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.

</routing>

<rules>

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()

</rules>

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

</references>

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