Async JavaScript

// Create a promise
const promise = new Promise((resolve, reject) => {
  const success = true;
  if (success) resolve("Done!");
  else reject(new Error("Failed"));
});

// Chain promises
promise
  .then(result => console.log(result))
  .catch(error => console.error(error))
  .finally(() => console.log("Cleanup"));

// Async function
async function fetchUser(id) {
  try {
    const response = await fetch(`/api/users/${id}`);
    if (!response.ok) throw new Error("Not found");
    return await response.json();
  } catch (error) {
    console.error("Fetch failed:", error);
    throw error;
  }
}

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

// Promise.all - wait for ALL (fails fast)
const [users, posts] = await Promise.all([
  fetchUsers(),
  fetchPosts()
]);

// Promise.allSettled - wait for ALL regardless
const results = await Promise.allSettled([
  fetchA(), fetchB(), fetchC()
]);

// Promise.race - first to settle
const fastest = await Promise.race([
  fetchFromCDN1(),
  fetchFromCDN2()
]);

// Promise.any - first to succeed
const first = await Promise.any([
  tryServer1(),
  tryServer2()
]);

// Retry with delay
async function retry(fn, retries = 3, delay = 1000) {
  for (let i = 0; i < retries; i++) {
    try {
      return await fn();
    } catch (err) {
      if (i === retries - 1) throw err;
      await new Promise(r => setTimeout(r, delay));
    }
  }
}

// Timeout wrapper
function withTimeout(promise, ms) {
  const timeout = new Promise((_, reject) =>
    setTimeout(() => reject(new Error("Timeout")), ms)
  );
  return Promise.race([promise, timeout]);
}

// Usage
const data = await retry(() => withTimeout(fetchData(), 5000));