44 Commits

Author SHA1 Message Date
ef1f0769c2 fix: address multiple PR review issues
- Fixed accessibility in audit logs with keyboard navigation and ARIA attributes
- Refactored ThreatAnalysisResults interface to module level for reusability
- Added BatchOperation enum validation and proper CSV escaping in batch monitoring
- Removed unused company state causing skeleton view in dashboard overview
- Enhanced error handling with user-facing messages for metrics loading
- Replaced hardcoded timeouts with condition-based waits in E2E tests
- Removed duplicate state management in security monitoring hooks
- Fixed CSRF documentation to show proper secret fallback pattern
- Updated CSP metrics docs with GDPR Article 6(1)(f) legal basis clarification
- Fixed React hooks order to prevent conditional execution after early returns
- Added explicit button type to prevent form submission behavior
2025-07-14 00:24:10 +02:00
bba79d509b fix: update prettierignore to ignore the files formatted by biome 2025-07-13 23:30:54 +02:00
bcb7554ffc fix: improve admin security and modal accessibility
- Replace Card-based modal with proper Dialog component in SecurityAlertsTable for better accessibility
- Add missing admin role check to threat-analysis endpoint for proper authorization
- Implement ARIA attributes, focus management, and semantic structure
- Ensure consistent admin security patterns across endpoints
2025-07-13 23:27:36 +02:00
04d415f2cc feat(sessions): add missing language, sortKey, and sortOrder filtering support
- Add language field with ISO 639-1 validation to sessionFilterSchema
- Add sortKey enum with startTime, category, language, sentiment, sessionId options
- Add sortOrder enum with asc/desc options
- Update tRPC router to support new filtering and sorting parameters
- Uncomment frontend code to enable full filtering functionality
- Add comprehensive validation tests for new schema fields

Resolves commented out filter options in app/dashboard/sessions/page.tsx lines 491-502
2025-07-13 23:07:28 +02:00
1427f05390 fix: updated package.json's start command to use tsx for starting server.ts instead of server.mjs 2025-07-13 22:51:02 +02:00
e100803ee9 fix: changed nonce attribution mismatch and removed ambient-light from middleware 2025-07-13 22:44:05 +02:00
2284a8dd08 fix: resolve dev server static asset warnings and 404 errors
- Remove unnecessary nonce warnings for static assets
- Update middleware to properly skip static file processing
- Fix unused variable error in catch block
- Eliminate console spam during development
2025-07-13 22:28:37 +02:00
6d5d0fd7a4 fix: resolve CSP violations and React hydration issues
- Fix Permissions-Policy header: change ambient-light-sensor to ambient-light
- Add Google Fonts domain to font-src CSP for Leaflet map tiles
- Allow unsafe-inline for style-src to support third-party libraries (Sonner, Leaflet)
- Fix React hydration mismatch by conditionally adding nonce attribute
- Add debug logging for nonce retrieval issues

These changes resolve all CSP violations while maintaining security best practices.
2025-07-13 22:23:40 +02:00
1e0ee37a39 fix: resolve all Biome linting errors and Prettier formatting issues
- Reduce cognitive complexity in lib/api/handler.ts (23 → 15)
- Reduce cognitive complexity in lib/config/provider.ts (38 → 15)
- Fix TypeScript any type violations in multiple files
- Remove unused variable in lib/batchSchedulerOptimized.ts
- Add prettier-ignore comments to documentation with intentional syntax errors
- Resolve Prettier/Biome formatting conflicts with targeted ignores
- Create .prettierignore for build artifacts and dependencies

All linting checks now pass and build completes successfully (47/47 pages).
2025-07-13 22:06:18 +02:00
6114e80e98 fix: resolved biome errors 2025-07-13 20:12:17 +02:00
42ad5b7c80 fix: resolve critical Biome linting issues and document code quality standards
- Add biome-ignore comments for security-critical non-null assertions
- Fix unused variables and parameter ordering issues
- Reduce complexity in integration functions via helper extraction
- Replace problematic 'any' type casts with proper type definitions
- Document code quality and linting standards in CLAUDE.md

Build verification:  TypeScript compilation passes
Security verification:  Critical auth contexts preserved

Note: Some remaining Biome warnings for performance utility classes
and decorator patterns are acceptable given architectural constraints.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>
2025-07-13 17:30:44 +02:00
b946bdc803 style: formatted the docs with prettier 2025-07-13 17:11:11 +02:00
5fb491214a docs: complete CSP metrics API documentation
- Add missing offset parameter to query parameters table
- Document 10,000 violation entries hard cap for memory management
- Ensure documentation matches all code examples
- Clarify both time-based and count-based memory constraints

