MENU


Promise.withResolvers()

Promise.withResolvers(), added in ECMAScript 2024, is a static convenience method that returns a plain object {promise, resolve, reject}: a new Promise together with its resolve and reject functions extracted right alongside it, instead of the executor-callback dance required by 'new Promise((resolve,reject)=>{...})' (5.1).

This is handy whenever the code that resolves/rejects a promise lives outside the scope where the promise itself is created, eg. storing pending requests in a Map keyed by request id.
const pending = new Map();

function request(id) {
  const {promise, resolve, reject} = Promise.withResolvers();
  pending.set(id, {resolve, reject});
  return promise;
}

function onServerMessage(id, err, data) {   // called later, from a totally different scope
  const {resolve, reject} = pending.get(id);
  pending.delete(id);
  err ? reject(err) : resolve(data);
}

request(1).then(data => console.log('got:', data));
onServerMessage(1, null, 'hello');

got: hello

Before this method existed, the same {promise, resolve, reject} shape had to be built manually:


let resolve, reject;
const promise = new Promise((res, rej) => {
  resolve = res;
  reject = rej;
});