React TypeScript

Type-safe React = compile-time guarantees = confident refactoring.

<when_to_use>

• Building typed React components

• Implementing generic components

• Typing event handlers, forms, refs

• Using React 19 features (Actions, Server Components, use())

• Router integration (TanStack Router, React Router)

• Custom hooks with proper typing

NOT for: non-React TypeScript, vanilla JS React

</when_to_use>

<react_19_changes>

React 19 breaking changes require migration. Key patterns:

ref as prop - forwardRef deprecated:

// React 19 - ref as regular prop type ButtonProps = { ref?: React.Ref<HTMLButtonElement>; } & React.ComponentPropsWithoutRef<'button'>;

function Button({ ref, children, ...props }: ButtonProps) { return <button ref={ref} {...props}>{children}</button>; }

useActionState - replaces useFormState:

import { useActionState } from 'react';

type FormState = { errors?: string[]; success?: boolean };

function Form() { const [state, formAction, isPending] = useActionState(submitAction, {}); return <form action={formAction}>...</form>; }

use() - unwraps promises/context:

function UserProfile({ userPromise }: { userPromise: Promise<User> }) { const user = use(userPromise); // Suspends until resolved return <div>{user.name}</div>; }

See react-19-patterns.md for useOptimistic, useTransition, migration checklist.

</react_19_changes>

<component_patterns>

Props - extend native elements:

type ButtonProps = { variant: 'primary' | 'secondary'; } & React.ComponentPropsWithoutRef<'button'>;

function Button({ variant, children, ...props }: ButtonProps) { return <button className={variant} {...props}>{children}</button>; }

Children typing:

type Props = { children: React.ReactNode; // Anything renderable icon: React.ReactElement; // Single element render: (data: T) => React.ReactNode; // Render prop };

Discriminated unions for variant props:

type ButtonProps = | { variant: 'link'; href: string } | { variant: 'button'; onClick: () => void };

function Button(props: ButtonProps) { if (props.variant === 'link') { return <a href={props.href}>Link</a>; } return <button onClick={props.onClick}>Button</button>; }

</component_patterns>

<event_handlers>

Use specific event types for accurate target typing:

// Mouse function handleClick(e: React.MouseEvent<HTMLButtonElement>) { e.currentTarget.disabled = true; }

// Form function handleSubmit(e: React.FormEvent<HTMLFormElement>) { e.preventDefault(); const formData = new FormData(e.currentTarget); }

// Input function handleChange(e: React.ChangeEvent<HTMLInputElement>) { console.log(e.target.value); }

// Keyboard function handleKeyDown(e: React.KeyboardEvent<HTMLInputElement>) { if (e.key === 'Enter') e.currentTarget.blur(); }

See event-handlers.md for focus, drag, clipboard, touch, wheel events.

</event_handlers>

<hooks_typing>

useState - explicit for unions/null:

const [user, setUser] = useState<User | null>(null); const [status, setStatus] = useState<'idle' | 'loading'>('idle');

useRef - null for DOM, value for mutable:

const inputRef = useRef<HTMLInputElement>(null); // DOM - use ?. const countRef = useRef<number>(0); // Mutable - direct access

useReducer - discriminated unions for actions:

type Action = | { type: 'increment' } | { type: 'set'; payload: number };

function reducer(state: State, action: Action): State { switch (action.type) { case 'set': return { ...state, count: action.payload }; default: return state; } }

Custom hooks - tuple returns with as const:

function useToggle(initial = false) { const [value, setValue] = useState(initial); const toggle = () => setValue(v => !v); return [value, toggle] as const; }

useContext - null guard pattern:

const UserContext = createContext<User | null>(null);

function useUser() { const user = useContext(UserContext); if (!user) throw new Error('useUser outside UserProvider'); return user; }

See hooks.md for useCallback, useMemo, useImperativeHandle, useSyncExternalStore.

</hooks_typing>

<generic_components>

Generic components infer types from props - no manual annotations at call site.