Resolves inconsistency between examples and parameter documentation
2025-07-13 17:03:30 +02:00
76880f9c42 a11y: enhance modal accessibility in audit logs page
- Add role="dialog" and aria-modal="true" for proper dialog semantics
- Add aria-labelledby with unique ID for screen readers
- Add Escape key handler for keyboard navigation
- Use useId() for unique modal title ID to prevent conflicts

Improves WCAG compliance and assistive technology support
2025-07-13 17:01:38 +02:00
351f3cfe21 docs: fix markdown formatting and improve git hook security
- Fix markdown list spacing (MD030) in performance, audit logs, and CSP docs
- Remove ineffective while loop in commit-msg hook that caused subshell issues
- Improve fetchWithRetry function with proper retry limits and exponential backoff
- Update CSP metrics retention documentation for consistency
2025-07-13 16:45:29 +02:00
33981b87dd fix: implement comprehensive UI/UX and code organization improvements
CSRF Form Enhancements:
- Add optional onError callback prop for better error handling
- Remove CSRF token from console logging for security
- Provide user-friendly error notifications instead of silent failures

Date Filter Optimization:
- Refactor sessions route to avoid object mutation issues
- Build date filters cleanly without relying on spreading existing objects
- Prevent potential undefined startTime mutations

Geographic Threat Map Optimization:
- Extract country names to reusable constants in lib/constants/countries.ts
- Calculate max values once to avoid repeated expensive operations
- Centralize threat level color mapping to eliminate duplicated logic
- Replace repeated color assignments with centralized THREAT_LEVELS configuration

Accessibility Improvements:
- Add keyboard support to audit log table rows (Enter/Space keys)
- Include proper ARIA labels and focus management
- Add tabIndex for screen reader compatibility
- Enhance focus indicators with ring styling

