf')
 )
 const logoData = readFileSync(
  join(process.cwd(), 'public/images/logo.png')
 )
 export async function GET(request: Request) {
  return new ImageResponse(
    <div style={{ fontFamily: 'Inter' }}>
      <img src={logoData} />
      Hello World
    </div>,
    { fonts: [{ name: 'Inter', data: fontData }] }
  )
 }
 ```
 **General Node.js example: loading config or templates**
 ```typescript
 // Incorrect: reads config on every call
 export async function processRequest(data: Data) {
  const config = JSON.parse(await fs.readFile("./config.json", "utf-8"));
  const template = await fs.readFile("./template.html", "utf-8");
  return render(template, data, config);
 }
 // Correct: loads once at module level
 const configPromise = fs.readFile("./config.json", "utf-8").then(JSON.parse);
 const templatePromise = fs.readFile("./template.html", "utf-8");
 export async function processRequest(data: Data) {
  const [config, template] = await Promise.all([configPromise, templatePromise]);
  return render(template, data, config);
 }
 ```
 **When to use this pattern:**
 - Loading fonts for OG image generation
 - Loading static logos, icons, or watermarks
 - Reading configuration files that don't change at runtime
 - Loading email templates or other static templates
 - Any static asset that's the same across all requests
 **When NOT to use this pattern:**
 - Assets that vary per request or user
 - Files that may change during runtime (use caching with TTL instead)
 - Large files that would consume too much memory if kept loaded
 - Sensitive data that shouldn't persist in memory
 **With Vercel's [Fluid Compute](https://vercel.com/docs/fluid-compute):** Module-level caching is especially effective because multiple concurrent requests share the same function instance. The static assets stay loaded in memory across requests without cold start penalties.
 **In traditional serverless:** Each cold start re-executes module-level code, but subsequent warm invocations reuse the loaded assets until the instance is recycled.
@@ -0,0 +1,83 @@
 ---
 title: Parallel Data Fetching with Component Composition
 impact: CRITICAL
 impactDescription: eliminates server-side waterfalls
 tags: server, rsc, parallel-fetching, composition
 ---
 ## Parallel Data Fetching with Component Composition
 React Server Components execute sequentially within a tree. Restructure with composition to parallelize data fetching.
 **Incorrect (Sidebar waits for Page's fetch to complete):**
 ```tsx
 export default async function Page() {
  const header = await fetchHeader();
  return (
    <div>
      <div>{header}</div>
      <Sidebar />
    </div>
  );
 }
 async function Sidebar() {
  const items = await fetchSidebarItems();
  return <nav>{items.map(renderItem)}</nav>;
 }
 ```
 **Correct (both fetch simultaneously):**
 ```tsx
 async function Header() {
  const data = await fetchHeader();
  return <div>{data}</div>;
 }
 async function Sidebar() {
  const items = await fetchSidebarItems();
  return <nav>{items.map(renderItem)}</nav>;
 }
 export default function Page() {
  return (
    <div>
      <Header />
      <Sidebar />
    </div>
  );
 }
 ```
 **Alternative with children prop:**
 ```tsx
 async function Header() {
  const data = await fetchHeader();
  return <div>{data}</div>;
 }
 async function Sidebar() {
  const items = await fetchSidebarItems();
  return <nav>{items.map(renderItem)}</nav>;
 }
 function Layout({ children }: { children: ReactNode }) {
  return (
    <div>
      <Header />
      {children}
    </div>
  );
 }
 export default function Page() {
  return (
    <Layout>
      <Sidebar />
    </Layout>
  );
 }
 ```
@@ -0,0 +1,38 @@
 ---
 title: Minimize Serialization at RSC Boundaries
 impact: HIGH
 impactDescription: reduces data transfer size
 tags: server, rsc, serialization, props
 ---
 ## Minimize Serialization at RSC Boundaries
 The React Server/Client boundary serializes all object properties into strings and embeds them in the HTML response and subsequent RSC requests. This serialized data directly impacts page weight and load time, so **size matters a lot**. Only pass fields that the client actually uses.
 **Incorrect (serializes all 50 fields):**
 ```tsx
 async function Page() {
  const user = await fetchUser(); // 50 fields
  return <Profile user={user} />;
 }
 ("use client");
 function Profile({ user }: { user: User }) {
  return <div>{user.name}</div>; // uses 1 field
 }
 ```
 **Correct (serializes only 1 field):**
 ```tsx
 async function Page() {
  const user = await fetchUser();
  return <Profile name={user.name} />;
 }
 ("use client");
 function Profile({ name }: { name: string }) {
  return <div>{name}</div>;
 }
 ```
@@ -0,0 +1,145 @@
 ---
 name: vercel-react-best-practices
 description: React and Next.js performance optimization guidelines from Vercel Engineering. This skill should be used when writing, reviewing, or refactoring React/Next.js code to ensure optimal performance patterns. Triggers on tasks involving React components, Next.js pages, data fetching, bundle optimization, or performance improvements.
 license: MIT
 metadata:
  author: vercel
  version: "1.0.0"
 ---
 # Vercel React Best Practices
 Comprehensive performance optimization guide for React and Next.js applications, maintained by Vercel. Contains 64 rules across 8 categories, prioritized by impact to guide automated refactoring and code generation.