Pattern - keyof T for column keys, render props for custom rendering:

type Column<T> = { key: keyof T; header: string; render?: (value: T[keyof T], item: T) => React.ReactNode; };

type TableProps<T> = { data: T[]; columns: Column<T>[]; keyExtractor: (item: T) => string | number; };

function Table<T>({ data, columns, keyExtractor }: TableProps<T>) { return ( <table> <thead> <tr>{columns.map(col => <th key={String(col.key)}>{col.header}</th>)}</tr> </thead> <tbody> {data.map(item => ( <tr key={keyExtractor(item)}> {columns.map(col => ( <td key={String(col.key)}> {col.render ? col.render(item[col.key], item) : String(item[col.key])} </td> ))} </tr> ))} </tbody> </table> ); }

Constrained generics for required properties:

type HasId = { id: string | number };

function List<T extends HasId>({ items }: { items: T[] }) { return <ul>{items.map(item => <li key={item.id}>...</li>)}</ul>; }

See generic-components.md for Select, List, Modal, FormField patterns.

</generic_components>

<server_components>

React 19 Server Components run on server, can be async.

Async data fetching:

export default async function UserPage({ params }: { params: { id: string } }) { const user = await fetchUser(params.id); return <div>{user.name}</div>; }

Server Actions - 'use server' for mutations:

'use server';

export async function updateUser(userId: string, formData: FormData) { await db.user.update({ where: { id: userId }, data: { ... } }); revalidatePath(/users/${userId}); }

Client + Server Action:

'use client';

import { useActionState } from 'react'; import { updateUser } from '@/actions/user';

function UserForm({ userId }: { userId: string }) { const [state, formAction, isPending] = useActionState( (prev, formData) => updateUser(userId, formData), {} ); return <form action={formAction}>...</form>; }

use() for promise handoff:

// Server: pass promise without await async function Page() { const userPromise = fetchUser('123'); return <UserProfile userPromise={userPromise} />; }

// Client: unwrap with use() 'use client'; function UserProfile({ userPromise }: { userPromise: Promise<User> }) { const user = use(userPromise); return <div>{user.name}</div>; }

See server-components.md for parallel fetching, streaming, error boundaries.

</server_components>

Both TanStack Router and React Router v7 provide type-safe routing solutions.

TanStack Router - Compile-time type safety with Zod validation:

import { createRoute } from '@tanstack/react-router'; import { z } from 'zod';

const userRoute = createRoute({ path: '/users/$userId', component: UserPage, loader: async ({ params }) => ({ user: await fetchUser(params.userId) }), validateSearch: z.object({ tab: z.enum(['profile', 'settings']).optional(), page: z.number().int().positive().default(1), }), });

function UserPage() { const { user } = useLoaderData({ from: userRoute.id }); const { tab, page } = useSearch({ from: userRoute.id }); const { userId } = useParams({ from: userRoute.id }); }

React Router v7 - Automatic type generation with Framework Mode:

import type { Route } from "./+types/user";

export async function loader({ params }: Route.LoaderArgs) { return { user: await fetchUser(params.userId) }; }

export default function UserPage({ loaderData }: Route.ComponentProps) { const { user } = loaderData; // Typed from loader return <h1>{user.name}</h1>; }

See tanstack-router.md for TanStack patterns and react-router.md for React Router patterns.

ALWAYS:

• Specific event types (MouseEvent, ChangeEvent, etc)

• Explicit useState for unions/null

• ComponentPropsWithoutRef for native element extension

• Discriminated unions for variant props

• as const for tuple returns

• ref as prop in React 19 (no forwardRef)

• useActionState for form actions

• Type-safe routing patterns (see routing section)

NEVER:

• any for event handlers

• JSX.Element for children (use ReactNode)

• forwardRef in React 19+

• useFormState (deprecated)

• Forget null handling for DOM refs

• Mix Server/Client components in same file

• Await promises when passing to use()

• hooks.md - useState, useRef, useReducer, useContext, custom hooks

• event-handlers.md - all event types, generic handlers

• react-19-patterns.md - useActionState, use(), useOptimistic, migration

• generic-components.md - Table, Select, List, Modal patterns

• server-components.md - async components, Server Actions, streaming

• tanstack-router.md - TanStack Router typed routes, search params, navigation

• react-router.md - React Router v7 loaders, actions, type generation, forms