const controller = new AbortController();
const timeout = setTimeout(() => controller.abort(), 8000); // 8s timeout

try {
  const res = await fetch('/api/items', { signal: controller.signal });
  if (!res.ok) throw new Error(`HTTP ${res.status}`);
  const data = await res.json();
  console.log(data);
} catch (e) {
  console.error('Request failed:', e.name === 'AbortError' ? 'timeout' : e);
} finally {
  clearTimeout(timeout);
}

const res = await fetch('/api/logs/stream');
const reader = res.body.getReader();
const decoder = new TextDecoder();

while (true) {
  const { value, done } = await reader.read();
  if (done) break;
  console.log(decoder.decode(value, { stream: true }));
}

const ws = new WebSocket('wss://example.com/ws');
ws.addEventListener('message', e => {
  const msg = JSON.parse(e.data);
  console.log('event:', msg);
});
ws.addEventListener('close', () => console.log('socket closed'));

const ch = new BroadcastChannel('cart');
ch.onmessage = e => console.log('Cart updated in another tab:', e.data);
// где-то в другой вкладке:
new BroadcastChannel('cart').postMessage({ type: 'ADD', id: 123 });

Service Worker регистрация + Cache Storage
if ('serviceWorker' in navigator) {
  navigator.serviceWorker.register('/sw.js').catch(console.error);
}

// sw.js (минимум кэша статики)
self.addEventListener('install', e => {
  e.waitUntil(caches.open('app-v1').then(c =>
    c.addAll(['/','/styles.css','/app.js','/logo.png'])
  ));
});
self.addEventListener('fetch', e => {
  e.respondWith(
    caches.match(e.request).then(r => r || fetch(e.request))
  );
});

function openDB(name, version = 1) {
  return new Promise((resolve, reject) => {
    const req = indexedDB.open(name, version);
    req.onupgradeneeded = () => req.result.createObjectStore('items', { keyPath: 'id' });
    req.onsuccess = () => resolve(req.result);
    req.onerror = () => reject(req.error);
  });
}

const db = await openDB('appdb');
const tx = db.transaction('items', 'readwrite');
tx.objectStore('items').put({ id: 1, title: 'Foo' });
await tx.complete?.(); // в некоторых браузерах не нужно
db.close();

Web Workers (не блокируем UI)
// worker.js
self.onmessage = ({ data }) => {
  const result = heavyCompute(data);
  self.postMessage(result);
};

// main
const worker = new Worker('/worker.js', { type: 'module' });
worker.postMessage({ n: 5e6 });
worker.onmessage = ({ data }) => console.log('Result:', data);

requestIdleCallback / Scheduler API (мягкая загрузка)
const schedule = cb =>
  ('scheduler' in window && scheduler.postTask)
    ? scheduler.postTask(cb, { priority: 'background' })
    : (window.requestIdleCallback || setTimeout)(cb);

schedule(() => warmCachesOrPrecompute());