Asynchronous Programming

A Promise is like a receipt for something that will happen in the future. When you order online, you get a tracking number (promise) - the package will either arrive (fulfilled) or get lost (rejected).

Promise States:

1. Pending - Still waiting for result 2. Fulfilled - Success! Got the result 3. Rejected - Failed, something went wrong

Creating a Promise:

const myPromise = new Promise((resolve, reject) => {
  // Async operation here
  if (everythingWentWell) {
    resolve("Success!");  // Promise fulfilled
  } else {
    reject("Error!");     // Promise rejected
  }
});

Using Promises:

• •.then() - Runs if promise succeeds
• •.catch() - Runs if promise fails
• •.finally() - Always runs at the end

myPromise
  .then(result => {
    console.log("Success:", result);
  })
  .catch(error => {
    console.log("Failed:", error);
  })
  .finally(() => {
    console.log("All done!");
  });

Chaining Promises (solving callback hell):

fetchUser() .then(user => fetchPosts(user.id)) .then(posts => displayPosts(posts)) .catch(error => showError(error));

Why Promises are better than callbacks:

• •Cleaner, more readable code
• •Better error handling
• •Can chain operations easily
• •Avoid deeply nested callbacks