React Dev

React TypeScript

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

• 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

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.

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
}

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.

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.

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.

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

Async data fetching:

export default async function UserPage({ params }: { params: Promise<{ id: string }> }) {
  const { id } = await params;          // Next.js 15+: params는 Promise (await 필수)
  const user = await fetchUser(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.

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()

Effects are an escape hatch from React. If no external system is involved, you don't need an Effect.

Decision Tree:

Need to respond to something?
├── User interaction (click, submit)? → EVENT HANDLER
├── Component appeared on screen? → EFFECT (external sync, analytics)
├── Props/state changed, need derived value? → CALCULATE DURING RENDER
│   └── Expensive? → useMemo
└── Reset state when prop changes? → KEY PROP

Quick Reference:

See useeffect-anti-patterns.md for 9 common mistakes with fixes. See useeffect-alternatives.md for useMemo, key prop, useSyncExternalStore patterns.

• useeffect-anti-patterns.md - 9 common useEffect mistakes with fixes
• useeffect-alternatives.md - useMemo, key prop, useSyncExternalStore, lifting state
• 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