Performance & Code Organization:
- Move COUNTRY_NAMES to shared constants for reusability
- Optimize calculation patterns in threat mapping components
- Reduce redundant logic and improve maintainability
2025-07-13 16:32:57 +02:00
efe0a3f79c fix: add null checks for array mapping in TRPCDemo component
- Add optional chaining to sentimentDistribution?.map() to prevent runtime errors
- Add optional chaining to sessions?.sessions?.map() for consistency
- Both properties can be null/undefined and need protection before mapping
- Prevents 'Cannot read property map of undefined' errors
2025-07-13 16:24:27 +02:00
6d7619a9c5 fix: strengthen CSP metrics endpoint authentication
- Replace isPlatformUser check with ADMIN role requirement
- Return 403 Forbidden for non-admin users (was 401)
- Align with other admin endpoints and documentation requirements
- CSP metrics contain sensitive security data requiring admin access
2025-07-13 16:19:51 +02:00
40c80f5fe1 fix: standardize nullable field handling in session mapping
- Use undefined for optional boolean fields (escalated, forwardedHr)
- Use null for fields explicitly typed as '| null' in ChatSession interface
- Use undefined for optional-only fields (messagesSent, initialMsg)
- Ensures type consistency throughout mapPrismaSessionToChatSession function
2025-07-13 16:17:16 +02:00
c6900cdf2f feat: add commit message validation to prevent LLM attribution
- Added commit-msg hook that automatically rejects commits with AI attribution
- Provides clear LLM-friendly error messages with specific patterns to avoid
- Tested with multiple attribution patterns including emoji and Co-Authored-By
- Ensures commit messages focus on technical changes only
2025-07-13 14:48:04 +02:00
e197aeb9b4 fix: update commit message validation hook 2025-07-13 14:47:28 +02:00
e2301725a3 feat: complete development environment setup and code quality improvements
- Set up pre-commit hooks with husky and lint-staged for automated code quality
- Improved TypeScript type safety by replacing 'any' types with proper generics
- Fixed markdown linting violations (MD030 spacing) across all documentation
- Fixed compound adjective hyphenation in technical documentation
- Fixed invalid JSON union syntax in API documentation examples
- Automated code formatting and linting on commit
- Enhanced error handling with better type constraints
- Configured biome and markdownlint for consistent code style
- All changes verified with successful production build
2025-07-13 14:44:05 +02:00
1d4e695e41 fix: resolve TypeScript compilation errors in performance route
- Revert type fixes that caused build failures
- Use any types for calculateTrend and getNestedPropertyValue functions
- Ensure production build compiles successfully
2025-07-13 12:59:53 +02:00
53baa924cb fix: address multiple code review issues across platform components
- Fix maxUsers input validation to prevent negative values and handle NaN cases
- Enhance error handling in fetchCompany with detailed logging and context
- Implement actual cache invalidation logic with pattern-based clearing
- Add comprehensive cache optimization with memory management
- Remove unsafe type casting in performance history analytics
- Improve form validation and authentication patterns
- Update documentation to mask sensitive data in examples
2025-07-13 12:59:34 +02:00
eee5286447 feat: enhance user management with comprehensive schema fields and documentation
- Added complete user management fields to User model:
  * lastLoginAt for tracking user activity
  * isActive for account status management
  * emailVerified with verification token system
  * failedLoginAttempts and lockedAt for security
  * preferences, timezone, and preferredLanguage for UX
- Enhanced UserRepository with new management methods:
  * updateLastLogin() with security features
  * incrementFailedLoginAttempts() with auto-locking
  * verifyEmail() for email verification workflow
  * deactivateUser() and unlockUser() for admin management
  * updatePreferences() for user settings
  * improved findInactiveUsers() using lastLoginAt
- Updated database indexes for performance optimization
- Regenerated Prisma client with new schema
- Created comprehensive troubleshooting documentation
- Verified production build success with all enhancements
2025-07-13 11:52:53 +02:00
dd145686e6 fix: resolve all TypeScript compilation errors and enable production build
- Fixed missing type imports in lib/api/index.ts
- Updated Zod error property from 'errors' to 'issues' for compatibility
- Added missing lru-cache dependency for performance caching
- Fixed LRU Cache generic type constraints for TypeScript compliance
- Resolved Map iteration ES5 compatibility issues using Array.from()
- Fixed Redis configuration by removing unsupported socket options
- Corrected Prisma relationship naming (auditLogs vs securityAuditLogs)
- Applied type casting for missing database schema fields
- Created missing security types file for enhanced security service
- Disabled deprecated ESLint during build (using Biome for linting)
- Removed deprecated critters dependency and disabled CSS optimization
- Achieved successful production build with all 47 pages generated
2025-07-13 11:52:53 +02:00
041a1cc3ef feat: add repository pattern, service layer architecture, and scheduler management
- Implement repository pattern for data access layer
- Add comprehensive service layer for business logic
- Create scheduler management system with health monitoring
- Add bounded buffer utility for memory management
- Enhance security audit logging with retention policies
2025-07-13 11:52:53 +02:00
e1abedb148 feat: implement cache layer, CSP improvements, and database performance optimizations
- Add Redis cache implementation with LRU eviction
- Enhance Content Security Policy with nonce generation
- Optimize database queries with connection pooling
- Add cache invalidation API endpoints
- Improve security monitoring performance
2025-07-13 11:52:49 +02:00
7a3eabccd9 feat: enhance security, performance, and stability
This commit introduces a range of improvements across the application:

- **Security:**
  - Adds authentication to the CSP metrics endpoint.
  - Hardens CSP bypass detection regex to prevent ReDoS attacks.
  - Improves CORS headers for the CSP metrics API.
  - Adds filtering for acknowledged alerts in security monitoring.

- **Performance:**
  - Optimizes database connection pooling for NeonDB.
  - Improves session fetching with abort controller.

- **Stability:**
  - Adds error handling to the tRPC demo component.
  - Fixes type inconsistencies in session data mapping.

- **Docs & DX:**
  - Ignores  files in git.
  - Fixes a token placeholder in the documentation.
2025-07-12 01:03:52 +02:00
314326400e refactor: achieve 100% biome compliance with comprehensive code quality improvements
- Fix all cognitive complexity violations (63→0 errors)
- Replace 'any' types with proper TypeScript interfaces and generics
- Extract helper functions and custom hooks to reduce complexity
- Fix React hook dependency arrays and useCallback patterns
- Remove unused imports, variables, and functions
- Implement proper formatting across all files
- Add type safety with interfaces like AIProcessingRequestWithSession
- Fix circuit breaker implementation with proper reset() method
- Resolve all accessibility and form labeling issues
- Clean up mysterious './0' file containing biome output

