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name: using-the-use-hook description: React 19's use() API for reading Promises/Context conditionally. For async data fetching, Suspense, conditional context access, useContext migration. allowed-tools: Read, Write, Edit, Glob, Grep version: 1.0.0

Using the use() Hook in React 19

Teaches when and how to use React 19's new use() API: reads Promises (suspending component) and Context (conditionally—inside if statements, loops, after early returns).

Activates when: User mentions use(), use hook, async patterns, Suspense + data fetching, conditional context, or useContext migration.

Key Capabilities vs. Traditional Hooks

Usage Patterns

Reading Promises:

Copyimport { use, Suspense } from 'react';

async function fetchUser(id) {
  const res = await fetch(`/api/users/${id}`);
  return res.json();
}

function UserProfile({ userId }) {
  return (
    <Suspense fallback={<div>Loading...</div>}>
      <UserData promise={fetchUser(userId)} />
    </Suspense>
  );
}

function UserData({ promise }) {
  const user = use(promise);
  return <div>{user.name}</div>;
}

Conditional Context Access:

Copyimport { use, createContext } from 'react';

const ThemeContext = createContext('light');

function Button({ isPrimary, children }) {
  let className = 'button';
  if (isPrimary) {
    const theme = use(ThemeContext);
    className += ` ${theme}`;
  }
  return <button className={className}>{children}</button>;
}

Reusable Custom Hook:

Copyimport { use, cache } from 'react';

const getUser = cache(async (id) => {
  const res = await fetch(`/api/users/${id}`);
  return res.json();
});

function useUser(userId) {
  if (!userId) return null;
  return use(getUser(userId));
}

Implementation Workflow

Async data: Pass Promise as prop from parent → wrap component in Suspense boundary → use use(promise) in child → catch errors with Error Boundary.

Conditional context: Replace useContext(Context) with use(Context) for flexibility across conditional branches, loops, post-early-return.

Migration from useContext: Find useContext(Context) calls → replace with use(Context) → benefits: now conditional if needed.

Requirements & Validation

MUST:

• Wrap use(promise) in Suspense boundary with fallback
• Use Error Boundary (not try-catch) for error handling
• Create Promises in parents/Server Components, pass as props
• Use cache() wrapper to prevent duplicate requests

NEVER:

• Call use() inside try-catch blocks
• Create new Promises inside component (causes infinite loops)
• Use use() outside Component/Hook functions
• Omit Suspense boundary for Promises

Verify:

1. Suspense boundary wraps every use(promise) with fallback
2. Error Boundary catches Promise errors
3. Promises created externally or with cache()—stable across renders
4. All conditional logic branches tested

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For comprehensive React 19 use() documentation:

research/react-19-comprehensive.md lines 241–311 (use API, Promise Patterns, useContext sections).