What is a Promise in JavaScript? What is it used for?

What is a Promise?

A Promise is an object representing an asynchronous operation that is not yet complete, but will eventually succeed or fail. It solves the "Callback Hell" problem.

Three States

State	Description
Pending	Initial state, operation not yet complete
Fulfilled	Operation succeeded, has a return value
Rejected	Operation failed, has an error reason

Once a state changes, it is irreversible.

Basic Usage

// Creating a Promise
const promise = new Promise((resolve, reject) => {
  setTimeout(() => {
    const success = true;
    if (success) {
      resolve('Operation succeeded!');
    } else {
      reject(new Error('Operation failed!'));
    }
  }, 1000);
});

// Consuming a Promise
promise
  .then(result => console.log(result))   // 'Operation succeeded!'
  .catch(error => console.error(error))
  .finally(() => console.log('Done'));

Promise Chaining

fetch('/api/user')
  .then(res => res.json())
  .then(user => fetch('/api/posts?userId=' + user.id))
  .then(res => res.json())
  .then(posts => console.log(posts))
  .catch(err => console.error(err));

async/await (Syntactic Sugar)

async function loadData() {
  try {
    const res = await fetch('/api/user');
    const user = await res.json();
    console.log(user);
  } catch (err) {
    console.error(err);
  }
}

Common Static Methods

Method	Description
Promise.all()	Resolves when ALL fulfill; rejects if any rejects
Promise.race()	Settles with the first to complete (success or failure)
Promise.allSettled()	Waits for ALL to settle, regardless of outcome
Promise.any()	Resolves when ANY fulfills