Explain Promises — states, chaining, and error handling.

A Promise is a value that may be available now or later. States: pending → fulfilled / rejected. Once settled, immutable.

fetch('/api/data')
  .then(res => res.json())
  .then(data => console.log(data))
  .catch(err => console.error(err))
  .finally(() => setLoading(false));

Combinators: Promise.all (all must resolve), Promise.allSettled (wait all), Promise.race (first settles), Promise.any (first resolves).