A demonstration of 3 methods running asynchronously with promises

concurrent-async.js

function sleep(ms) {
  return new Promise(resolve => {
    console.log(`starting ${ms}`);
    setTimeout(() => {
      console.log(`done ${ms}`);
      resolve(ms);
    }, ms);
  });
}

(async () => {
  console.log('aPromise, bPromise, cPromise executed concurrently as promises are in an array');
  const start = new Date();
  const aPromise = sleep(2000);
  const bPromise = sleep(500);
  const cPromise = sleep(5);
  const [a, b, c] = [await aPromise, await bPromise, await cPromise];
  console.log(`sleeping done - got ${a} ${b} ${c} in ${new Date()-start}`);
})();

// Now, let's look at rejecting promises that go beyond a certain amount of time:
var errorSleep = function(para) {
    return new Promise(function(resolve, reject) {
        setTimeout(function() {
            reject(' ErrorSleep')
        }, 1000)
    })
}

try {
    var result1 = await sleep(1);
    var result2 = await errorSleep(4);
    var result3 = await sleep(1);
    console.log('result1: ', result1)
    console.log('result2: ', result2)
    console.log('result3: ', result3)
} catch (err) {
    console.log('err: ', err)
    console.log('result1: ', result1)
    console.log('result2: ', result2)
    console.log('result3: ', result3)
}
//err:   ErrorSleep
//result1:  1
//result2:  undefined
//result3:  undefined


/* How about some references?
https://stackoverflow.com/a/54526132/1098368
https://techbrij.com/javascript-promises-error-handling-chain
https://techbrij.com/javascript-async-await-parallel-sequence
http://exploringjs.com/es6/ch_promises.html
https://medium.com/@piotrkarpaa/async-await-and-parallel-code-in-node-js-6de6501eea48
https://hackernoon.com/asycn-await-bible-sequential-parallel-and-nest-4d1db7b8b95c
*/

This code was originally written to satisfy this question:

How do you get 3 promises to run asynchronously, while each one fetches something over REST, returning the added values from the results of all 3 REST calls as long as they finish in under 30 seconds, or the results of all those calls that finished within 30 seconds if the timer expires?

On further evaluation, it seems like this is a pretty straightforward use case for generator functions. I'll have to try my hand at another revision of this problem and see if I can come up with a generator-based solution.