What are Promises and what are their three states?

A Promise is an object representing the eventual result of an asynchronous operation. It provides a structured alternative to callback pyramids.

Three states:

Once settled (fulfilled or rejected), a Promise cannot change state.

const p = fetch('/api/user')
  .then(res => res.json())
  .then(user => console.log(user.name))
  .catch(err => console.error(err))
  .finally(() => console.log('done'))

Key rules (A+ / Promises spec)

• .then / .catch / .finally return new Promises, enabling chaining.
• Handlers run as microtasks, after current sync code.
• Unhandled rejections can be silently lost — always attach .catch or use try/catch with await.

Common static methods: Promise.all, Promise.race, Promise.allSettled, Promise.any.