What are Promises in JavaScript? | JavaScript Interview Questions

What You'll Learn

What Promises are
Promise states
How to use Promises

What is a Promise?

A Promise is an object representing the eventual completion or failure of an asynchronous operation.

Promise States

State	Description
Pending	Initial state, not fulfilled or rejected
Fulfilled	Operation completed successfully
Rejected	Operation failed

Creating a Promise

code.jsJavaScript

const promise = new Promise((resolve, reject) => {
  const success = true;

  if (success) {
    resolve('Data fetched!');
  } else {
    reject('Error occurred');
  }
});

promise
  .then(data => console.log(data))
  .catch(error => console.error(error))
  .finally(() => console.log('Done'));

Promise Methods

code.jsJavaScript

// Wait for all promises
Promise.all([p1, p2, p3])
  .then(results => console.log(results));

// First to settle
Promise.race([p1, p2, p3])
  .then(result => console.log(result));

// All settled (success or failure)
Promise.allSettled([p1, p2, p3])
  .then(results => console.log(results));