 ## When to Apply
 Reference these guidelines when:
 - Writing new React components or Next.js pages
 - Implementing data fetching (client or server-side)
 - Reviewing code for performance issues
 - Refactoring existing React/Next.js code
 - Optimizing bundle size or load times
 ## Rule Categories by Priority
 | Priority | Category                  | Impact      | Prefix       |
 | -------- | ------------------------- | ----------- | ------------ |
 | 1        | Eliminating Waterfalls    | CRITICAL    | `async-`     |
 | 2        | Bundle Size Optimization  | CRITICAL    | `bundle-`    |
 | 3        | Server-Side Performance   | HIGH        | `server-`    |
 | 4        | Client-Side 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-`  |
 ## 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-auth-actions` - Authenticate server actions like API routes
 - `server-cache-react` - Use React.cache() for per-request deduplication
 - `server-cache-lru` - Use LRU cache for cross-request caching
 - `server-dedup-props` - Avoid duplicate serialization in RSC props
 - `server-hoist-static-io` - Hoist static I/O (fonts, logos) to module level
 - `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
 - `client-passive-event-listeners` - Use passive listeners for scroll
 - `client-localstorage-schema` - Version and minimize localStorage data
 ### 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-memo-with-default-value` - Hoist default non-primitive props
 - `rerender-dependencies` - Use primitive dependencies in effects
 - `rerender-derived-state` - Subscribe to derived booleans, not raw values
 - `rerender-derived-state-no-effect` - Derive state during render, not effects
 - `rerender-functional-setstate` - Use functional setState for stable callbacks
 - `rerender-lazy-state-init` - Pass function to useState for expensive values
 - `rerender-simple-expression-in-memo` - Avoid memo for simple primitives
 - `rerender-split-combined-hooks` - Split hooks with independent dependencies
 - `rerender-move-effect-to-event` - Put interaction logic in event handlers
 - `rerender-transitions` - Use startTransition for non-urgent updates
 - `rerender-use-deferred-value` - Defer expensive renders to keep input responsive
 - `rerender-use-ref-transient-values` - Use refs for transient frequent values
 - `rerender-no-inline-components` - Don't define components inside components
 ### 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-hydration-suppress-warning` - Suppress expected mismatches
 - `rendering-activity` - Use Activity component for show/hide
 - `rendering-conditional-render` - Use ternary, not && for conditionals
 - `rendering-usetransition-loading` - Prefer useTransition for loading state
 - `rendering-resource-hints` - Use React DOM resource hints for preloading
 - `rendering-script-defer-async` - Use defer or async on script tags
 ### 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
 - `js-flatmap-filter` - Use flatMap to map and filter in one pass
 ### 8. Advanced Patterns (LOW)
 - `advanced-event-handler-refs` - Store event handlers in refs
 - `advanced-init-once` - Initialize app once per app load
 - `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
 ```
 Each rule file contains:
 - Brief explanation of why it matters
 - Incorrect code example with explanation
 - Correct code example with explanation
 - Additional context and references
 ## Full Compiled Document
 For the complete guide with all rules expanded: `AGENTS.md`
@@ -0,0 +1,46 @@
 # Sections
 This file defines all sections, their ordering, impact levels, and descriptions.
 The section ID (in parentheses) is the filename prefix used to group rules.
 ---
 ## 1. Eliminating Waterfalls (async)
 **Impact:** CRITICAL  
 **Description:** Waterfalls are the #1 performance killer. Each sequential await adds full network latency. Eliminating them yields the largest gains.
 ## 2. Bundle Size Optimization (bundle)
 **Impact:** CRITICAL  
 **Description:** Reducing initial bundle size improves Time to Interactive and Largest Contentful Paint.
 ## 3. Server-Side Performance (server)
 **Impact:** HIGH  
 **Description:** Optimizing server-side rendering and data fetching eliminates server-side waterfalls and reduces response times.
 ## 4. Client-Side Data Fetching (client)
 **Impact:** MEDIUM-HIGH  
 **Description:** Automatic deduplication and efficient data fetching patterns reduce redundant network requests.
 ## 5. Re-render Optimization (rerender)
 **Impact:** MEDIUM  
 **Description:** Reducing unnecessary re-renders minimizes wasted computation and improves UI responsiveness.
 ## 6. Rendering Performance (rendering)
 **Impact:** MEDIUM  
 **Description:** Optimizing the rendering process reduces the work the browser needs to do.
 ## 7. JavaScript Performance (js)
 **Impact:** LOW-MEDIUM  
 **Description:** Micro-optimizations for hot paths can add up to meaningful improvements.
 ## 8. Advanced Patterns (advanced)
 **Impact:** LOW  
 **Description:** Advanced patterns for specific cases that require careful implementation.
@@ -0,0 +1,28 @@
 ---
 title: Rule Title Here
 impact: MEDIUM
 impactDescription: Optional description of impact (e.g., "20-50% improvement")
 tags: tag1, tag2
 ---
 ## Rule Title Here
 **Impact: MEDIUM (optional impact description)**
 Brief explanation of the rule and why it matters. This should be clear and concise, explaining the performance implications.
 **Incorrect (description of what's wrong):**
 ```typescript
 // Bad code example here
 const bad = example();
 ```
 **Correct (description of what's right):**
 ```typescript
 // Good code example here
 const good = example();
 ```
 Reference: [Link to documentation or resource](https://example.com)
@@ -0,0 +1,55 @@
 ---
 title: Store Event Handlers in Refs
 impact: LOW
 impactDescription: stable subscriptions
 tags: advanced, hooks, refs, event-handlers, optimization
 ---
 ## Store Event Handlers in Refs
 Store callbacks in refs when used in effects that shouldn't re-subscribe on callback changes.
 **Incorrect (re-subscribes on every render):**
 ```tsx
 function useWindowEvent(event: string, handler: (e) => void) {
  useEffect(() => {
    window.addEventListener(event, handler);
    return () => window.removeEventListener(event, handler);
  }, [event, handler]);
 }
 ```
 **Correct (stable subscription):**
 ```tsx
 function useWindowEvent(event: string, handler: (e) => void) {
  const handlerRef = useRef(handler);
  useEffect(() => {
    handlerRef.current = handler;
  }, [handler]);
  useEffect(() => {
    const listener = (e) => handlerRef.current(e);
    window.addEventListener(event, listener);
    return () => window.removeEventListener(event, listener);
  }, [event]);
 }
 ```
 **Alternative: use `useEffectEvent` if you're on latest React:**
 ```tsx
 import { useEffectEvent } from "react";
 function useWindowEvent(event: string, handler: (e) => void) {
  const onEvent = useEffectEvent(handler);
  useEffect(() => {
    window.addEventListener(event, onEvent);
    return () => window.removeEventListener(event, onEvent);
  }, [event]);
 }
 ```
 `useEffectEvent` provides a cleaner API for the same pattern: it creates a stable function reference that always calls the latest version of the handler.
@@ -0,0 +1,42 @@
 ---
 title: Initialize App Once, Not Per Mount
 impact: LOW-MEDIUM
 impactDescription: avoids duplicate init in development
 tags: initialization, useEffect, app-startup, side-effects
 ---
 ## Initialize App Once, Not Per Mount
 Do not put app-wide initialization that must run once per app load inside `useEffect([])` of a component. Components can remount and effects will re-run. Use a module-level guard or top-level init in the entry module instead.
 **Incorrect (runs twice in dev, re-runs on remount):**
 ```tsx
 function Comp() {
  useEffect(() => {
    loadFromStorage();
    checkAuthToken();
  }, []);
  // ...
