How do you implement a “raceToSuccess” helper, given a list of promises?

Question

I\'m puzzled by something in the ES6 Promise API. I can see a clear use case for submitting multiple async jobs concurrently, and \"resolving\" on the first success. This wo

Answer 1

I expanded the @loganfsmyth approach with timeouts and I wrote a small function that:

• runs all the promises,
• wait for the promises to succeed for no more than a specified amount of time (options.timeOutMs),
• return the first that succeed.

In the following snippet, you can test it:

const firstThatCompleteSuccessfullyES6 = (options) => {

    // return the first promise that resolve
    const oneSuccess = (promises) => Promise.all(promises.map(p => {
                    // If a request fails, count that as a resolution so it will keep
                    // waiting for other possible successes. If a request succeeds,
                    // treat it as a rejection so Promise.all immediately bails out.
                    return p.then(
                        (val) => { return Promise.reject(val); },
                        (err) => { return Promise.resolve(err); }
                    );
            })
            ).then(
                // If '.all' resolved, we've just got an array of errors.
                (errors) => { return Promise.reject(errors); },

                // If '.all' rejected, we've got the result we wanted.
                (val) => { return Promise.resolve(val); }
            );
    

    // return the promise or reect it if timeout occur first
    const timeoutPromise = (ms, promise) => new Promise(function(resolve, reject) {
            setTimeout(() => reject(new Error('timeout')), ms);
            promise.then(resolve, reject);
        });
    

    if (options.subsystems.length < 1) {
        return Promise.reject('Parameters error, no subSystems specified');
    }

    const timedOutSubsystems = options.subsystems.map(function(subsystem){
        return timeoutPromise(options.timeOutMs, subsystem(options));
    });

    const startDate = Date.now();

    return oneSuccess(
        timedOutSubsystems
    )
    .then((result) => {
        const elapsedTime = Math.abs((startDate - Date.now()) / 1000);
        console.log('firstThatCompleteSuccessfully() done, after s: ' + elapsedTime + ': '+ result);
        return result;
    })
    .catch((error) => {
        const elapsedTime = Math.abs((startDate - Date.now()) / 1000);
        console.error('firstThatCompleteSuccessfully() error/nodata: ' + error);
    });

}



// example of use with two promises (subsystem1 & subsystem2) that resolves after a fixed amount of time

const subsystem1 = (options) => new Promise(function(resolve, reject) {
        setTimeout(function(){
            console.log('subsystem1 finished');
            resolve('subsystem 1 OK');
        }, 1000);
    });



const subsystem2 = (options) => new Promise(function(resolve, reject) {
        setTimeout(function(){
            console.log('subsystem2 finished');
            resolve('subsystem 2 OK');
        }, 2000);
    });


firstThatCompleteSuccessfullyES6({
    subsystems: [subsystem1, subsystem2],
    timeOutMs: 2000
})
.then((result) => console.log("Finished: "+result));

Answer 2

This is a classic example where inverting your logic makes it much clearer. Your "race" in this case is that you want your rejection behavior to in fact be success behavior.

function oneSuccess(promises){
  return Promise.all(promises.map(p => {
    // If a request fails, count that as a resolution so it will keep
    // waiting for other possible successes. If a request succeeds,
    // treat it as a rejection so Promise.all immediately bails out.
    return p.then(
      val => Promise.reject(val),
      err => Promise.resolve(err)
    );
  })).then(
    // If '.all' resolved, we've just got an array of errors.
    errors => Promise.reject(errors),
    // If '.all' rejected, we've got the result we wanted.
    val => Promise.resolve(val)
  );
}

Answer 3

Old topic but here's my entry; it's essentially @loganfsmyth's solution, but with a few more checks to conform to conventions established by Promise.all():

• Empty array as input returns (synchronously) an already resolved promise
• Non-promise entries in the array results in the 1st such entry to be used as the resolved value

Promise.any = a => {
  return !a.length ?
    Promise.resolve() :
    Promise.all(a.map(
      e => (typeof e.then !== 'function') ?
        Promise.reject(e) :
        e.then(
          result => Promise.reject(result),
          failure => Promise.resolve(failure)
        )
    )).then(
      allRejected => Promise.reject(allRejected),
      firstResolved => Promise.resolve(firstResolved)
    );
};

// Testing...

function delayed(timeout, result, rejected) {
  return new Promise((resolve, reject) => {
    setTimeout(
      () => rejected ? reject(result) : resolve(result),
      timeout);
  });
}

