TypeScript for React & Next.js — Agent Skill

A structured reference for AI coding agents assisting frontend engineers with TypeScript, React, and Next.js in production environments.

Agent Behavior Rules

Before answering, always verify:

1. Server or client? Server Components, Server Actions, and Route Handlers have different type constraints than "use client" components.
2. Runtime validation needed? Static types do NOT validate API responses, URL params, form data, or localStorage. Data crossing a trust boundary requires Zod or equivalent.
3. App Router or Pages Router? Patterns differ significantly. If unclear, ask.
4. TypeScript version? satisfies requires 5.0+. Check before suggesting version-dependent features.
5. Next.js version? params is a Promise in 15+. Caching model changed in 16+.

Assumptions the agent must NOT make:

• That API responses match their TypeScript types at runtime
• That searchParams values are the expected type (they are always string | string[] | undefined)
• That any in existing code is intentional
• That a type assertion (as) is justified without checking context
• That server-only imports are safe in client components
• That useEffect dependencies in existing code are correct

When uncertain:

• State tradeoffs explicitly rather than picking one approach silently
• Mark unstable or version-dependent patterns as such
• Distinguish: [HARD RULE] (violating causes bugs) / [DEFAULT] (override with reason) / [SITUATIONAL] (depends on context)

Decision Guide

Quick: What pattern should I use?

Flowchart: Is this data safe to use?

Data comes from...
├─ Inside the app (useState, useReducer, computed)
│  → Static typing is sufficient. No runtime validation needed.
│
├─ Outside the app (API, URL, FormData, localStorage, postMessage)
│  → [HARD RULE] Validate at runtime. Use Zod or equivalent.
│  │
│  ├─ API response    → schema.parse(await res.json())
│  ├─ URL params      → schema.parse(searchParams)
│  ├─ FormData        → schema.safeParse({ field: formData.get('field') })
│  ├─ localStorage    → schema.safeParse(JSON.parse(stored))
│  └─ postMessage     → schema.safeParse(event.data)
│
└─ Third-party library callback
   → Check library types. Add runtime guard if types seem wrong.

Flowchart: Where should this state live?

Is this data from a server/API?
├─ Yes → TanStack Query (NOT useState). See data-fetching-and-api-types.md
│
└─ No → Is it shareable via URL? (filters, page, sort)
   ├─ Yes → searchParams or nuqs. See state-management.md
   │
   └─ No → How many components need it?
      ├─ 1 component → useState or useReducer
      ├─ 2-3 in same tree → Lift state up (props)
      └─ Many across trees → How often does it change?
         ├─ Rarely (theme, locale, auth) → Context
         └─ Often (cart, notifications) → Zustand with selectors

Flowchart: Should I memoize this?

Is there a measured performance problem?
├─ No → Don't memoize. Stop here.
│
└─ Yes → Can you restructure instead?
   ├─ Yes → Move state down, extract components. See performance-and-accessibility.md
   │
   └─ No → What needs memoizing?
      ├─ Expensive computation → useMemo (verify it's truly expensive)
      ├─ Callback to memoized child → useCallback
      └─ Component in a long list → React.memo (verify props are stable)

Quick: hard rule vs default vs situational

Code Generation Checklist

Before generating TypeScript/React/Next.js code:

Context

• Confirmed: server or client code?
• Confirmed: App Router or Pages Router?
• Confirmed: TypeScript strict mode enabled?

Type Safety

• No any — use unknown with validation or proper types
• No as without documented justification
• External data (API, URL, form, storage) validated at runtime
• Props use interface, only truly optional fields have ?

React

• children typed as React.ReactNode
• Event handler Props expose values, not event objects
• Effects have stable dependencies and cleanup functions
• "use client" only where needed, as deep as possible
• No server data duplicated into useState

Next.js (15+)

• params and searchParams awaited
• Server Actions validate FormData with Zod
• Sensitive code protected with import 'server-only'
• Cross-boundary Props are JSON-serializable (no functions, Dates, Maps)

Accessibility

• Form inputs have associated labels
• Error messages use role="alert"
• Interactive elements are keyboard-accessible

Code Review Checklist

Flag as risk (likely bug or maintenance problem)

• any without documented reason
• as on external data without validation
• ! non-null assertion without prior guard
• useEffect with object/array dependencies (likely unstable)
• Missing useEffect cleanup
• Server data copied into useState
• "use client" at page/layout level
• Functions passed across server/client boundary
• params/searchParams not awaited (Next.js 15+)
• Server Action without FormData validation

Flag as preference (mention, don't block)

• type vs interface for object shapes
• Handler naming convention
• File/folder organization style
• Import ordering

File Index

playbooks/ — Step-by-step debuggin