 }
 ```
 **Correct (once per app load):**
 ```tsx
 let didInit = false;
 function Comp() {
  useEffect(() => {
    if (didInit) return;
    didInit = true;
    loadFromStorage();
    checkAuthToken();
  }, []);
  // ...
 }
 ```
 Reference: [Initializing the application](https://react.dev/learn/you-might-not-need-an-effect#initializing-the-application)
@@ -0,0 +1,39 @@
 ---
 title: useEffectEvent for Stable Callback Refs
 impact: LOW
 impactDescription: prevents effect re-runs
 tags: advanced, hooks, useEffectEvent, refs, optimization
 ---
 ## useEffectEvent for Stable Callback Refs
 Access latest values in callbacks without adding them to dependency arrays. Prevents effect re-runs while avoiding stale closures.
 **Incorrect (effect re-runs on every callback change):**
 ```tsx
 function SearchInput({ onSearch }: { onSearch: (q: string) => void }) {
  const [query, setQuery] = useState("");
  useEffect(() => {
    const timeout = setTimeout(() => onSearch(query), 300);
    return () => clearTimeout(timeout);
  }, [query, onSearch]);
 }
 ```
 **Correct (using React's useEffectEvent):**
 ```tsx
 import { useEffectEvent } from "react";
 function SearchInput({ onSearch }: { onSearch: (q: string) => void }) {
  const [query, setQuery] = useState("");
  const onSearchEvent = useEffectEvent(onSearch);
  useEffect(() => {
    const timeout = setTimeout(() => onSearchEvent(query), 300);
    return () => clearTimeout(timeout);
  }, [query]);
 }
 ```
@@ -0,0 +1,35 @@
 ---
 title: Prevent Waterfall Chains in API Routes
 impact: CRITICAL
 impactDescription: 2-10× improvement
 tags: api-routes, server-actions, waterfalls, parallelization
 ---
 ## Prevent Waterfall Chains in API Routes
 In API routes and Server Actions, start independent operations immediately, even if you don't await them yet.
 **Incorrect (config waits for auth, data waits for both):**
 ```typescript
 export async function GET(request: Request) {
  const session = await auth();
  const config = await fetchConfig();
  const data = await fetchData(session.user.id);
  return Response.json({ data, config });
 }
 ```
 **Correct (auth and config start immediately):**
 ```typescript
 export async function GET(request: Request) {
  const sessionPromise = auth();
  const configPromise = fetchConfig();
  const session = await sessionPromise;
  const [config, data] = await Promise.all([configPromise, fetchData(session.user.id)]);
  return Response.json({ data, config });
 }
 ```
 For operations with more complex dependency chains, use `better-all` to automatically maximize parallelism (see Dependency-Based Parallelization).
@@ -0,0 +1,80 @@
 ---
 title: Defer Await Until Needed
 impact: HIGH
 impactDescription: avoids blocking unused code paths
 tags: async, await, conditional, optimization
 ---
 ## Defer Await Until Needed
 Move `await` operations into the branches where they're actually used to avoid blocking code paths that don't need them.
 **Incorrect (blocks both branches):**
 ```typescript
 async function handleRequest(userId: string, skipProcessing: boolean) {
  const userData = await fetchUserData(userId);
  if (skipProcessing) {
    // Returns immediately but still waited for userData
    return { skipped: true };
  }
  // Only this branch uses userData
  return processUserData(userData);
 }
 ```
 **Correct (only blocks when needed):**
 ```typescript
 async function handleRequest(userId: string, skipProcessing: boolean) {
  if (skipProcessing) {
    // Returns immediately without waiting
    return { skipped: true };
  }
  // Fetch only when needed
  const userData = await fetchUserData(userId);
  return processUserData(userData);
 }
 ```
 **Another example (early return optimization):**
 ```typescript
 // Incorrect: always fetches permissions
 async function updateResource(resourceId: string, userId: string) {
  const permissions = await fetchPermissions(userId);
  const resource = await getResource(resourceId);
  if (!resource) {
    return { error: "Not found" };
  }
  if (!permissions.canEdit) {
    return { error: "Forbidden" };
  }
  return await updateResourceData(resource, permissions);
 }
 // Correct: fetches only when needed
 async function updateResource(resourceId: string, userId: string) {
  const resource = await getResource(resourceId);
  if (!resource) {
    return { error: "Not found" };
  }
  const permissions = await fetchPermissions(userId);
  if (!permissions.canEdit) {
    return { error: "Forbidden" };
  }
  return await updateResourceData(resource, permissions);
 }
 ```
 This optimization is especially valuable when the skipped branch is frequently taken, or when the deferred operation is expensive.
@@ -0,0 +1,48 @@
 ---
 title: Dependency-Based Parallelization
 impact: CRITICAL
 impactDescription: 2-10× improvement
 tags: async, parallelization, dependencies, better-all
 ---
 ## Dependency-Based Parallelization
 For operations with partial dependencies, use `better-all` to maximize parallelism. It automatically starts each task at the earliest possible moment.
 **Incorrect (profile waits for config unnecessarily):**
 ```typescript
 const [user, config] = await Promise.all([fetchUser(), fetchConfig()]);
 const profile = await fetchProfile(user.id);
 ```
 **Correct (config and profile run in parallel):**
 ```typescript
 import { all } from "better-all";
 const { user, config, profile } = await all({
  async user() {
    return fetchUser();
  },
  async config() {
    return fetchConfig();
  },
  async profile() {
    return fetchProfile((await this.$.user).id);
  },
 });
 ```
 **Alternative without extra dependencies:**
 We can also create all the promises first, and do `Promise.all()` at the end.
 ```typescript
 const userPromise = fetchUser();
 const profilePromise = userPromise.then((user) => fetchProfile(user.id));
 const [user, config, profile] = await Promise.all([userPromise, fetchConfig(), profilePromise]);
 ```
 Reference: [https://github.com/shuding/better-all](https://github.com/shuding/better-all)
@@ -0,0 +1,24 @@
 ---
 title: Promise.all() for Independent Operations
 impact: CRITICAL
 impactDescription: 2-10× improvement
 tags: async, parallelization, promises, waterfalls
 ---
 ## Promise.all() for Independent Operations
 When async operations have no interdependencies, execute them concurrently using `Promise.all()`.
