অ্যাসিঙ্ক্রোনাস প্রোগ্রামিং (Async Programming)

// setTimeout — the simplest async operation
console.log("1. Start");

setTimeout(() => {
  console.log("2. This runs after 1 second");
}, 1000);

console.log("3. End (runs BEFORE the timeout!)");

// Callback pattern — pass a function to run when done
function fetchUser(id, callback) {
  setTimeout(() => {
    const user = { id, name: "Laboni", level: 42 };
    callback(user);
  }, 500);
}

fetchUser(1, (user) => {
  console.log(`Got user: ${user.name}`);
});

// Callback hell — nested callbacks get ugly fast
// fetchUser(1, (user) => {
//   fetchPosts(user.id, (posts) => {
//     fetchComments(posts[0].id, (comments) => {
//       // 😱 This is called "callback hell" or "pyramid of doom"
//     });
//   });
// });

1. Start
3. End (runs BEFORE the timeout!)
Got user: Laboni
2. This runs after 1 second

// Creating a Promise
function delay(ms) {
  return new Promise((resolve) => {
    setTimeout(() => resolve(`Done after ${ms}ms`), ms);
  });
}

delay(1000).then(msg => console.log(msg));

// A Promise that can fail
function fetchScore(playerId) {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      if (playerId > 0) {
        resolve({ playerId, score: 9500 });
      } else {
        reject(new Error("Invalid player ID"));
      }
    }, 300);
  });
}

// .then() for success, .catch() for errors
fetchScore(1)
  .then(data => console.log(`Score: ${data.score}`))
  .catch(err => console.log(`Error: ${err.message}`));

fetchScore(-1)
  .then(data => console.log(`Score: ${data.score}`))
  .catch(err => console.log(`Error: ${err.message}`));

Done after 1000ms
Score: 9500
Error: Invalid player ID

// async/await — clean and readable
async function getPlayerInfo(id) {
  try {
    const response = await fetch(`https://api.example.com/players/${id}`);

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

    const player = await response.json();
    console.log(`Player: ${player.name}, Level: ${player.level}`);
    return player;
  } catch (error) {
    console.log(`Failed to fetch player: ${error.message}`);
    return null;
  }
}

// Call it
getPlayerInfo(42);

// Promise.all — run multiple async tasks in PARALLEL
async function loadDashboard() {
  try {
    const [users, posts, stats] = await Promise.all([
      fetch("/api/users").then(r => r.json()),
      fetch("/api/posts").then(r => r.json()),
      fetch("/api/stats").then(r => r.json())
    ]);

    console.log(`Loaded: ${users.length} users, ${posts.length} posts`);
    return { users, posts, stats };
  } catch (error) {
    console.log("One of the requests failed!");
  }
}

// Promise.allSettled — don't fail if ONE fails
async function loadData() {
  const results = await Promise.allSettled([
    fetch("/api/critical").then(r => r.json()),
    fetch("/api/optional").then(r => r.json())
  ]);

  results.forEach((result, i) => {
    if (result.status === "fulfilled") {
      console.log(`Request ${i}: Success`);
    } else {
      console.log(`Request ${i}: Failed — ${result.reason}`);
    }
  });
}

Player: Laboni, Level: 42
Loaded: 50 users, 120 posts
Request 0: Success
Request 1: Failed — TypeError: Failed to fetch