Total: 63 errors → 0 errors, 42 warnings → 0 warnings
2025-07-12 00:28:12 +02:00
1eea2cc3e4 refactor: fix biome linting issues and update project documentation
- Fix 36+ biome linting issues reducing errors/warnings from 227 to 191
- Replace explicit 'any' types with proper TypeScript interfaces
- Fix React hooks dependencies and useCallback patterns
- Resolve unused variables and parameter assignment issues
- Improve accessibility with proper label associations
- Add comprehensive API documentation for admin and security features
- Update README.md with accurate PostgreSQL setup and current tech stack
- Create complete documentation for audit logging, CSP monitoring, and batch processing
- Fix outdated project information and missing developer workflows
2025-07-12 00:28:09 +02:00
3e9e75e854 feat: implement comprehensive CSRF protection 2025-07-12 00:28:07 +02:00
e7818f5e4f fix: resolve user invitation unique constraint error and add integration tests
- Fix platform user invitation to handle global email uniqueness properly
- Replace findFirst with findUnique for email validation
- Add clear error messages for email conflicts across companies
- Create comprehensive CSV import workflow integration tests
- Create comprehensive session processing pipeline integration tests
- Cover end-to-end flows from import to AI analysis completion
2025-07-12 00:28:04 +02:00
fa7e815a3b feat: complete tRPC integration and fix platform UI issues
- Implement comprehensive tRPC setup with type-safe API
- Create tRPC routers for dashboard, admin, and auth endpoints
- Migrate frontend components to use tRPC client
- Fix platform dashboard Settings button functionality
- Add platform settings page with profile and security management
- Create OpenAI API mocking infrastructure for cost-safe testing
- Update tests to work with new tRPC architecture
- Sync database schema to fix AIBatchRequest table errors
2025-07-12 00:27:57 +02:00
f2a3d87636 fix: resolve TypeScript compilation errors in batch processing
- Fix type assertion for session access in batchProcessor.ts
- Add explicit type annotation for batchRequests array in processingScheduler.ts
- Import AIProcessingRequest type from Prisma client
- All TypeScript compilation errors resolved
2025-07-12 00:27:55 +02:00
7d80ffe704 docs: update CLAUDE.md with batch API and security features
- Document new batch processing architecture and libraries
- Add batch scheduler to scheduler system documentation
- Document 50% AI cost reduction with OpenAI Batch API
- Add comprehensive security features documentation
- Document rate limiting configuration for all auth endpoints
- Add input validation and session security details
2025-07-12 00:27:53 +02:00
7cc5cad14f security: enhance authentication rate limiting and add comprehensive security tests
- Add rate limiting middleware for NextAuth login endpoints
- Implement authRateLimitMiddleware for /api/auth/* routes
- Add comprehensive security tests covering:
  - Rate limiter functionality (5 tests)
  - IP extraction from headers (5 tests)
  - Input validation and sanitization (10 tests)
  - Password strength requirements
  - XSS and SQL injection prevention
- All 21 security tests passing
- Rate limits configured: 5 login attempts per 15 minutes
2025-07-12 00:27:51 +02:00
25f6625c4f test: add comprehensive dashboard component tests
- Configure vitest with jsdom environment for React component testing
- Add comprehensive tests for TopQuestionsChart component (6 tests)
- Add comprehensive tests for TranscriptViewer component (7 tests)
- Mock all necessary dependencies (ReactMarkdown, shadcn/ui components)
- All 13 component tests passing successfully
- GeographicMap excluded due to react-leaflet test environment issues
2025-07-12 00:27:48 +02:00
8c8f360936 feat: implement OpenAI Batch API for cost-efficient AI processing
- Add AIBatchRequest and AIRequestStatus models to Prisma schema
- Create comprehensive batch processing system (lib/batchProcessor.ts)
- Add intelligent batch scheduler with automated management
- Update processing pipeline to use batch requests instead of direct API calls
- Integrate batch scheduler into main server startup
- Achieve 50% cost reduction on OpenAI API usage
- Improve rate limiting and processing reliability
2025-07-12 00:27:46 +02:00
5798988012 fix: resolve TypeScript errors and eliminate manual coordinate hardcoding
- Fix sendEmail function call to use proper EmailOptions object
- Improve GeographicMap by replacing 52 hardcoded coordinates with automatic extraction from @rapideditor/country-coder library
- Fix test imports to use correct exported functions from lib modules
- Add missing required properties to Prisma mock objects in tests
- Properly type all mock objects with correct enum values and required fields
- Simplify rate limiter mock to avoid private property conflicts
- Fix linting issues with variable declarations and useEffect dependencies
2025-07-12 00:27:40 +02:00
a0ac60cf04 feat: implement comprehensive email system with rate limiting and extensive test suite
- Add robust email service with rate limiting and configuration management
- Implement shared rate limiter utility for consistent API protection
- Create comprehensive test suite for core processing pipeline
- Add API tests for dashboard metrics and authentication routes
- Fix date range picker infinite loop issue
- Improve session lookup in refresh sessions API
- Refactor session API routing with better code organization
- Update processing pipeline status monitoring
- Clean up leftover files and improve code formatting
2025-07-12 00:26:30 +02:00
19628233ea fix: set execute permissions on pre-commit hook 2025-06-30 20:04:39 +02:00
2e3e028579 Removed eslint disable comments and removed pullrequest comments/reviews json files 2025-06-30 19:59:27 +02:00
38aff21c3a fix: comprehensive security and type improvements from PR #20 review
Security Enhancements:
- Implemented proper rate limiting with automatic cleanup for /register and /forgot-password endpoints
- Added memory usage protection with MAX_ENTRIES limit (10000)
- Fixed rate limiter memory leaks by adding cleanup intervals
- Improved IP extraction with x-real-ip and x-client-ip header support

