Frontend Typescript Rules Agent Skill

TypeScript Development Rules (Frontend)

Frontend-specific React/TypeScript rules for implementation: thresholds, boundary type safety, component/state design, error handling, and project conventions.

Prerequisite Detection

Inspect TypeScript, bundler/framework, lint/format, path-alias, React compiler, and representative component configuration before applying a project convention. Treat a convention as observed when configuration or an established repository pattern supports it; label a conclusion from limited examples as inferred. When conflicting patterns affect public behavior, compatibility, or component boundaries, stop and name the required source or decision.

Anti-patterns and Thresholds

Signals that trigger a design change:

• Prop drilling through 3+ levels → lift to Context or state management
• Component over 300 lines → split
• Props count over 10 → split the component (3-7 is the working range)
• Optional props over 50% → introduce defaults or Context
• Props nesting deeper than 2 levels → flatten
• The same as assertion appearing 3+ times → revisit the type design

Type Safety at Boundaries

Receive untrusted or unavailable types as unknown and narrow them with a type guard. Use as only when a runtime/framework invariant proves the asserted type and record that invariant in a nearby comment. Existing generated or third-party declarations that contain any are boundary inputs to wrap, not justification for spreading any into application contracts.

Inside the app, React Props/State are type-guaranteed — no unknown needed. At every external boundary, receive as unknown and narrow with a type guard before use: API responses, localStorage/sessionStorage, URL parameters, parsed JSON. Controlled-component form input stays type-safe through React synthetic events.

const raw: unknown = await (await fetch(url)).json()
if (!isUser(raw)) throw new ValidationError('invalid user')
const user = raw // narrowed to User

Component and State Design

• Function components only. Class components are allowed solely for Error Boundaries (no hook equivalent exists).
• Type Props explicitly with a named type and destructure: function UserCard({ user, onSelect }: UserCardProps). Type props directly on the function so the props contract stays explicit.
• Props-driven: pass dependencies as props when one explicit parent owns them. Use Context or established global state when multiple non-adjacent descendants share the value and prop forwarding would add intermediate components with no ownership role.
• Custom hooks are the unit of logic reuse and dependency injection (inject collaborators through the hook for testability).
• Function parameters: 0-2 positional; for 3+ take a single options object.
• State shape: type state explicitly; for multi-field state with discrete transitions, use useReducer with a discriminated-union action type rather than many useState calls.
• Server/Client boundary (RSC frameworks only — e.g. Next.js App Router): default to server components for data fetching/rendering and isolate interactivity behind a "use client" boundary at the smallest scope that needs it; keep browser-only APIs (window, localStorage, event handlers) inside client components, since calling them in a server component breaks the render. N/A for client-only SPAs (e.g. Vite) — skip when the project has no server-component runtime.

Error Handling

• Give every error one explicit outcome: convert it to a typed expected failure, handle it at the owning UI boundary, or propagate it with its diagnostic context. Log at the layer that owns observability so the same failure is not logged repeatedly.
• Fail fast: on an invalid state, throw rather than returning a silent fallback.
• Represent expected failures as values with a Result type; reserve throw for unexpected/unrecoverable cases.
• Use purpose-specific error classes extending a base AppError carrying a code (e.g. ValidationError, ApiError, NotFoundError).
• Layer responsibilities: the API layer converts transport errors into domain errors; hooks propagate AppError upward; an Error Boundary catches render-time errors and shows fallback UI.
• Effect race/cleanup: guard useEffect data fetches against out-of-order responses and post-unmount state updates — abort or ignore stale results (AbortController or a mounted flag), or use a server-state library (React Query/SWR) that cancels and dedupes. try-catch alone does not cover this.
• Log only diagnostic fields approved for the current trust boundary; redact credentials, tokens, payment data, and other sensitive values before logging.

type Result<T, E> = { ok: true; value: T } | { ok: false; error: E }

class AppError extends Error {
  constructor(message: string, readonly code: string, readonly statusCode = 500) {
    super(message); this.name = this.constructor.name
  }
}

Error Boundary — the one place a class component is required:

class ErrorBoundary extends React.Component<{ children: React.ReactNode; fallback: React.ReactNode }, { hasError: boolean }> {
  state = { hasError: false }
  static getDerivedStateFromError() { return { hasError: true } }
  render() { return this.state.hasError ? this.props.fallback : this.props.children }
}

Project Conventions

• Environment variables: read client-side env through the configured bundler's exposed accessor. Match the observed bundler: Vite import.meta.env.VITE_*, Next.js public process.env.NEXT_PUBLIC_*, CRA process.env.REACT_APP_*. Frontend bundles contain public configuration; secret values remain behind a server-side boundary.
• Bundle & performance: monitor bundle size with the build script against the project's budget; code-split with React.lazy + Suspense; structure state to minimize re-renders. Memoization: when React Compiler is enabled, rely on it; reach for manual React.memo/useMemo/useCallback only as a profiler- or identity-justified escape hatch (a measured bottleneck, or stable reference identity for third-party APIs / effect dependencies).
• Naming: components/types PascalCase; variables/functions camelCase; hooks use-prefixed; constants SCREAMING_SNAKE_CASE.
• Imports: follow the alias and import-order rules observed in tsconfig, lint configuration, and representative files. Use src/ absolute paths only when the configured alias supports them.
• Formatting: follow the repository's configured formatter; when Biome is present, semicolons and style come from its project configuration.