जावास्क्रिप्ट एसिंक/प्रतीक्षा: संपूर्ण 2026 गाइड

// Promise chain — works, but nests deeply
fetch('https://api.example.com/users/1')
  .then(response => response.json())
  .then(user => fetch('https://api.example.com/posts?userId=' + user.id))
  .then(response => response.json())
  .then(posts => console.log(posts))
  .catch(error => console.error('Failed:', error));

// Same logic with async/await — flat and readable
async function getUserPosts(userId) {
  const userRes = await fetch('https://api.example.com/users/' + userId);
  const user = await userRes.json();
  const postsRes = await fetch('https://api.example.com/posts?userId=' + user.id);
  const posts = await postsRes.json();
  return posts;
}

async function getUser(id) {
  const response = await fetch('https://jsonplaceholder.typicode.com/users/' + id);
  if (!response.ok) throw new Error('HTTP error: ' + response.status);
  return response.json();
}

const user = await getUser(1);
console.log(user.name); // "Leanne Graham"

async function fetchUserData(userId) {
  try {
    const response = await fetch('/api/users/' + userId);
    if (!response.ok) throw new Error('Server error: ' + response.status);
    return await response.json();
  } catch (error) {
    if (error.name === 'TypeError') {
      console.error('Network error:', error.message);
    } else {
      console.error('API error:', error.message);
    }
    return null;
  }
}

// Per-await error handling
async function loadDashboard() {
  const user   = await fetchUserData(1).catch(() => null);
  const stats  = await fetchStats().catch(() => ({ views: 0 }));
  const alerts = await fetchAlerts().catch(() => []);
  return { user, stats, alerts };
}

// ❌ Sequential — 3 seconds total (1s + 1s + 1s)
async function loadSequential() {
  const users  = await fetchUsers();   // waits 1s
  const posts  = await fetchPosts();   // waits another 1s
  const photos = await fetchPhotos();  // waits another 1s
  return { users, posts, photos };
}

// ✅ Parallel — ~1 second total (all fire at once)
async function loadParallel() {
  const [users, posts, photos] = await Promise.all([
    fetchUsers(),
    fetchPosts(),
    fetchPhotos()
  ]);
  return { users, posts, photos };
}

// Promise.allSettled — get results even if some fail
async function loadWithFallbacks() {
  const results = await Promise.allSettled([fetchUsers(), fetchPosts(), fetchPhotos()]);
  return results.map(r => r.status === 'fulfilled' ? r.value : null);
}

// Promise.race — first wins (timeout pattern)
async function fetchWithTimeout(url, ms = 5000) {
  const timeout = new Promise((_, reject) =>
    setTimeout(() => reject(new Error('Request timed out')), ms)
  );
  return Promise.race([fetch(url), timeout]);
}

const userIds = [1, 2, 3, 4, 5];

// ❌ forEach DOES NOT await — requests fire but nothing waits
userIds.forEach(async (id) => {
  const user = await fetchUser(id); // fires immediately without waiting
  console.log(user.name);
});

// ✅ for...of — sequential (each awaits before next starts)
for (const id of userIds) {
  const user = await fetchUser(id);
  console.log(user.name);
}

// ✅ Promise.all + map — parallel (all fire simultaneously)
const users = await Promise.all(userIds.map(id => fetchUser(id)));
users.forEach(u => console.log(u.name));

// ✅ Batched parallel (avoid overwhelming the server)
async function batchProcess(ids, batchSize = 5) {
  const results = [];
  for (let i = 0; i < ids.length; i += batchSize) {
    const batch = ids.slice(i, i + batchSize);
    const batchResults = await Promise.all(batch.map(id => fetchUser(id)));
    results.push(...batchResults);
  }
  return results;
}

// Async IIFE — top-level await not available? Use this
(async () => {
  const config = await loadConfig();
  await startServer(config);
  console.log('Server running on port', config.port);
})();

async function* streamLargeDataset(url) {
  let page = 1;
  while (true) {
    const response = await fetch(url + '?page=' + page);
    const data = await response.json();
    if (!data.items.length) break;
    yield data.items;
    page++;
  }
}

// Consume with for-await-of
for await (const items of streamLargeDataset('/api/records')) {
  processItems(items);
}

async function fetchWithRetry(url, options = {}, retries = 3, backoff = 1000) {
  try {
    const res = await fetch(url, options);
    if (!res.ok && res.status >= 500 && retries > 0) {
      await new Promise(r => setTimeout(r, backoff));
      return fetchWithRetry(url, options, retries - 1, backoff * 2);
    }
    return res;
  } catch (err) {
    if (retries > 0) {
      await new Promise(r => setTimeout(r, backoff));
      return fetchWithRetry(url, options, retries - 1, backoff * 2);
    }
    throw err;
  }
}

// ❌ Missing await — user is a Promise, not an object
async function broken() {
  const user = fetchUser(1);   // missing await!
  console.log(user.name);      // undefined — user is a Promise
}

// ✅ Fixed
async function fixed() {
  const user = await fetchUser(1);
  console.log(user.name);      // "Leanne Graham"
}

class ApiClient {
  constructor(baseUrl, options = {}) {
    this.baseUrl = baseUrl;
    this.timeout = options.timeout || 10000;
  }

  async request(path, options = {}) {
    const controller = new AbortController();
    const timeoutId = setTimeout(() => controller.abort(), this.timeout);
    try {
      const response = await fetch(this.baseUrl + path, {
        ...options,
        signal: controller.signal,
        headers: { 'Content-Type': 'application/json', ...options.headers }
      });
      clearTimeout(timeoutId);
      if (!response.ok) {
        const err = await response.json().catch(() => ({}));
        throw Object.assign(new Error(err.message || 'HTTP ' + response.status), {
          status: response.status, body: err
        });
      }
      return response.json();
    } catch (error) {
      clearTimeout(timeoutId);
      if (error.name === 'AbortError') throw new Error('Request timed out');
      throw error;
    }
  }

  async get(path)        { return this.request(path); }
  async post(path, body) { return this.request(path, { method: 'POST', body: JSON.stringify(body) }); }
}

const api = new ApiClient('https://api.example.com', { timeout: 8000 });
const user = await api.get('/users/1');