Code Quality Improvements:
- Refactored ProcessingStatusManager from individual functions to class-based architecture
- Maintained backward compatibility with singleton instance pattern
- Fixed TypeScript strict mode violations across the codebase
- Resolved all build errors and type mismatches

UI Component Fixes:
- Removed unused chart components (Charts.tsx, DonutChart.tsx)
- Fixed calendar component type issues by removing unused custom implementations
- Resolved theme provider type imports
- Fixed confetti component default options handling
- Corrected pointer component coordinate type definitions

Type System Improvements:
- Extended NextAuth types to support dual auth systems (regular and platform users)
- Fixed nullable type handling throughout the codebase
- Resolved Prisma JSON field type compatibility issues
- Corrected SessionMessage and ImportRecord interface definitions
- Fixed ES2015 iteration compatibility issues

Database & Performance:
- Updated database pool configuration for Prisma adapter compatibility
- Fixed pagination response structure in user management endpoints
- Improved error handling with proper error class usage

Testing & Build:
- All TypeScript compilation errors resolved
- ESLint warnings remain but no errors
- Build completes successfully with proper static generation
2025-06-30 19:15:25 +02:00
397 changed files with 77293 additions and 19996 deletions

10
.biomeignore Normal file
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node_modules/
.next/
dist/
build/
coverage/
.git/
*.min.js
public/
prisma/migrations/
.claude/

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Use pnpm to manage this project, not npm!

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# To get started with Dependabot version updates, you'll need to specify which
# package ecosystems to update and where the package manifests are located.
# Please see the documentation for all configuration options:
# https://docs.github.com/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file
version: 2 version: 2
updates: updates:
- package-ecosystem: "bun" - package-ecosystem: "npm" # See documentation for possible values
directory: "/" directory: "/" # Location of package manifests
schedule: schedule:
interval: "monthly" interval: "weekly"
open-pull-requests-limit: 2 day: "tuesday"
groups: time: "03:00"
all-dependencies: timezone: "Europe/Amsterdam"
patterns:
- "*"
- package-ecosystem: "github-actions" - package-ecosystem: "github-actions" # See documentation for possible values
directory: "/" directory: "/" # Location of package manifests
schedule: schedule:
interval: "monthly" interval: "weekly"
open-pull-requests-limit: 2 day: "tuesday"
groups: time: "03:00"
all-actions: timezone: "Europe/Amsterdam"
patterns:
- "*"
- package-ecosystem: "docker" - package-ecosystem: "docker" # See documentation for possible values
directory: "/" directory: "/" # Location of package manifests
schedule: schedule:
interval: "monthly" interval: "weekly"
open-pull-requests-limit: 2 day: "tuesday"
groups: time: "03:00"
all-docker: timezone: "Europe/Amsterdam"
patterns:
- "*"
- package-ecosystem: "docker-compose" - package-ecosystem: "docker-compose" # See documentation for possible values
directory: "/" directory: "/" # Location of package manifests
schedule: schedule:
interval: "monthly" interval: "weekly"
open-pull-requests-limit: 2 day: "tuesday"
groups: time: "03:00"
all-docker-compose: timezone: "Europe/Amsterdam"
patterns:
- "*"

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---
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 45 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-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`

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---
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.

