react-best-practices

# Vercel React Best Practices Comprehensive performance optimization guide for React applications, maintained by Vercel. Contains 45 rules across 8 categories, prioritized by impact. ## Applicability **This codebase is a client-side SPA: Vite + TanStack Router, React 19, TanStack Query, a Spring Boot backend. There is no Next.js, no RSC and no SSR.** The pack is written for Next.js, so read this section before any rule file. ### Skip — the API or the runtime does not exist here | Rule | Why | |------|-----| | `async-api-routes` | Next.js API routes; the backend is Spring Boot | | `async-suspense-boundaries` | About streaming an RSC tree; nothing in the SPA renders on a server | | `server-cache-react` | `React.cache()` is RSC-only | | `server-cache-lru` | Server-side caching, which is the backend's problem | | `server-serialization` | RSC client/server boundary | | `server-parallel-fetching` | RSC data fetching; TanStack Query is the equivalent here | | `server-after-nonblocking` | `after()` from `next/server` | | `rendering-hydration-no-flicker` | SSR hydration; the build is pure CSR | | `client-swr-dedup` | SWR; this repo uses TanStack Query, which dedups by query key | | `rerender-memo` | React Compiler is on — see below | ### Apply, with the API substituted | Rule | Read it as | |------|-----------| | `bundle-dynamic-imports` | The reasoning holds and the repo has the rule's own example (`@monaco-editor/react`); the mechanism is `React.lazy()` plus TanStack Router's `autoCodeSplitting`, not `next/dynamic` | ### Apply as written `async-defer-await`, `async-parallel`, `async-dependencies`; `bundle-barrel-imports`, `bundle-defer-third-party`, `bundle-conditional`, `bundle-preload`; `client-event-listeners`; every `rerender-*` except `rerender-memo`; every `rendering-*` except `rendering-hydration-no-flicker` (this includes `rendering-activity` — React is 19.2+, so `<Activity>` exists); every `js-*`; every `advanced-*`. ### React Compiler `webapp/vite.shared.ts` passes `compiler: true` to `@vitejs/plugin-react` for every build of app source, so **do not add `useMemo`, `useCallback` or `React.memo`**. A hand-written one that survives in the tree is load-bearing; deleting it breaks something that still type-checks (`webapp/AGENTS.md` § React Compiler). ## When to Apply Reference these guidelines when: - Writing new React components - Implementing client-side data fetching - Reviewing code for performance issues - Optimizing bundle size or load times - **NOT** when working with server-side patterns (this is a SPA) ## 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-cache-react` - Use React.cache() for per-request deduplication - `server-cache-lru` - Use LRU cache for cross-request caching - `server-serialization` - Minimize data passed to client components - `server-parallel-fetching` - Restructure components to parallelize fetches - `server-after-nonblocking` - Use after() for non-blocking operations ### 4. Client-Side Data Fetching (MEDIUM-HIGH) - `client-swr-dedup` - Use SWR for automatic request deduplication - `client-event-listeners` - Deduplicate global event listeners ### 5. Re-render Optimization (MEDIUM) - `rerender-defer-reads` - Don't subscribe to state only used in callbacks - `rerender-memo` - Extract expensive work into memoized components - `rerender-dependencies` - Use primitive dependencies in effects - `rerender-derived-state` - Subscribe to derived booleans, not raw values - `rerender-functional-setstate` - Use functional setState for stable callbacks - `rerender-lazy-state-init` - Pass function to useState for expensive values - `rerender-transitions` - Use startTransition for non-urgent updates ### 6. Rendering Performance (MEDIUM) - `rendering-animate-svg-wrapper` - Animate div wrapper, not SVG element - `rendering-content-visibility` - Use content-visibility for long lists - `rendering-hoist-jsx` - Extract static JSX outside components - `rendering-svg-precision` - Reduce SVG coordinate precision - `rendering-hydration-no-flicker` - Use inline script for client-only data - `rendering-activity` - Use Activity component for show/hide - `rendering-conditional-render` - Use ternary, not && for conditionals ### 7. JavaScript Performance (LOW-MEDIUM) - `js-batch-dom-css` - Group CSS changes via classes or cssText - `js-index-maps` - Build Map for repeated lookups - `js-cache-property-access` - Cache object properties in loops - `js-cache-function-results` - Cache function results in module-level Map - `js-cache-storage` - Cache localStorage/sessionStorage reads - `js-combine-iterations` - Combine multiple filter/map into one loop - `js-length-check-first` - Check array length before expensive comparison - `js-early-exit` - Return early from functions - `js-hoist-regexp` - Hoist RegExp creation outside loops - `js-min-max-loop` - Use loop for min/max instead of sort - `js-set-map-lookups` - Use Set/Map for O(1) lookups - `js-tosorted-immutable` - Use toSorted() for immutability ### 8. Advanced Patterns (LOW) - `advanced-event-handler-refs` - Store event handlers in refs - `advanced-use-latest` - useLatest for stable callback refs ## How to Use Read individual rule files for detailed explanations and code examples: ``` rules/async-parallel.md rules/bundle-barrel-imports.md rules/_sections.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`