A Promise is an object that represents the eventual completion (or failure) of an asynchronous operation and its resulting value.

Think of it like a "promise" someone makes:

“I promise I’ll give you the data... just wait a moment!”

🔁 Why Promises?

JavaScript is asynchronous — it doesn’t wait for things like API calls or file reads. Before Promises, we used callbacks, which could lead to callback hell 😵

Promises make async code cleaner, more readable, and easier to manage.

🔧 Syntax 

const promise = new Promise((resolve, reject) => {
  // async task
  if (/* everything is good */) {
    resolve('Success!');
  } else {
    reject('Error!');
  }
});
  • resolve(value) → when async task is successful
  • reject(error) → when async task fails

✅ Consuming a Promise 

promise
  .then((result) => {
    console.log('Resolved:', result);
  })
  .catch((error) => {
    console.log('Rejected:', error);
  })
  .finally(() => {
    console.log('Always runs');
  });

🕰 Real Example (setTimeout) 

const wait = new Promise((resolve) => {
  setTimeout(() => {
    resolve('Done waiting!');
  }, 2000);
});

wait.then(console.log); // After 2 sec → "Done waiting!"

📦 Built-in Promises Example (Fetch) 

fetch('https://jsonplaceholder.typicode.com/posts/1')
  .then(response => response.json())
  .then(data => console.log(data))
  .catch(err => console.error(err));

🧠 Promise States

  1. Pending – Initial state
  2. Fulfilled – Resolved with a value
  3. Rejected – Failed with an error

⛓ Chaining Promises 

doSomething()
  .then(result => doSomethingElse(result))
  .then(newResult => doThirdThing(newResult))
  .catch(error => console.error(error));

✨ Bonus: Async/Await (Built on Promises) 

async function fetchData() {
  try {
    const res = await fetch('https://api.example.com/data');
    const data = await res.json();
    console.log(data);
  } catch (err) {
    console.error(err);
  }
}