Promises

Creating & consuming

resolve fulfills the promise, reject fails it; .then() and .catch() react to each outcome.

const promise = new Promise((resolve, reject) => {
  setTimeout(() => resolve("Done!"), 1000);
});
promise.then((result) => console.log(result))
       .catch((err) => console.log(err));

Three states

A Promise is pending, then settles as fulfilled or rejected — once settled, it never changes state again.

Chaining

Each .then() returns a new promise, so async steps can be chained in a flat, readable sequence.

fetchUser()
  .then((user) => fetchPosts(user.id))
  .then((posts) => console.log(posts))
  .catch((err) => console.error(err));

Quick check: What does .catch() handle in a Promise chain?

• Only network errors
• Any rejection from earlier in the chain
• Nothing — it's just for logging