Asynchronous JavaScript

Node.js Promises

A promise represents a value that is not available yet. It is pending, then either fulfilled with a value or rejected with an error, and .then/.catch/.finally attach handlers to those outcomes.

What is Promises in Node.js?

A promise represents a value that is not available yet. It is pending, then either fulfilled with a value or rejected with an error, and .then/.catch/.finally attach handlers to those outcomes.

Promises example

Creating and consuming a promise

JavaScript

function delay(ms) {
  return new Promise(resolve => setTimeout(resolve, ms));
}

fetchJob(id)
  .then(job => enrich(job))     // return a promise to chain
  .then(job => save(job))
  .catch(err => console.error(err))
  .finally(() => console.log('finished'));

Key points to remember

Returning a promise inside .then chains it; forgetting to return breaks the chain.
A rejection with no .catch becomes an unhandled rejection and terminates the process.
Promise.allSettled is right when partial failure is acceptable.

Promise combinators

Method	Resolves when	Rejects when
Promise.all	every promise fulfils	any one rejects (fails fast)
Promise.allSettled	all settle, whatever the outcome	never
Promise.race	the first promise settles	if the first to settle rejects
Promise.any	the first promise fulfils	only if all reject

Common mistakes with Promises

Creating a promise around code that already returns one — the "promise constructor anti-pattern".
Using Promise.all when one failure should not discard every other result.

Node.js Promises— Interview Questions & FAQs

What is the difference between Promise.all and Promise.allSettled?+

Promise.all rejects as soon as any promise rejects, discarding the other results. allSettled waits for every promise and returns an array describing each outcome, which is what you want when partial success is useful.