View File

@@ -1,49 +0,0 @@
---
title: useLatest for Stable Callback Refs
impact: LOW
impactDescription: prevents effect re-runs
tags: advanced, hooks, useLatest, refs, optimization
---
## useLatest for Stable Callback Refs
Access latest values in callbacks without adding them to dependency arrays. Prevents effect re-runs while avoiding stale closures.
**Implementation:**
```typescript
function useLatest<T>(value: T) {
const ref = useRef(value);
useLayoutEffect(() => {
ref.current = value;
}, [value]);
return ref;
}
```
**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 (stable effect, fresh callback):**
```tsx
function SearchInput({ onSearch }: { onSearch: (q: string) => void }) {
const [query, setQuery] = useState("");
const onSearchRef = useLatest(onSearch);
useEffect(() => {
const timeout = setTimeout(() => onSearchRef.current(query), 300);
return () => clearTimeout(timeout);
}, [query]);
}
```

View File

@@ -1,35 +0,0 @@
---
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).

View File

@@ -1,80 +0,0 @@
---
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.

View File

@@ -1,37 +0,0 @@
---
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);
},
});
```
Reference: [https://github.com/shuding/better-all](https://github.com/shuding/better-all)

View File

@@ -1,24 +0,0 @@
---
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()]);
```

View File

@@ -1,99 +0,0 @@
---
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.

View File

@@ -1,59 +0,0 @@
---
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)

View File

@@ -1,37 +0,0 @@
---
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.

View File

@@ -1,48 +0,0 @@
---
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>
);
}
```

View File

@@ -1,34 +0,0 @@
---
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} />;
}
```

View File

@@ -1,44 +0,0 @@
---
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.

View File

@@ -1,78 +0,0 @@
---
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", () => {
/* ... */
});
// ...
}
```

View File

@@ -1,74 +0,0 @@
---
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.

View File

@@ -1,48 +0,0 @@
---
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()`.

View File

@@ -1,56 +0,0 @@
---
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)

View File

@@ -1,57 +0,0 @@
---
title: Batch DOM CSS Changes
impact: MEDIUM
impactDescription: reduces reflows/repaints
tags: javascript, dom, css, performance, reflow
---
## Batch DOM CSS Changes
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.
**Incorrect (interleaved reads and writes force reflows):**
```typescript
function updateElementStyles(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();
}
```
**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();
}
```
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.

View File

@@ -1,80 +0,0 @@
---
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):**
```tsx
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):**
```tsx
// 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)

View File

@@ -1,28 +0,0 @@
---
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);
}
```

View File

@@ -1,68 +0,0 @@
---
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();
}
});
```

View File

@@ -1,32 +0,0 @@
---
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);
}
```

View File

@@ -1,50 +0,0 @@
---
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 };
}
```

View File

@@ -1,42 +0,0 @@
---
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(`(${query})`, "gi");
const parts = text.split(regex);
return <>{parts.map((part, i) => part)}</>;
}
```
**Correct (memoize or hoist):**
```tsx
const EMAIL_REGEX = /^[^\s@]+@[^\s@]+\.[^\s@]+$/;
function Highlighter({ text, query }: Props) {
const regex = useMemo(() => new RegExp(`(${escapeRegex(query)})`, "gi"), [query]);
const parts = text.split(regex);
return <>{parts.map((part, i) => part)}</>;
}
```
**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
```

View File

@@ -1,37 +0,0 @@
---
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.

View File

@@ -1,50 +0,0 @@
---
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

View File

@@ -1,82 +0,0 @@
---
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.

View File

@@ -1,24 +0,0 @@
---
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));
```

View File

@@ -1,51 +0,0 @@
---
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):**
```tsx
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):**
```tsx
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

View File

@@ -1,26 +0,0 @@
---
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.

View File

@@ -1,38 +0,0 @@
---
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.

View File

@@ -1,32 +0,0 @@
---
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>
```

View File

@@ -1,38 +0,0 @@
---
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).

View File

@@ -1,36 +0,0 @@
---
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.

View File

@@ -1,72 +0,0 @@
---
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.

View File

@@ -1,28 +0,0 @@
---
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
```

View File

@@ -1,39 +0,0 @@
---
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>;
}
```

View File

@@ -1,45 +0,0 @@
---
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]);
```

View File

@@ -1,29 +0,0 @@
---
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"} />;
}
```

View File

@@ -1,77 +0,0 @@
---
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.

View File

@@ -1,56 +0,0 @@
---
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.

View File

@@ -1,44 +0,0 @@
---
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.