 **Incorrect (sequential execution, 3 round trips):**
 ```typescript
 const user = await fetchUser();
 const posts = await fetchPosts();
 const comments = await fetchComments();
 ```
 **Correct (parallel execution, 1 round trip):**
 ```typescript
 const [user, posts, comments] = await Promise.all([fetchUser(), fetchPosts(), fetchComments()]);
 ```
@@ -0,0 +1,99 @@
 ---
 title: Strategic Suspense Boundaries
 impact: HIGH
 impactDescription: faster initial paint
 tags: async, suspense, streaming, layout-shift
 ---
 ## Strategic Suspense Boundaries
 Instead of awaiting data in async components before returning JSX, use Suspense boundaries to show the wrapper UI faster while data loads.
 **Incorrect (wrapper blocked by data fetching):**
 ```tsx
 async function Page() {
  const data = await fetchData(); // Blocks entire page
  return (
    <div>
      <div>Sidebar</div>
      <div>Header</div>
      <div>
        <DataDisplay data={data} />
      </div>
      <div>Footer</div>
    </div>
  );
 }
 ```
 The entire layout waits for data even though only the middle section needs it.
 **Correct (wrapper shows immediately, data streams in):**
 ```tsx
 function Page() {
  return (
    <div>
      <div>Sidebar</div>
      <div>Header</div>
      <div>
        <Suspense fallback={<Skeleton />}>
          <DataDisplay />
        </Suspense>
      </div>
      <div>Footer</div>
    </div>
  );
 }
 async function DataDisplay() {
  const data = await fetchData(); // Only blocks this component
  return <div>{data.content}</div>;
 }
 ```
 Sidebar, Header, and Footer render immediately. Only DataDisplay waits for data.
 **Alternative (share promise across components):**
 ```tsx
 function Page() {
  // Start fetch immediately, but don't await
  const dataPromise = fetchData();
  return (
    <div>
      <div>Sidebar</div>
      <div>Header</div>
      <Suspense fallback={<Skeleton />}>
        <DataDisplay dataPromise={dataPromise} />
        <DataSummary dataPromise={dataPromise} />
      </Suspense>
      <div>Footer</div>
    </div>
  );
 }
 function DataDisplay({ dataPromise }: { dataPromise: Promise<Data> }) {
  const data = use(dataPromise); // Unwraps the promise
  return <div>{data.content}</div>;
 }
 function DataSummary({ dataPromise }: { dataPromise: Promise<Data> }) {
  const data = use(dataPromise); // Reuses the same promise
  return <div>{data.summary}</div>;
 }
 ```
 Both components share the same promise, so only one fetch occurs. Layout renders immediately while both components wait together.
 **When NOT to use this pattern:**
 - Critical data needed for layout decisions (affects positioning)
 - SEO-critical content above the fold
 - Small, fast queries where suspense overhead isn't worth it
 - When you want to avoid layout shift (loading → content jump)
 **Trade-off:** Faster initial paint vs potential layout shift. Choose based on your UX priorities.
@@ -0,0 +1,59 @@
 ---
 title: Avoid Barrel File Imports
 impact: CRITICAL
 impactDescription: 200-800ms import cost, slow builds
 tags: bundle, imports, tree-shaking, barrel-files, performance
 ---
 ## Avoid Barrel File Imports
 Import directly from source files instead of barrel files to avoid loading thousands of unused modules. **Barrel files** are entry points that re-export multiple modules (e.g., `index.js` that does `export * from './module'`).
 Popular icon and component libraries can have **up to 10,000 re-exports** in their entry file. For many React packages, **it takes 200-800ms just to import them**, affecting both development speed and production cold starts.
 **Why tree-shaking doesn't help:** When a library is marked as external (not bundled), the bundler can't optimize it. If you bundle it to enable tree-shaking, builds become substantially slower analyzing the entire module graph.
 **Incorrect (imports entire library):**
 ```tsx
 import { Check, X, Menu } from "lucide-react";
 // Loads 1,583 modules, takes ~2.8s extra in dev
 // Runtime cost: 200-800ms on every cold start
 import { Button, TextField } from "@mui/material";
 // Loads 2,225 modules, takes ~4.2s extra in dev
 ```
 **Correct (imports only what you need):**
 ```tsx
 import Check from "lucide-react/dist/esm/icons/check";
 import X from "lucide-react/dist/esm/icons/x";
 import Menu from "lucide-react/dist/esm/icons/menu";
 // Loads only 3 modules (~2KB vs ~1MB)
 import Button from "@mui/material/Button";
 import TextField from "@mui/material/TextField";
 // Loads only what you use
 ```
 **Alternative (Next.js 13.5+):**
 ```js
 // next.config.js - use optimizePackageImports
 module.exports = {
  experimental: {
    optimizePackageImports: ["lucide-react", "@mui/material"],
  },
 };
 // Then you can keep the ergonomic barrel imports:
 import { Check, X, Menu } from "lucide-react";
 // Automatically transformed to direct imports at build time
 ```
 Direct imports provide 15-70% faster dev boot, 28% faster builds, 40% faster cold starts, and significantly faster HMR.
 Libraries commonly affected: `lucide-react`, `@mui/material`, `@mui/icons-material`, `@tabler/icons-react`, `react-icons`, `@headlessui/react`, `@radix-ui/react-*`, `lodash`, `ramda`, `date-fns`, `rxjs`, `react-use`.
 Reference: [How we optimized package imports in Next.js](https://vercel.com/blog/how-we-optimized-package-imports-in-next-js)
@@ -0,0 +1,37 @@
 ---
 title: Conditional Module Loading
 impact: HIGH
 impactDescription: loads large data only when needed
 tags: bundle, conditional-loading, lazy-loading
 ---
 ## Conditional Module Loading
 Load large data or modules only when a feature is activated.
 **Example (lazy-load animation frames):**
 ```tsx
 function AnimationPlayer({
  enabled,
  setEnabled,
 }: {
  enabled: boolean;
  setEnabled: React.Dispatch<React.SetStateAction<boolean>>;
 }) {
  const [frames, setFrames] = useState<Frame[] | null>(null);
  useEffect(() => {
    if (enabled && !frames && typeof window !== "undefined") {
      import("./animation-frames.js")
        .then((mod) => setFrames(mod.frames))
        .catch(() => setEnabled(false));
    }
  }, [enabled, frames, setEnabled]);
  if (!frames) return <Skeleton />;
  return <Canvas frames={frames} />;
 }
 ```
 The `typeof window !== 'undefined'` check prevents bundling this module for SSR, optimizing server bundle size and build speed.
