Explain the concept of promises in JavaScript.

A Promise in JavaScript is an object that represents the eventual completion or failure of an asynchronous operation. It can be in one of three states: pending, fulfilled, or rejected. When a promise is fulfilled, it resolves with a value; when it is rejected, it resolves with a reason (an error). Promises allow you to attach callbacks using .then() for success and .catch() for failure, enabling a cleaner, more readable flow than nested callbacks.

Example:

const fetchData = () => new Promise((resolve, reject) => {
  setTimeout(() => {
    const data = { id: 1, name: 'Alice' };
    resolve(data);
  }, 1000);
});

fetchData()
  .then(result => console.log(result))
  .catch(err => console.error(err));

Promises are especially useful for chaining asynchronous operations, handling errors in a single place, and integrating with async/await syntax, which internally uses promises to pause execution until the promise resolves.