nextjs-code-reviewer

masanao-ohba/nextjs-code-reviewer

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Comprehensive code review guidelines for Next.js 15 App Router projects

SKILL.md

Next.js Code Review Guidelines

App Router Structure

File Organization

Issues:

page.tsx missing in route directory (route won't work)
layout.tsx not at app root (required)
Client Component marked with 'use client' unnecessarily
API route in wrong location (should be in app/api/)
Special files (loading, error, not-found) in wrong location

Verify:

Root layout.tsx exists and returns <html> and <body>
Each route directory has page.tsx for that route
loading.tsx and error.tsx placed at appropriate levels
API routes follow app/api/* convention

Server vs Client

Critical Issues:

'use client' on component that doesn't need it
Server Component trying to use hooks
Client Component doing data fetching (should be Server Component)
Sensitive data (API keys, secrets) in Client Components

Verify:

Default to Server Components
'use client' only for: hooks, event handlers, browser APIs, Context
Data fetching in Server Components when possible
Client Components receive data as props

Data Fetching

Server Components

Issues:

Using useEffect for data fetching (should be async/await)
Not handling errors with try-catch
Fetching inside loops (N+1 problem)
Not using Promise.all for parallel fetches
Missing revalidation strategy

Verify:

Async functions for data fetching
Proper error handling
Parallel fetching with Promise.all when possible
Appropriate cache strategy (force-cache, no-store, revalidate)

Client Components

Issues:

Using fetch in useEffect instead of React Query
Not handling loading and error states
Race conditions in useEffect
Missing error boundaries

Verify:

React Query for client-side data fetching
Loading, error, and success states all handled
Proper query key structure
Error boundaries in place

Routing

Dynamic Routes

Issues:

Not validating params before use
Not handling missing data (404 cases)
Missing generateStaticParams for static export
Params not properly typed

Verify:

Params validated and typed correctly
notFound() called for missing resources
generateStaticParams implemented for SSG
Type safety: { params: { slug: string } }

Navigation

Issues:

Using <a> instead of <Link>
Using router.push with full URL (should be relative)
Not using prefetch for important links
Client-side navigation for external links

Verify:

<Link> component for internal navigation
Relative paths in navigation
prefetch={true} for critical routes
<a> with rel="noopener noreferrer" for external links

Loading States

Suspense

Issues:

No loading.tsx at route level
No fallback for Suspense boundaries
Loading state only shows after delay (missing instant feedback)
Suspense boundaries too coarse-grained

Verify:

loading.tsx provides meaningful loading UI
Suspense boundaries around async components
Instant loading feedback
Multiple Suspense boundaries for granular loading

Error Handling

Error Boundaries

Issues:

No error.tsx at route level
Error component not marked 'use client'
Error messages expose sensitive information
No retry mechanism in error UI
Not using error.digest for error tracking

Verify:

error.tsx in place for each route segment
'use client' directive on error components
User-friendly error messages
Reset function provided to users
Error logging/tracking implemented

Not Found

Issues:

404 errors not handled gracefully
notFound() not called for missing resources
Generic 404 page for all routes

Verify:

notFound() called when resource doesn't exist
Custom not-found.tsx at route level if needed
Clear messaging about what was not found

API Routes

Route Handlers

Issues:

Not validating request body
Missing error handling
Not using proper HTTP status codes
CORS not configured for cross-origin requests
No rate limiting for public endpoints

Verify:

Request validation with Zod or similar
Proper error responses with status codes
NextResponse.json() for responses
CORS headers if needed
Authentication checks before sensitive operations

Security

Critical Issues:

SQL injection vulnerabilities
No authentication on protected routes
API keys exposed in client code
CSRF vulnerabilities in mutations
No input sanitization

Verify:

Parameterized queries (no string concatenation)
Authentication middleware on protected routes
Environment variables for secrets
CSRF protection for mutations
Input validation and sanitization

Server Actions

Implementation

Issues:

'use server' directive missing
Not revalidating cache after mutations
No error handling
Not returning serializable data
Complex logic in server action (should be in service layer)

Verify:

'use server' at file or function level
revalidatePath() or revalidateTag() after data changes
Try-catch with user-friendly error messages
Returned data is JSON-serializable
Business logic in separate service functions

Metadata and SEO

Metadata

Issues:

No metadata exported
Missing Open Graph tags
No Twitter Card metadata
Dynamic pages missing generateMetadata
Duplicate titles across pages

Verify:

Metadata object or generateMetadata function present
Title, description, and OG tags complete
Dynamic metadata for dynamic routes
Unique, descriptive titles for each page
Appropriate meta tags for social sharing

Images

Issues:

Using <img> instead of <Image>
Missing alt text
Not specifying width and height
Not using priority for LCP images
Remote images not configured in next.config.js

Verify:

next/image for all images
Descriptive alt text (or empty string if decorative)
Width and height specified (or fill mode)
priority prop on above-the-fold images
Remote image domains configured

Performance

Bundle Size

Issues:

Heavy libraries imported in Client Components
No code splitting for large features
All components in single file
Not using dynamic imports for heavy components

Verify:

Heavy dependencies in Server Components when possible
Dynamic imports for large, conditional components
Components split into separate files
Bundle analysis done regularly

Caching

Issues:

No caching strategy defined
Using no-store for everything (too aggressive)
Not using revalidation for stale-while-revalidate
Not leveraging Next.js automatic request deduplication

Verify:

Appropriate cache strategy per request
force-cache for static data
revalidate for time-based freshness
no-store only for user-specific or sensitive data

Rendering

Issues:

Not using Static Site Generation (SSG) when possible
Generating all pages at build time (slow builds)
Not implementing Incremental Static Regeneration (ISR)

Verify:

generateStaticParams for known routes
ISR (revalidate) for frequently changing content
On-demand revalidation for critical updates

TypeScript

Type Safety

Issues:

Params and searchParams not typed
API route request/response not typed
Server action return values not typed
Using 'any' for Next.js types

Verify:

PageProps interface for params and searchParams
NextRequest and NextResponse properly typed
Server actions have return type annotations
Imported Next.js types used correctly

Middleware

Implementation

Issues:

Heavy logic in middleware (runs on every request)
Not returning NextResponse
Middleware running on static assets
No matcher config (runs on all routes)

Verify:

Lightweight logic only (auth checks, redirects)
NextResponse.next() or redirect returned
Matcher excludes _next/static and other assets
Matcher config specific to needed routes

Internationalization

next-intl

Issues:

Locale not validated in params
Messages not loaded for server components
Hardcoded strings instead of translations
Missing translations causing runtime errors

Verify:

Locale param validated against supported locales
getMessages() called in server components
All user-facing text uses t() function
Fallback locale configured

Code Quality

Best Practices

Issues:

Mixing Server and Client code in same file
Deep nesting of route groups
No consistent naming convention
Component files > 500 lines

Verify:

Clear separation of Server and Client code
Reasonable route group nesting (< 3 levels)
Consistent file naming (kebab-case or PascalCase)
Components split appropriately

Review Checklist

Critical

Server/Client component separation correct
No security vulnerabilities (auth, input validation)
Error handling comprehensive
Type safety maintained

High Priority

Loading states implemented
SEO metadata complete
Images optimized
Caching strategy appropriate

Medium Priority

Code organization clean
No console.logs left in
Consistent code style
Performance optimizations justified

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