@@ -0,0 +1,48 @@
 ---
 title: Defer Non-Critical Third-Party Libraries
 impact: MEDIUM
 impactDescription: loads after hydration
 tags: bundle, third-party, analytics, defer
 ---
 ## Defer Non-Critical Third-Party Libraries
 Analytics, logging, and error tracking don't block user interaction. Load them after hydration.
 **Incorrect (blocks initial bundle):**
 ```tsx
 import { Analytics } from "@vercel/analytics/react";
 export default function RootLayout({ children }) {
  return (
    <html>
      <body>
        {children}
        <Analytics />
      </body>
    </html>
  );
 }
 ```
 **Correct (loads after hydration):**
 ```tsx
 import dynamic from "next/dynamic";
 const Analytics = dynamic(() => import("@vercel/analytics/react").then((m) => m.Analytics), {
  ssr: false,
 });
 export default function RootLayout({ children }) {
  return (
    <html>
      <body>
        {children}
        <Analytics />
      </body>
    </html>
  );
 }
 ```
@@ -0,0 +1,34 @@
 ---
 title: Dynamic Imports for Heavy Components
 impact: CRITICAL
 impactDescription: directly affects TTI and LCP
 tags: bundle, dynamic-import, code-splitting, next-dynamic
 ---
 ## Dynamic Imports for Heavy Components
 Use `next/dynamic` to lazy-load large components not needed on initial render.
 **Incorrect (Monaco bundles with main chunk ~300KB):**
 ```tsx
 import { MonacoEditor } from "./monaco-editor";
 function CodePanel({ code }: { code: string }) {
  return <MonacoEditor value={code} />;
 }
 ```
 **Correct (Monaco loads on demand):**
 ```tsx
 import dynamic from "next/dynamic";
 const MonacoEditor = dynamic(() => import("./monaco-editor").then((m) => m.MonacoEditor), {
  ssr: false,
 });
 function CodePanel({ code }: { code: string }) {
  return <MonacoEditor value={code} />;
 }
 ```
@@ -0,0 +1,44 @@
 ---
 title: Preload Based on User Intent
 impact: MEDIUM
 impactDescription: reduces perceived latency
 tags: bundle, preload, user-intent, hover
 ---
 ## Preload Based on User Intent
 Preload heavy bundles before they're needed to reduce perceived latency.
 **Example (preload on hover/focus):**
 ```tsx
 function EditorButton({ onClick }: { onClick: () => void }) {
  const preload = () => {
    if (typeof window !== "undefined") {
      void import("./monaco-editor");
    }
  };
  return (
    <button onMouseEnter={preload} onFocus={preload} onClick={onClick}>
      Open Editor
    </button>
  );
 }
 ```
 **Example (preload when feature flag is enabled):**
 ```tsx
 function FlagsProvider({ children, flags }: Props) {
  useEffect(() => {
    if (flags.editorEnabled && typeof window !== "undefined") {
      void import("./monaco-editor").then((mod) => mod.init());
    }
  }, [flags.editorEnabled]);
  return <FlagsContext.Provider value={flags}>{children}</FlagsContext.Provider>;
 }
 ```
 The `typeof window !== 'undefined'` check prevents bundling preloaded modules for SSR, optimizing server bundle size and build speed.
@@ -0,0 +1,78 @@
 ---
 title: Deduplicate Global Event Listeners
 impact: LOW
 impactDescription: single listener for N components
 tags: client, swr, event-listeners, subscription
 ---
 ## Deduplicate Global Event Listeners
 Use `useSWRSubscription()` to share global event listeners across component instances.
 **Incorrect (N instances = N listeners):**
 ```tsx
 function useKeyboardShortcut(key: string, callback: () => void) {
  useEffect(() => {
    const handler = (e: KeyboardEvent) => {
      if (e.metaKey && e.key === key) {
        callback();
      }
    };
    window.addEventListener("keydown", handler);
    return () => window.removeEventListener("keydown", handler);
  }, [key, callback]);
 }
 ```
 When using the `useKeyboardShortcut` hook multiple times, each instance will register a new listener.
 **Correct (N instances = 1 listener):**
 ```tsx
 import useSWRSubscription from "swr/subscription";
 // Module-level Map to track callbacks per key
 const keyCallbacks = new Map<string, Set<() => void>>();
 function useKeyboardShortcut(key: string, callback: () => void) {
  // Register this callback in the Map
  useEffect(() => {
    if (!keyCallbacks.has(key)) {
      keyCallbacks.set(key, new Set());
    }
    keyCallbacks.get(key)!.add(callback);
    return () => {
      const set = keyCallbacks.get(key);
      if (set) {
        set.delete(callback);
        if (set.size === 0) {
          keyCallbacks.delete(key);
        }
      }
    };
  }, [key, callback]);
  useSWRSubscription("global-keydown", () => {
    const handler = (e: KeyboardEvent) => {
      if (e.metaKey && keyCallbacks.has(e.key)) {
        keyCallbacks.get(e.key)!.forEach((cb) => cb());
      }
    };
    window.addEventListener("keydown", handler);
    return () => window.removeEventListener("keydown", handler);
  });
 }
 function Profile() {
  // Multiple shortcuts will share the same listener
  useKeyboardShortcut("p", () => {
    /* ... */
  });
  useKeyboardShortcut("k", () => {
    /* ... */
  });
  // ...
 }
 ```
@@ -0,0 +1,74 @@
 ---
 title: Version and Minimize localStorage Data
 impact: MEDIUM
 impactDescription: prevents schema conflicts, reduces storage size
 tags: client, localStorage, storage, versioning, data-minimization
 ---
 ## Version and Minimize localStorage Data
 Add version prefix to keys and store only needed fields. Prevents schema conflicts and accidental storage of sensitive data.
 **Incorrect:**
 ```typescript
 // No version, stores everything, no error handling
 localStorage.setItem("userConfig", JSON.stringify(fullUserObject));
 const data = localStorage.getItem("userConfig");
 ```
 **Correct:**
 ```typescript
 const VERSION = "v2";
 function saveConfig(config: { theme: string; language: string }) {
  try {
    localStorage.setItem(`userConfig:${VERSION}`, JSON.stringify(config));
  } catch {
    // Throws in incognito/private browsing, quota exceeded, or disabled
  }
 }
 function loadConfig() {
  try {
    const data = localStorage.getItem(`userConfig:${VERSION}`);
    return data ? JSON.parse(data) : null;
  } catch {
    return null;
  }
 }
 // Migration from v1 to v2
 function migrate() {
  try {
    const v1 = localStorage.getItem("userConfig:v1");
    if (v1) {
      const old = JSON.parse(v1);
      saveConfig({ theme: old.darkMode ? "dark" : "light", language: old.lang });
      localStorage.removeItem("userConfig:v1");
    }
  } catch {}
 }
 ```
 **Store minimal fields from server responses:**
 ```typescript
 // User object has 20+ fields, only store what UI needs
 function cachePrefs(user: FullUser) {
  try {
    localStorage.setItem(
      "prefs:v1",
      JSON.stringify({
        theme: user.preferences.theme,
        notifications: user.preferences.notifications,
      }),
    );
  } catch {}
 }
 ```
 **Always wrap in try-catch:** `getItem()` and `setItem()` throw in incognito/private browsing (Safari, Firefox), when quota exceeded, or when disabled.
