Explain Promises in Node — states, chaining, and the combinators (all/race/allSettled/any). | Node.js Interview Question

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Answer:

States: a Promise is pending, then settles to fulfilled (with a value) or rejected (with a reason). Once settled it never changes.

Chaining: each .then returns a new promise, so you can transform values and funnel all errors to a single .catch:

fetchUser(id)
  .then(user => fetchPosts(user.id))
  .then(posts => render(posts))
  .catch(err => handle(err))   // catches any rejection above
  .finally(() => stopSpinner());

Combinators:

Method	Settles when	Result	Use case
`Promise.all`	all fulfilled, or first rejection	array of values / first error	parallel work, all required
`Promise.allSettled`	all settled	array of `{status, value/reason}`	independent tasks, want every outcome
`Promise.race`	first to settle (either way)	that value/reason	timeouts, fastest wins
`Promise.any`	first fulfillment, or all rejected	first value / `AggregateError`	fallbacks/redundancy

// Fetch three resources in parallel; fail if any fails
const [a, b, c] = await Promise.all([getA(), getB(), getC()]);

// Add a timeout to any promise
const result = await Promise.race([
  doWork(),
  new Promise((_, rej) => setTimeout(() => rej(new Error('timeout')), 5000)),
]);

Gotchas:

Promise.all is fail-fast: one rejection rejects the whole thing (the others still run but their results are discarded). Use allSettled when you need every outcome.
Always attach a .catch (or wrap await in try/catch) — an unhandled rejection can crash the process.

Explain Promises in Node — states, chaining, and the combinators (all/race/allSettled/any).

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