Tech React

Core Challenges

React patterns evolve with each major version. React 19 introduced the use() hook for promise handling and formalized Server Component boundaries with clearer client/server semantics. Common pitfalls include:

• Hook ordering violations: Conditionally calling hooks breaks React's tracking system
• Unnecessary memoization: useMemo/useCallback/React.memo add overhead without measurement
• Oversized components: Mixing Server and Client logic prevents streaming optimization
• Missing Suspense boundaries: Async data without Suspense blocks entire render
• Stale closures: Effects with incorrect dependency arrays or missing cleanup
• Key misuse: Index keys cause state to attach to wrong list items after reorder

React 19 improves these patterns: use() hook for consuming promises, Server Components for data fetching without extra requests, and useTransition() for non-blocking updates.

Workflow

1. Identify component responsibilities: Determine if component should be Server (data-heavy) or Client (interactive)
2. Define hooks at top level: All hooks must execute unconditionally before any returns
3. Use Suspense for async: Wrap use() hook calls with Suspense boundaries for loading fallback
4. Compose with keys: Use stable, unique keys for list items; use key prop to reset component state
5. Measure before optimizing: Profile with React DevTools before adding memoization
6. Clean up subscriptions: Always return cleanup function from effects that subscribe to systems
7. Keep components small: Extract Client Components for interactivity, let Server Components handle data

Rules

See rules index for detailed patterns.

Examples

Positive Trigger

User: "Refactor this React component to reduce re-renders and clarify hook usage."

Expected behavior: Use tech-react guidance, follow its workflow, and return actionable output.

Positive Trigger: Server Component Boundary

User: "I'm fetching data on the client with useEffect. How should I refactor this with Server Components?"

Expected behavior: Move data fetch to Server Component, pass promise to Client Component via use() hook, wrap with Suspense boundary.

Non-Trigger

User: "Write a Bash script to package release artifacts."

Expected behavior: Do not prioritize tech-react; choose a more relevant skill or proceed without it.

Troubleshooting

Hook Call Violations ("Rendered more hooks than during the previous render")

• Error: React throws "Rendered fewer/more hooks than during the previous render"
• Cause: Hooks called inside conditions, loops, or early returns
• Solution: Move hook calls before any conditional logic; use enabled option in data hooks to skip execution

Suspense Fallback Never Shows

• Error: Loading fallback doesn't appear when use() suspends
• Cause: Suspense boundary is not wrapping the component that calls use()
• Solution: Ensure <Suspense> is a parent of the component, not a sibling

Server Component Can't Use State/Events

• Error: useState, onClick handlers don't work in Server Components
• Cause: Server Components render once on server; they can't respond to client interaction
• Solution: Extract interactive parts to Client Components with "use client" directive; Server Component handles data fetching and passes to Client Component

useEffect Runs Twice or Creates Memory Leaks

• Error: Effect side effect runs multiple times; cleanup doesn't execute
• Cause: Missing dependency array or missing cleanup return function
• Solution: Include dependency array; return cleanup function for subscriptions/timers/listeners

List Items Lose Focus or Appear in Wrong Order

• Error: Input focus jumps between rows; items appear shuffled after sort
• Cause: Using array index as key instead of stable unique identifier
• Solution: Change key={index} to key={item.id} with unique property from data

Examples: Error Patterns

Error 1: Conditional Hook

Incorrect:

function UserProfile({ userId }: { userId: string | null }) {
  if (!userId) {
    return <div>Select a user</div>;
  }

  // Hook called conditionally - React can't track it!
  const [profile, setProfile] = useState(null);
  return <div>{profile?.name}</div>;
}

Correct:

function UserProfile({ userId }: { userId: string | null }) {
  const [profile, setProfile] = useState(null);

  // Early return AFTER hooks
  if (!userId) {
    return <div>Select a user</div>;
  }

  return <div>{profile?.name}</div>;
}

Error 2: Suspense Without use() Hook

Incorrect:

function Comments({ id }: { id: string }) {
  const [comments, setComments] = useState([]);

  useEffect(() => {
    fetchComments(id).then(setComments);
  }, [id]);

  return <ul>{comments.map(c => <li key={c.id}>{c.text}</li>)}</ul>;
}

Correct:

function Comments({ commentsPromise }: { commentsPromise: Promise<Comment[]> }) {
  const comments = use(commentsPromise);
  return <ul>{comments.map(c => <li key={c.id}>{c.text}</li>)}</ul>;
}

function CommentsSection({ id }: { id: string }) {
  return (
    <Suspense fallback={<div>Loading comments...</div>}>
      <Comments commentsPromise={fetchComments(id)} />
    </Suspense>
  );
}

Error 3: Server/Client Boundary Confusion

Incorrect:

// BAD: Server Component with useState
async function NotesPage() {
  const [selectedNote, setSelectedNote] = useState(null); // ERROR: Can't use state
  const notes = await db.notes.getAll();

  return (
    <div>
      {notes.map(note => (
        <button => setSelectedNote(note)}>
          {note.title}
        </button>
      ))}
    </div>
  );
}

Correct:

// Server Component - fetch data
async function NotesPage() {
  const notes = await db.notes.getAll();

  return (
    <div>
      <NotesList notes={notes} />
    </div>
  );
}

// Client Component - handle interaction
"use client";

function NotesList({ notes }: { notes: Note[] }) {
  const [selectedNote, setSelectedNote] = useState<Note | null>(null);

  return (
    <div>
      {notes.map(note => (
        <button
          key={note.id}
          => setSelectedNote(note)}
        >
          {note.title}
        </button>
      ))}
    </div>
  );
}