 **Benefits:** Schema evolution via versioning, reduced storage size, prevents storing tokens/PII/internal flags.
@@ -0,0 +1,48 @@
 ---
 title: Use Passive Event Listeners for Scrolling Performance
 impact: MEDIUM
 impactDescription: eliminates scroll delay caused by event listeners
 tags: client, event-listeners, scrolling, performance, touch, wheel
 ---
 ## Use Passive Event Listeners for Scrolling Performance
 Add `{ passive: true }` to touch and wheel event listeners to enable immediate scrolling. Browsers normally wait for listeners to finish to check if `preventDefault()` is called, causing scroll delay.
 **Incorrect:**
 ```typescript
 useEffect(() => {
  const handleTouch = (e: TouchEvent) => console.log(e.touches[0].clientX);
  const handleWheel = (e: WheelEvent) => console.log(e.deltaY);
  document.addEventListener("touchstart", handleTouch);
  document.addEventListener("wheel", handleWheel);
  return () => {
    document.removeEventListener("touchstart", handleTouch);
    document.removeEventListener("wheel", handleWheel);
  };
 }, []);
 ```
 **Correct:**
 ```typescript
 useEffect(() => {
  const handleTouch = (e: TouchEvent) => console.log(e.touches[0].clientX);
  const handleWheel = (e: WheelEvent) => console.log(e.deltaY);
  document.addEventListener("touchstart", handleTouch, { passive: true });
  document.addEventListener("wheel", handleWheel, { passive: true });
  return () => {
    document.removeEventListener("touchstart", handleTouch);
    document.removeEventListener("wheel", handleWheel);
  };
 }, []);
 ```
 **Use passive when:** tracking/analytics, logging, any listener that doesn't call `preventDefault()`.
 **Don't use passive when:** implementing custom swipe gestures, custom zoom controls, or any listener that needs `preventDefault()`.
@@ -0,0 +1,56 @@
 ---
 title: Use SWR for Automatic Deduplication
 impact: MEDIUM-HIGH
 impactDescription: automatic deduplication
 tags: client, swr, deduplication, data-fetching
 ---
 ## Use SWR for Automatic Deduplication
 SWR enables request deduplication, caching, and revalidation across component instances.
 **Incorrect (no deduplication, each instance fetches):**
 ```tsx
 function UserList() {
  const [users, setUsers] = useState([]);
  useEffect(() => {
    fetch("/api/users")
      .then((r) => r.json())
      .then(setUsers);
  }, []);
 }
 ```
 **Correct (multiple instances share one request):**
 ```tsx
 import useSWR from "swr";
 function UserList() {
  const { data: users } = useSWR("/api/users", fetcher);
 }
 ```
 **For immutable data:**
 ```tsx
 import { useImmutableSWR } from "@/lib/swr";
 function StaticContent() {
  const { data } = useImmutableSWR("/api/config", fetcher);
 }
 ```
 **For mutations:**
 ```tsx
 import { useSWRMutation } from "swr/mutation";
 function UpdateButton() {
  const { trigger } = useSWRMutation("/api/user", updateUser);
  return <button onClick={() => trigger()}>Update</button>;
 }
 ```
 Reference: [https://swr.vercel.app](https://swr.vercel.app)
@@ -0,0 +1,110 @@
 ---
 title: Avoid Layout Thrashing
 impact: MEDIUM
 impactDescription: prevents forced synchronous layouts and reduces performance bottlenecks
 tags: javascript, dom, css, performance, reflow, layout-thrashing
 ---
 ## Avoid Layout Thrashing
 Avoid interleaving style writes with layout reads. When you read a layout property (like `offsetWidth`, `getBoundingClientRect()`, or `getComputedStyle()`) between style changes, the browser is forced to trigger a synchronous reflow.
 **This is OK (browser batches style changes):**
 ```typescript
 function updateElementStyles(element: HTMLElement) {
  // Each line invalidates style, but browser batches the recalculation
  element.style.width = "100px";
  element.style.height = "200px";
  element.style.backgroundColor = "blue";
  element.style.border = "1px solid black";
 }
 ```
 **Incorrect (interleaved reads and writes force reflows):**
 ```typescript
 function layoutThrashing(element: HTMLElement) {
  element.style.width = "100px";
  const width = element.offsetWidth; // Forces reflow
  element.style.height = "200px";
  const height = element.offsetHeight; // Forces another reflow
 }
 ```
 **Correct (batch writes, then read once):**
 ```typescript
 function updateElementStyles(element: HTMLElement) {
  // Batch all writes together
  element.style.width = "100px";
  element.style.height = "200px";
  element.style.backgroundColor = "blue";
  element.style.border = "1px solid black";
  // Read after all writes are done (single reflow)
  const { width, height } = element.getBoundingClientRect();
 }
 ```
 **Correct (batch reads, then writes):**
 ```typescript
 function avoidThrashing(element: HTMLElement) {
  // Read phase - all layout queries first
  const rect1 = element.getBoundingClientRect();
  const offsetWidth = element.offsetWidth;
  const offsetHeight = element.offsetHeight;
  // Write phase - all style changes after
  element.style.width = "100px";
  element.style.height = "200px";
 }
 ```
 **Better: use CSS classes**
 ```css
 .highlighted-box {
  width: 100px;
  height: 200px;
  background-color: blue;
  border: 1px solid black;
 }
 ```
 ```typescript
 function updateElementStyles(element: HTMLElement) {
  element.classList.add("highlighted-box");
  const { width, height } = element.getBoundingClientRect();
 }
 ```
 **React example:**
 ```tsx
 // Incorrect: interleaving style changes with layout queries
 function Box({ isHighlighted }: { isHighlighted: boolean }) {
  const ref = useRef<HTMLDivElement>(null);
  useEffect(() => {
    if (ref.current && isHighlighted) {
      ref.current.style.width = "100px";
      const width = ref.current.offsetWidth; // Forces layout
      ref.current.style.height = "200px";
    }
  }, [isHighlighted]);
  return <div ref={ref}>Content</div>;
 }
 // Correct: toggle class
 function Box({ isHighlighted }: { isHighlighted: boolean }) {
  return <div className={isHighlighted ? "highlighted-box" : ""}>Content</div>;
 }
 ```
 Prefer CSS classes over inline styles when possible. CSS files are cached by the browser, and classes provide better separation of concerns and are easier to maintain.