Promise.any([
  delayed(800, 'a'),
  delayed(500, 'b'),
  delayed(250, 'c', true)
]).then(e => {
  console.log('First resolved (expecting b):', e);
});

Promise.any([
  delayed(800, 'a', true),
  delayed(500, 'b', true),
  delayed(250, 'c', true)
]).then(null, e => {
  console.log('All rejected (expecting array of failures):', e);
});

Promise.any([
  delayed(800, 'a'),
  delayed(500, 'b'),
  delayed(250, 'c', true),
  'd',
  'e'
]).then(e => {
  console.log('First non-promise (expecting d):', e);
});

// Because this is the only case to resolve synchronously,
// its output should appear before the others
Promise.any([]).then(e => {
  console.log('Empty input (expecting undefined):', e);
});

Answer 4

Is there something in the API that permits a "raceToSuccess" kind of behavior

Soon, there almost certainly will be. There is a Stage 3 proposal for Promise.any:

Promise.any() takes an iterable of Promise objects and, as soon as one of the promises in the iterable fulfills, returns a single promise that resolves with the value from that promise.

So, the following syntax will be valid:

// assume getApi returns a Promise

const promises = [
  getApi('url1'),
  getApi('url2'),
  getApi('url3'),
  getApi('url4'),
];
Promise.any(promises)
  .then((result) => {
    // result will contain the resolve value of the first Promise to resolve
  })
  .catch((err) => {
    // Every Promise rejected
  });

Promise.any has been implemented in Spidermonkey, and there are some polyfills available.

Answer 5

I'm using a function based on Promise.race() but with a twist: it ignores rejects, unless all given promises reject:

// ignores any rejects except if all promises rejects
Promise.firstResolve = function (promises) {
    return new Promise(function (fulfil, reject) {
        var rejectCount = 0;
        promises.forEach(function (promise) {
            promise.then(fulfil, () => {
                rejectCount++;
                if(rejectCount == promises.length) {
                    reject('All promises were rejected');
                } 
            });
        });
    });
};

It's based on Rich Harris's Promise polyfill race method. I just made the looping promise reject conditional: it only rejects the main promise, if all given promises failed, otherwise it ignores rejects and resolves the first success.

Usage:

// fastest promise to end, but is a reject (gets ignored)
var promise1 = new Promise((resolve, reject) => {
    setTimeout(() => {
        reject("foo")
    }, 100);
})

// fastest promise to resolve (wins the race)
var promise2 = new Promise((resolve, reject) => {
    setTimeout(() => {
        resolve("bar")
    }, 200);
})

// Another, slower resolve (gets ignored)
var promise3 = new Promise((resolve, reject) => {
    setTimeout(() => {
        resolve("baz")
    }, 300);
})

Promise.firstResolve([promise1, promise2, promise3])
    .then((res) => {
        console.log(res) // "bar"
    })
    .catch(err => {
        console.log(err) // "All promises were rejected" (if all promises were to fail)
    })

The reason I use this instead of the promise inverting approach, is because in my opinion this is more readable.

To please the question in the strictest way, below there is a version that resolves the first successful promise but doesn't do anything if all given promises fail:

// ignores any and all rejects
Promise.firstResolve = function (promises) {
    return new Promise(function (fulfil) {
        promises.forEach(function (promise) {
            promise.then(fulfil, () => {});
        });
    });
};

(usage same as above)

Edit: This is in fact the same as @user663031's suggestion. Which I haven't realized until just now.

Answer 6

To resolve this problem i used a Promise.rice with a Promise.allSettled.

The next code wait with the Promise.rice a success value. but if no hay a success result. return a array with all errors.

const PromiseRiceSuccess = <T = unknown>(promises: Promise<T>[]) => {
  let done: (reason?: T) => void;
  const waitEndAllPromises = new Promise((resolve, reject) => done = reject);
  const waitCatchs = promise => Promise.resolve(promise).catch(() => waitEndAllPromises);

  Promise.allSettled(promises).then(r => done(r));

  return Promise.race(promises.map(waitCatchs));
};

Example:

PromiseRiceSuccess([
  Promise.reject(1),
  new Promise((r) => setTimeout(() => r(2), 4000)),
]);
// 2

PromiseRiceSuccess([
  Promise.reject(1),
  new Promise((resolve, reject) => setTimeout(() => reject(2), 4000)),
]);
// Uncaught (in promise) (2) [{…}, {…}]