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.

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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 needs 5.0+, NoInfer 5.4+, inferred type predicates 5.5+. TS 6.0 is the final JS-based compiler; TS 7 is the Go-native rewrite — ~10x faster, same language, but tooling that consumes the old compiler API may lag.
5. Next.js version? params is a Promise in 15+. Caching model changed in 16+ ("use cache"), and 16 renames middleware.ts → proxy.ts (Node runtime).
6. React version? 19 makes ref a regular prop (forwardRef deprecated), renders <Context> directly, and replaces useFormState with useActionState. useEffectEvent and <Activity> need 19.2+.

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)

After context compaction:

If this session was compacted and the rules content survives only as a summary:

1. Do NOT answer from the summary — it drops the reasoning, examples, and edge cases.
2. Re-read the relevant file from the Decision Guide below before continuing. One file read is enough; this hub is deliberately thin so recovery is cheap.
3. If mid-task and unsure which file applies, re-read HARD-RULES.md first — every non-negotiable rule in compressed form.

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Decision Guide

Quick: What pattern should I use?

Situation	Start here
Typing component Props, children, events, refs	→ react-typescript-patterns.md
Narrowing unions, unknown, type guards, utility types	→ typescript-core.md
Next.js params, searchParams, Server Actions, RSC boundary	→ nextjs-typescript.md
Discriminated unions, conditional props, compound components	→ component-patterns.md
API responses, fetch typing, TanStack Query, caching	→ data-fetching-and-api-types.md
Form state, validation, controlled vs uncontrolled	→ forms-and-validation.md
Local state vs context vs server state vs Zustand	→ state-management.md
Re-renders, memoization, accessibility	→ performance-and-accessibility.md
Type errors, hydration, stale state, effect bugs	→ debugging-checklists.md (hub) + playbooks/
PR review, risk vs preference, architecture smells	→ code-review-rules.md
Common mistakes, cargo-cult patterns	→ anti-patterns.md

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

Label	Meaning	Example
[HARD RULE]	Violating causes bugs or security issues. No exceptions.	"Validate API responses at runtime"
[DEFAULT]	Recommended unless you have a documented reason to deviate.	"Use interface for Props"
[SITUATIONAL]	Depends on context. Both options are valid. Explain your choice.	"Polymorphic components — only for design-system foundations"

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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 RSC-serializable (no plain callbacks or class instances; Date/Map/Set/Promise are fine)

Accessibility

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

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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
• Non-Server-Function callbacks or class instances 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

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File Index

File	Scope
HARD-RULES.md	All [HARD RULE]s in one page — compaction recovery, CLAUDE.md embedding, hook injection
typescript-core.md	Narrowing, unions, generics, utility types, inference, unknown vs any, as const, satisfies
react-typescript-patterns.md	Props, children, events, refs (19 ref-as-prop / 18 forwardRef), hooks, context
nextjs-typescript.md	App Router types, params, searchParams, Server Actions, RSC boundaries, Cache Components (16), proxy.ts, metadata
component-patterns.md	Discriminated union Props, compound components, controlled/uncontrolled, polymorphic
data-fetching-and-api-types.md	Fetch typing, Zod validation, TanStack Query, safe response handling
forms-and-validation.md	Form state, Zod, react-hook-form, Server Actions, progressive enhancement
state-management.md	Local state, Context, Zustand, TanStack Query, URL state, decision matrix
performance-and-accessibility.md	Memoization tradeoffs, effect stability, semantic HTML, ARIA patterns
debugging-checklists.md	Quick diagnosis router, serialization issues, null access, re-render errors
code-review-rules.md	Risk vs preference, architecture smells, review comment templates
anti-patterns.md	13 common mistakes with root causes and fixes

playbooks/ — Step-by-step debugging guides (consult when diagnosing specific bugs)

File	Scope
type-error-debugging.md	Systematic type error resolution with React/Next.js-specific errors
hydration-issues.md	SSR/CSR mismatch diagnosis flowchart and fix patterns
effect-dependency-bugs.md	Infinite loops, stale closures, missing cleanups, real-world debounce example