View File

@@ -1,40 +0,0 @@
---
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);
}, []);
}
```

View File

@@ -1,73 +0,0 @@
---
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)

View File

@@ -1,41 +0,0 @@
---
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)

View File

@@ -1,76 +0,0 @@
---
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)

View File

@@ -1,83 +0,0 @@
---
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>
);
}
```

View File

@@ -1,38 +0,0 @@
---
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>;
}
```

View File

@@ -1,40 +0,0 @@
---
name: web-design-guidelines
description: Review UI code for Web Interface Guidelines compliance. Use when asked to "review my UI", "check accessibility", "audit design", "review UX", or "check my site against best practices".
metadata:
author: vercel
version: "1.0.0"
argument-hint: <file-or-pattern>
---
# Web Interface Guidelines
Review files for compliance with Web Interface Guidelines.
## How It Works
1. Fetch the latest guidelines from the source URL below
2. Read the specified files (or prompt user for files/pattern)
3. Check against all rules in the fetched guidelines
4. Output findings in the terse `file:line` format
## Guidelines Source
Fetch fresh guidelines before each review:
```
https://raw.githubusercontent.com/vercel-labs/web-interface-guidelines/main/command.md
```
Use WebFetch to retrieve the latest rules. The fetched content contains all the rules and output format instructions.
## Usage
When a user provides a file or pattern argument:
1. Fetch guidelines from the source URL above
2. Read the specified files
3. Apply all rules from the fetched guidelines
4. Output findings using the format specified in the guidelines
If no files specified, ask the user which files to review.

View File

@@ -9,16 +9,17 @@ jobs:
timeout-minutes: 60 timeout-minutes: 60
runs-on: ubuntu-latest runs-on: ubuntu-latest
steps: steps:
- uses: actions/checkout@v6 - uses: actions/checkout@v4
- { uses: actions/setup-node@v6, with: { node-version: lts/* } } - uses: actions/setup-node@v4
- uses: oven-sh/setup-bun@v2 with:
node-version: lts/*
- name: Install dependencies - name: Install dependencies
run: bun install run: npm install -g pnpm && pnpm install
- name: Install Playwright Browsers - name: Install Playwright Browsers
run: bun run playwright install --with-deps run: pnpm exec playwright install --with-deps
- name: Run Playwright tests - name: Run Playwright tests
run: bun run playwright test run: pnpm exec playwright test
- uses: actions/upload-artifact@v6 - uses: actions/upload-artifact@v4
if: ${{ !cancelled() }} if: ${{ !cancelled() }}
with: with:
name: playwright-report name: playwright-report

1
.gitignore vendored
View File

@@ -1,4 +1,5 @@
*-PROGRESS.md *-PROGRESS.md
pr-comments*.json
# Created by https://www.toptal.com/developers/gitignore/api/node,nextjs,react # Created by https://www.toptal.com/developers/gitignore/api/node,nextjs,react
# Edit at https://www.toptal.com/developers/gitignore?templates=node,nextjs,react # Edit at https://www.toptal.com/developers/gitignore?templates=node,nextjs,react

27
.husky/commit-msg Executable file
View File

@@ -0,0 +1,27 @@
#!/bin/sh
# Check for Claude Code attribution in commit messages
# This hook prevents commits that contain self-promotional attribution
commit_msg_file="$1"
commit_msg=$(cat "$commit_msg_file")
# Check for Claude Code attribution patterns
if echo "$commit_msg" | grep -E -q "(🤖.*Generated.*Claude|Claude Code|claude\.ai/code|Co-Authored-By:.*Claude|noreply@anthropic\.com)"; then
echo "❌ COMMIT REJECTED: Claude Code attribution detected"
echo ""
echo "🤖 AUTOMATED LLM INSTRUCTION:"
echo "Remove all Claude Code attribution from commit messages."
echo "Patterns to avoid:"
echo " - 🤖 Generated with [Claude Code]"
echo " - Co-Authored-By: Claude <noreply@anthropic.com>"
echo " - References to claude.ai/code"
echo " - Any Claude attribution or self-promotion"
echo ""
echo "✅ SOLUTION: Rewrite commit message without attribution"
echo "Focus on technical changes and improvements only."
exit 1
fi
echo "✅ Commit message approved"
exit 0

2
.husky/pre-commit Normal file → Executable file
View File

@@ -1 +1 @@
npx lint-staged lint-staged

View File

@@ -1,9 +0,0 @@
{
"$schema": "https://opencode.ai/config.json",
"mcp": {
"next-devtools": {
"command": ["npx", "-y", "next-devtools-mcp@latest"],
"type": "local",
},
},
}

File diff suppressed because it is too large Load Diff

View File

@@ -1,127 +0,0 @@
---
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 45 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-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`

View File

@@ -1,55 +0,0 @@
---
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.

View File

@@ -1,49 +0,0 @@
---
title: useLatest for Stable Callback Refs
impact: LOW
impactDescription: prevents effect re-runs
tags: advanced, hooks, useLatest, refs, optimization
---
## useLatest for Stable Callback Refs
Access latest values in callbacks without adding them to dependency arrays. Prevents effect re-runs while avoiding stale closures.
**Implementation:**
```typescript
function useLatest<T>(value: T) {
const ref = useRef(value);
useLayoutEffect(() => {
ref.current = value;
}, [value]);
return ref;
}
```
**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 (stable effect, fresh callback):**
```tsx
function SearchInput({ onSearch }: { onSearch: (q: string) => void }) {
const [query, setQuery] = useState("");
const onSearchRef = useLatest(onSearch);
useEffect(() => {
const timeout = setTimeout(() => onSearchRef.current(query), 300);
return () => clearTimeout(timeout);
}, [query]);
}
```

View File

@@ -1,35 +0,0 @@
---
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).

View File

@@ -1,80 +0,0 @@
---
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.

View File

@@ -1,37 +0,0 @@
---
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);
},
});
```
Reference: [https://github.com/shuding/better-all](https://github.com/shuding/better-all)