 See [this gist](https://gist.github.com/paulirish/5d52fb081b3570c81e3a) and [CSS Triggers](https://csstriggers.com/) for more information on layout-forcing operations.
@@ -0,0 +1,80 @@
 ---
 title: Cache Repeated Function Calls
 impact: MEDIUM
 impactDescription: avoid redundant computation
 tags: javascript, cache, memoization, performance
 ---
 ## Cache Repeated Function Calls
 Use a module-level Map to cache function results when the same function is called repeatedly with the same inputs during render.
 **Incorrect (redundant computation):**
 ```typescript
 function ProjectList({ projects }: { projects: Project[] }) {
  return (
    <div>
      {projects.map(project => {
        // slugify() called 100+ times for same project names
        const slug = slugify(project.name)
        return <ProjectCard key={project.id} slug={slug} />
      })}
    </div>
  )
 }
 ```
 **Correct (cached results):**
 ```typescript
 // Module-level cache
 const slugifyCache = new Map<string, string>()
 function cachedSlugify(text: string): string {
  if (slugifyCache.has(text)) {
    return slugifyCache.get(text)!
  }
  const result = slugify(text)
  slugifyCache.set(text, result)
  return result
 }
 function ProjectList({ projects }: { projects: Project[] }) {
  return (
    <div>
      {projects.map(project => {
        // Computed only once per unique project name
        const slug = cachedSlugify(project.name)
        return <ProjectCard key={project.id} slug={slug} />
      })}
    </div>
  )
 }
 ```
 **Simpler pattern for single-value functions:**
 ```typescript
 let isLoggedInCache: boolean | null = null;
 function isLoggedIn(): boolean {
  if (isLoggedInCache !== null) {
    return isLoggedInCache;
  }
  isLoggedInCache = document.cookie.includes("auth=");
  return isLoggedInCache;
 }
 // Clear cache when auth changes
 function onAuthChange() {
  isLoggedInCache = null;
 }
 ```
 Use a Map (not a hook) so it works everywhere: utilities, event handlers, not just React components.
 Reference: [How we made the Vercel Dashboard twice as fast](https://vercel.com/blog/how-we-made-the-vercel-dashboard-twice-as-fast)
@@ -0,0 +1,28 @@
 ---
 title: Cache Property Access in Loops
 impact: LOW-MEDIUM
 impactDescription: reduces lookups
 tags: javascript, loops, optimization, caching
 ---
 ## Cache Property Access in Loops
 Cache object property lookups in hot paths.
 **Incorrect (3 lookups × N iterations):**
 ```typescript
 for (let i = 0; i < arr.length; i++) {
  process(obj.config.settings.value);
 }
 ```
 **Correct (1 lookup total):**
 ```typescript
 const value = obj.config.settings.value;
 const len = arr.length;
 for (let i = 0; i < len; i++) {
  process(value);
 }
 ```
@@ -0,0 +1,68 @@
 ---
 title: Cache Storage API Calls
 impact: LOW-MEDIUM
 impactDescription: reduces expensive I/O
 tags: javascript, localStorage, storage, caching, performance
 ---
 ## Cache Storage API Calls
 `localStorage`, `sessionStorage`, and `document.cookie` are synchronous and expensive. Cache reads in memory.
 **Incorrect (reads storage on every call):**
 ```typescript
 function getTheme() {
  return localStorage.getItem("theme") ?? "light";
 }
 // Called 10 times = 10 storage reads
 ```
 **Correct (Map cache):**
 ```typescript
 const storageCache = new Map<string, string | null>();
 function getLocalStorage(key: string) {
  if (!storageCache.has(key)) {
    storageCache.set(key, localStorage.getItem(key));
  }
  return storageCache.get(key);
 }
 function setLocalStorage(key: string, value: string) {
  localStorage.setItem(key, value);
  storageCache.set(key, value); // keep cache in sync
 }
 ```
 Use a Map (not a hook) so it works everywhere: utilities, event handlers, not just React components.
 **Cookie caching:**
 ```typescript
 let cookieCache: Record<string, string> | null = null;
 function getCookie(name: string) {
  if (!cookieCache) {
    cookieCache = Object.fromEntries(document.cookie.split("; ").map((c) => c.split("=")));
  }
  return cookieCache[name];
 }
 ```
 **Important (invalidate on external changes):**
 If storage can change externally (another tab, server-set cookies), invalidate cache:
 ```typescript
 window.addEventListener("storage", (e) => {
  if (e.key) storageCache.delete(e.key);
 });
 document.addEventListener("visibilitychange", () => {
  if (document.visibilityState === "visible") {
    storageCache.clear();
  }
 });
 ```
@@ -0,0 +1,32 @@
 ---
 title: Combine Multiple Array Iterations
 impact: LOW-MEDIUM
 impactDescription: reduces iterations
 tags: javascript, arrays, loops, performance
 ---
 ## Combine Multiple Array Iterations
 Multiple `.filter()` or `.map()` calls iterate the array multiple times. Combine into one loop.
 **Incorrect (3 iterations):**
 ```typescript
 const admins = users.filter((u) => u.isAdmin);
 const testers = users.filter((u) => u.isTester);
 const inactive = users.filter((u) => !u.isActive);
 ```
 **Correct (1 iteration):**
 ```typescript
 const admins: User[] = [];
 const testers: User[] = [];
 const inactive: User[] = [];
 for (const user of users) {
  if (user.isAdmin) admins.push(user);
  if (user.isTester) testers.push(user);
  if (!user.isActive) inactive.push(user);
 }
 ```
@@ -0,0 +1,50 @@
 ---
 title: Early Return from Functions
 impact: LOW-MEDIUM
 impactDescription: avoids unnecessary computation
 tags: javascript, functions, optimization, early-return
 ---
 ## Early Return from Functions
 Return early when result is determined to skip unnecessary processing.
