react

name: react description: React component-based UI with hooks, context, and state management. Use for .jsx/.tsx files.

React is the standard library for building user interfaces. React 19 (2025) introduces a new era with the React Compiler, Server Components, and Actions.

When to Use

Single Page Apps (SPA): Rich, interactive dashboards.
Complex UI: Applications with many moving parts and state.
Ecosystem: When you need the largest library of 3rd party components.

Quick Start (React 19)

import { use, Suspense } from "react";

// New: 'use' hook for promises
function Comments({ commentsPromise }) {
  const comments = use(commentsPromise);
  return comments.map((c) => <p key={c.id}>{c.text}</p>);
}

export default function Page({ id }) {
  const commentsPromise = fetchComments(id);

  return (
    <Suspense fallback={<p>Loading...</p>}>
      <Comments commentsPromise={commentsPromise} />
    </Suspense>
  );
}

Core Concepts

React Compiler

React 19 introduces an auto-memoizing compiler. You no longer need useMemo or useCallback manually in 99% of cases. The compiler treats code as "memoized by default".

Server Components (RSC)

Components that run only on the server. They don't send JS to the client.

'use server': Marks a function as a Server Action (callable from client).
'use client': Marks a component as interactive (hydrated on client).

Actions and `useActionState`

Native support for async form submission.

function Form() {
  const [error, submitAction, isPending] = useActionState(
    async (prev, formData) => {
      const error = await updateProfile(formData);
      if (error) return error;
      return null;
    },
    null,
  );

  return (
    <form action={submitAction}>
      <input name="name" />
      <button disabled={isPending}>Save</button>
      {error && <p>{error}</p>}
    </form>
  );
}

Best Practices (2025)

Do:

Trust the Compiler: Stop writing useMemo/useCallback unless you are building a library or strictly optimizing.
Use Server Actions: Replace manual fetch('/api/...') with robust Server Actions for data mutations.
Use ref as a prop: In React 19, ref is a plain prop. No more forwardRef.

Don't:

Don't overuse useEffect: Effects are for synchronization with external systems, not for data fetching or derived state.
Don't spread props blindly: Pass explicit props to make components easier to debug.