ECMAScript proposal: Promise.any + AggregateError

Author: Mathias Bynens, Kevin Gibbons, Sergey Rubanov

Champion: Mathias Bynens

Stage: Stage 4 of the TC39 process.

Motivation

There are four main combinators in the Promise landscape.

name	description
Promise.allSettled	does not short-circuit	added in ES2020 ✅
Promise.all	short-circuits when an input value is rejected	added in ES2015 ✅
Promise.race	short-circuits when an input value is settled	added in ES2015 ✅
Promise.any	short-circuits when an input value is fulfilled	this proposal 🆕 scheduled for ES2021

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

Proposed solution

Promise.any accepts an iterable of promises and returns a promise that is fulfilled by the first given promise to be fulfilled, or rejected with an AggregateError holding the rejection reasons if all of the given promises are rejected. (If something more fundamental goes wrong, e.g. iterating over the iterable results in an exception, Promise.any returns a rejected promise with that exception.)

High-level API

try {
  const first = await Promise.any(promises);
  // Any of the promises was fulfilled.
} catch (error) {
  // All of the promises were rejected.
}

Or, without async/await:

Promise.any(promises).then(
  (first) => {
    // Any of the promises was fulfilled.
  },
  (error) => {
    // All of the promises were rejected.
  }
);

In the above examples, error is an AggregateError, a new Error subclass that groups together individual errors. Every AggregateError instance contains a pointer to an array of exceptions.

FAQ

Why choose the name any?

It clearly describes what it does, and there’s precedent for the name any in userland libraries offering this functionality:

• https://github.com/kriskowal/q#combination
• http://bluebirdjs.com/docs/api/promise.any.html
• https://github.com/m0ppers/promise-any
• https://github.com/cujojs/when/blob/master/docs/api.md#whenany
• https://github.com/sindresorhus/p-any

Why throw an AggregateError instead of an array?

The prevailing practice within the ECMAScript language is to only throw exception types. Existing code in the ecosystem likely relies on the fact that currently, all exceptions thrown by built-in methods and syntax are instanceof Error. Adding a new language feature that can throw a plain array would break that invariant, and could be a web compatibility issue. Additionally, by using an Error instance (or a subclass), a stack trace can be provided — something that’s easy to discard if not needed, but impossible to obtain later if it is needed.

Illustrative examples

This snippet checks which endpoint responds the fastest, and then logs it.

Promise.any([
  fetch('https://v8.dev/').then(() => 'home'),
  fetch('https://v8.dev/blog').then(() => 'blog'),
  fetch('https://v8.dev/docs').then(() => 'docs')
]).then((first) => {
  // Any of the promises was fulfilled.
  console.log(first);
  // → 'home'
}).catch((error) => {
  // All of the promises were rejected.
  console.log(error);
});

TC39 meeting notes

• March 2019
• June 2019
• July 2019
• October 2019 part one and part two
• June 2020 part one and part two
• July 2020

Specification

• Ecmarkup source
• HTML version

Implementations

• JavaScript engines:

  • JavaScriptCore, shipping in Safari 14
  • SpiderMonkey, shipping in Firefox 79
  • V8, shipping in Chrome 85
  • XS
  • engine262

• Polyfills:

  • core-js
  • es-shims

• TypeScript