View File

@@ -1,24 +0,0 @@
---
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()]);
```

View File

@@ -1,99 +0,0 @@
---
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.

View File

@@ -1,59 +0,0 @@
---
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)

View File

@@ -1,37 +0,0 @@
---
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.

View File

@@ -1,48 +0,0 @@
---
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>
);
}
```

View File

@@ -1,34 +0,0 @@
---
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} />;
}
```

View File

@@ -1,44 +0,0 @@
---
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.

View File

@@ -1,78 +0,0 @@
---
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", () => {
/* ... */
});
// ...
}
```

View File

@@ -1,74 +0,0 @@
---
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.

View File

@@ -1,48 +0,0 @@
---
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()`.

View File

@@ -1,56 +0,0 @@
---
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)

View File

@@ -1,57 +0,0 @@
---
title: Batch DOM CSS Changes
impact: MEDIUM
impactDescription: reduces reflows/repaints
tags: javascript, dom, css, performance, reflow
---
## Batch DOM CSS Changes
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.
**Incorrect (interleaved reads and writes force reflows):**
```typescript
function updateElementStyles(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();
}
```
**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();
}
```
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.

View File

@@ -1,80 +0,0 @@
---
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):**
```tsx
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):**
```tsx
// 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)

View File

@@ -1,28 +0,0 @@
---
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);
}
```

View File

@@ -1,68 +0,0 @@
---
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();
}
});
```

View File

@@ -1,32 +0,0 @@
---
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);
}
```

View File

@@ -1,50 +0,0 @@
---
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 };
}
```

View File

@@ -1,42 +0,0 @@
---
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(`(${query})`, "gi");
const parts = text.split(regex);
return <>{parts.map((part, i) => part)}</>;
}
```
**Correct (memoize or hoist):**
```tsx
const EMAIL_REGEX = /^[^\s@]+@[^\s@]+\.[^\s@]+$/;
function Highlighter({ text, query }: Props) {
const regex = useMemo(() => new RegExp(`(${escapeRegex(query)})`, "gi"), [query]);
const parts = text.split(regex);
return <>{parts.map((part, i) => part)}</>;
}
```
**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
```

View File

@@ -1,37 +0,0 @@
---
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.

View File

@@ -1,50 +0,0 @@
---
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

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@@ -1,82 +0,0 @@
---
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.

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@@ -1,24 +0,0 @@
---
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));
```

View File

@@ -1,51 +0,0 @@
---
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):**
```tsx
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):**
```tsx
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

View File

@@ -1,26 +0,0 @@
---
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.

View File

@@ -1,38 +0,0 @@
---
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.

View File

@@ -1,32 +0,0 @@
---
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>
```

View File

@@ -1,38 +0,0 @@
---
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).

View File

@@ -1,36 +0,0 @@
---
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.

View File

@@ -1,72 +0,0 @@
---
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.

View File

@@ -1,28 +0,0 @@
---
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
```

View File

@@ -1,39 +0,0 @@
---
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>;
}
```

View File

@@ -1,45 +0,0 @@
---
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]);
```

View File

@@ -1,29 +0,0 @@
---
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"} />;
}
```

View File

@@ -1,77 +0,0 @@
---
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.

View File

@@ -1,56 +0,0 @@
---
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.

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