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      • Promise.allSettled S4

      Proposal details
      Proposal overview

      This proposal introduces Promise.allSettled, a combinator that waits for all input promises to settle (fulfill or reject) and returns an array of their state snapshots, without short-circuiting. It addresses the need to handle multiple async operations where complete results are required regardless of individual failures.

      Note

      The README below comes from the upstream repository and may contain outdated stage or status metadata. Use the proposal details above as the current source of truth.

      Promise.allSettled

      ECMAScript proposal and reference implementation for Promise.allSettled.

      Author: Jason Williams (BBC), Robert Pamely (Bloomberg), Mathias Bynens (Google)

      Champion: Mathias Bynens (Google)

      Stage: 4

      Overview and motivation

      There are four main combinators in the Promise landscape.

      namedescription
      Promise.allSettleddoes not short-circuitthis proposal 🆕
      Promise.allshort-circuits when an input value is rejectedadded in ES2015 ✅
      Promise.raceshort-circuits when an input value is settledadded in ES2015 ✅
      Promise.anyshort-circuits when an input value is fulfilledseparate proposal 🔜

      These are all commonly available in userland promise libraries, and they’re all independently useful, each one serving different use cases.

      A common use case for this combinator is wanting to take an action after multiple requests have completed, regardless of their success or failure. Other Promise combinators can short-circuit, discarding the results of input values that lose the race to reach a certain state. Promise.allSettled is unique in always waiting for all of its input values.

      Promise.allSettled returns a promise that is fulfilled with an array of promise state snapshots, but only after all the original promises have settled, i.e. become either fulfilled or rejected.

      Why allSettled?

      We say that a promise is settled if it is not pending, i.e. if it is either fulfilled or rejected. See promise states and fates for more background on the relevant terminology.

      Furthermore, the name allSettled is commonly used in userland libraries implementing this functionality. See below.

      Examples

      Currently you would need to iterate through the array of promises and return a new value with the status known (either through the resolved branch or the rejected branch.

      function reflect(promise) {
  return promise.then(
    (v) => {
      return { status: 'fulfilled', value: v };
    },
    (error) => {
      return { status: 'rejected', reason: error };
    }
  );
}

const promises = [ fetch('index.html'), fetch('https://does-not-exist/') ];
const results = await Promise.all(promises.map(reflect));
const successfulPromises = results.filter(p => p.status === 'fulfilled');

      The proposed API allows a developer to handle these cases without creating a reflect function and/or assigning intermediate results in temporary objects to map through:

      const promises = [ fetch('index.html'), fetch('https://does-not-exist/') ];
const results = await Promise.allSettled(promises);
const successfulPromises = results.filter(p => p.status === 'fulfilled');

      Collecting errors example:

      Here we are only interested in the promises which failed, and thus collect the reasons. allSettled allows us to do this quite easily.

      const promises = [ fetch('index.html'), fetch('https://does-not-exist/') ];

const results = await Promise.allSettled(promises);
const errors = results
  .filter(p => p.status === 'rejected')
  .map(p => p.reason);

      Real-world scenarios

      A common operation is knowing when all requests have completed regardless of the state of each request. This allows developers to build with progressive enhancement in mind. Not all API responses will be mandatory.

      Without Promise.allSettled this is tricker than it could be:

      const urls = [ /* ... */ ];
const requests = urls.map(x => fetch(x)); // Imagine some of these will fail, and some will succeed.

// Short-circuits on first rejection, all other responses are lost
try {
  await Promise.all(requests);
  console.log('All requests have completed; now I can remove the loading indicator.');
} catch {
  console.log('At least one request has failed, but some of the requests still might not be finished! Oops.');
}

      Using Promise.allSettled would be more suitable for the operation we wish to perform:

      // We know all API calls have finished. We use finally but allSettled will never reject.
Promise.allSettled(requests).finally(() => {
  console.log('All requests are completed: either failed or succeeded, I don’t care');
  removeLoadingIndicator();
});

      Userland implementations

      Naming in other languages

      Similar functionality exists in other languages with different names. Since there is no uniform naming mechanism across languages, this proposal follows the naming precedent of userland JavaScript libraries shown above. The following examples were contributed by jasonwilliams and benjamingr.

      Rust

      futures::join (similar to Promise.allSettled). "Polls multiple futures simultaneously, returning a tuple of all results once complete."

      futures::try_join (similar to Promise.all)

      C#

      Task.WhenAll (similar to ECMAScript Promise.all). You can use either try/catch or TaskContinuationOptions.OnlyOnFaulted to achieve the same behavior as allSettled.

      Task.WhenAny (similar to ECMAScript Promise.race)

      Python

      asyncio.wait using the ALL_COMPLETED option (similar to Promise.allSettled). Returns task objects which are akin to the allSettled inspection results.

      Java

      allOf (similar to Promise.all)

      Dart

      Future.wait (similar to ECMAScript Promise.all)

      Further reading

      TC39 meeting notes

      Specification

      Implementations