 **Incorrect (processes all items even after finding answer):**
 ```typescript
 function validateUsers(users: User[]) {
  let hasError = false;
  let errorMessage = "";
  for (const user of users) {
    if (!user.email) {
      hasError = true;
      errorMessage = "Email required";
    }
    if (!user.name) {
      hasError = true;
      errorMessage = "Name required";
    }
    // Continues checking all users even after error found
  }
  return hasError ? { valid: false, error: errorMessage } : { valid: true };
 }
 ```
 **Correct (returns immediately on first error):**
 ```typescript
 function validateUsers(users: User[]) {
  for (const user of users) {
    if (!user.email) {
      return { valid: false, error: "Email required" };
    }
    if (!user.name) {
      return { valid: false, error: "Name required" };
    }
  }
  return { valid: true };
 }
 ```
@@ -0,0 +1,51 @@
 ---
 title: Use flatMap to Map and Filter in One Pass
 impact: LOW-MEDIUM
 impactDescription: eliminates intermediate array
 tags: javascript, arrays, flatMap, filter, performance
 ---
 ## Use flatMap to Map and Filter in One Pass
 **Impact: LOW-MEDIUM (eliminates intermediate array)**
 Chaining `.map().filter(Boolean)` creates an intermediate array and iterates twice. Use `.flatMap()` to transform and filter in a single pass.
 **Incorrect (2 iterations, intermediate array):**
 ```typescript
 const userNames = users.map((user) => (user.isActive ? user.name : null)).filter(Boolean);
 ```
 **Correct (1 iteration, no intermediate array):**
 ```typescript
 const userNames = users.flatMap((user) => (user.isActive ? [user.name] : []));
 ```
 **More examples:**
 ```typescript
 // Extract valid emails from responses
 // Before
 const emails = responses.map((r) => (r.success ? r.data.email : null)).filter(Boolean);
 // After
 const emails = responses.flatMap((r) => (r.success ? [r.data.email] : []));
 // Parse and filter valid numbers
 // Before
 const numbers = strings.map((s) => parseInt(s, 10)).filter((n) => !isNaN(n));
 // After
 const numbers = strings.flatMap((s) => {
  const n = parseInt(s, 10);
  return isNaN(n) ? [] : [n];
 });
 ```
 **When to use:**
 - Transforming items while filtering some out
 - Conditional mapping where some inputs produce no output
 - Parsing/validating where invalid inputs should be skipped
@@ -0,0 +1,45 @@
 ---
 title: Hoist RegExp Creation
 impact: LOW-MEDIUM
 impactDescription: avoids recreation
 tags: javascript, regexp, optimization, memoization
 ---
 ## Hoist RegExp Creation
 Don't create RegExp inside render. Hoist to module scope or memoize with `useMemo()`.
 **Incorrect (new RegExp every render):**
 ```tsx
 function Highlighter({ text, query }: Props) {
  const regex = new RegExp(`(${query})`, 'gi')
  const parts = text.split(regex)
  return <>{parts.map((part, i) => ...)}</>
 }
 ```
 **Correct (memoize or hoist):**
 ```tsx
 const EMAIL_REGEX = /^[^\s@]+@[^\s@]+\.[^\s@]+$/
 function Highlighter({ text, query }: Props) {
  const regex = useMemo(
    () => new RegExp(`(${escapeRegex(query)})`, 'gi'),
    [query]
  )
  const parts = text.split(regex)
  return <>{parts.map((part, i) => ...)}</>
 }
 ```
 **Warning (global regex has mutable state):**
 Global regex (`/g`) has mutable `lastIndex` state:
 ```typescript
 const regex = /foo/g;
 regex.test("foo"); // true, lastIndex = 3
 regex.test("foo"); // false, lastIndex = 0
 ```
@@ -0,0 +1,37 @@
 ---
 title: Build Index Maps for Repeated Lookups
 impact: LOW-MEDIUM
 impactDescription: 1M ops to 2K ops
 tags: javascript, map, indexing, optimization, performance
 ---
 ## Build Index Maps for Repeated Lookups
 Multiple `.find()` calls by the same key should use a Map.
 **Incorrect (O(n) per lookup):**
 ```typescript
 function processOrders(orders: Order[], users: User[]) {
  return orders.map((order) => ({
    ...order,
    user: users.find((u) => u.id === order.userId),
  }));
 }
 ```
 **Correct (O(1) per lookup):**
 ```typescript
 function processOrders(orders: Order[], users: User[]) {
  const userById = new Map(users.map((u) => [u.id, u]));
  return orders.map((order) => ({
    ...order,
    user: userById.get(order.userId),
  }));
 }
 ```
 Build map once (O(n)), then all lookups are O(1).
 For 1000 orders × 1000 users: 1M ops → 2K ops.
@@ -0,0 +1,50 @@
 ---
 title: Early Length Check for Array Comparisons
 impact: MEDIUM-HIGH
 impactDescription: avoids expensive operations when lengths differ
 tags: javascript, arrays, performance, optimization, comparison
 ---
 ## Early Length Check for Array Comparisons
 When comparing arrays with expensive operations (sorting, deep equality, serialization), check lengths first. If lengths differ, the arrays cannot be equal.
 In real-world applications, this optimization is especially valuable when the comparison runs in hot paths (event handlers, render loops).
 **Incorrect (always runs expensive comparison):**
 ```typescript
 function hasChanges(current: string[], original: string[]) {
  // Always sorts and joins, even when lengths differ
  return current.sort().join() !== original.sort().join();
 }
 ```
 Two O(n log n) sorts run even when `current.length` is 5 and `original.length` is 100. There is also overhead of joining the arrays and comparing the strings.
 **Correct (O(1) length check first):**
 ```typescript
 function hasChanges(current: string[], original: string[]) {
  // Early return if lengths differ
  if (current.length !== original.length) {
    return true;
  }
  // Only sort when lengths match
  const currentSorted = current.toSorted();
  const originalSorted = original.toSorted();
  for (let i = 0; i < currentSorted.length; i++) {
    if (currentSorted[i] !== originalSorted[i]) {
      return true;
    }
  }
  return false;
 }
 ```
 This new approach is more efficient because:
 - It avoids the overhead of sorting and joining the arrays when lengths differ
 - It avoids consuming memory for the joined strings (especially important for large arrays)
 - It avoids mutating the original arrays
 - It returns early when a difference is found
--- a/Show More
+++ b/Show More