Async JavaScript

JavaScript is single-threaded but handles async operations (network requests, timers) without blocking the UI using the event loop. The modern way to write async code is with async/await built on Promises.

// ── async/await with fetch() ────────────────────────────────────────
async function getUser(userId) {
  try {
    const response = await fetch(`https://jsonplaceholder.typicode.com/users/${userId}`);

    if (!response.ok) {
      throw new Error(`HTTP ${response.status}: ${response.statusText}`);
    }

    const user = await response.json(); // parse JSON body
    console.log(user.name, user.email);
    return user;

  } catch (error) {
    console.error("Failed:", error.message);
    return null;
  }
}

getUser(1); // call the async function (returns a Promise)

// ── Promise methods ─────────────────────────────────────────────────
// Promise.all — run in PARALLEL, wait for ALL to finish
const [posts, users] = await Promise.all([
  fetch("/api/posts").then(r => r.json()),
  fetch("/api/users").then(r => r.json()),
]);

// Promise.race — resolve/reject with FIRST settled
// Promise.allSettled — wait for ALL regardless of outcome
// Promise.any — resolve with FIRST fulfilled

// ── Timers ──────────────────────────────────────────────────────────
// Run once after a delay
const timerId = setTimeout(() => {
  console.log("Runs after 2 seconds");
}, 2000);
clearTimeout(timerId); // cancel before it fires

// Run repeatedly
const intervalId = setInterval(() => {
  console.log("Runs every 1 second");
}, 1000);
clearInterval(intervalId); // stop it

// ── Promise chaining (older style, still valid) ─────────────────────
fetch("https://api.github.com/users/torvalds")
  .then(response => response.json())
  .then(user => console.log(user.name))
  .catch(error => console.error(error));