Tuesday, November 27, 2018

await in turn by mistake

I really enjoy working with the new async functions but it is really easy to set up a situation where code that could be running in parallel is force to run in sequence. Consider this simple invocation of a javascript function that takes two values returned from other async functions:

   const combination = await combine(await value(1), await value(2));

The problem here is that unless the JS environment does some optimisation for you, the actions contained in the two called to await are performed in series.

function action() {
   return new Promise(resolve => {
       setTimeout(resolve, 5000)

async function value(v) {
   console.log('started ' + v);
   return action('broken ' + v).then(()=> {
      console.log("finished " + v);
      return "ok "  + v

async function test() {
 console.log(await value('1') + " " + await value('2'));

This will result in an output that look something like this, and will take about ten seconds:

(index):39 started 1
(index):39 finished 1
(index):37 started 2
(index):39 finished 2
(index):44 ok 1 ok 2

So there are a number of way to re-write this code in order to ensure that the original processes are all started at the same time, I think the second is the best form, just wish they had put some syntactic sugar so we could do away with Promise.all references.

async function test() {
  // Worst
  const three = value('3');
  const four = value('4');
  console.log(await three + " " + await four);

  // Better
  const [five, six] = await Promise.all([value('5'), value('6')]; 
  console.log(five + " " + six));

  // Better?
  console.log(...await Promise.all([value('7'), value('8')]))

So you would expect a test output to be similar to this, with each block taking about the minimum 5 seconds.

(index):37 started 3
(index):37 started 4
(index):39 finished 3
(index):39 finished 4
(index):47 ok 3 ok 4
(index):37 started 5
(index):37 started 6
(index):39 finished 5
(index):39 finished 6
(index):48 ok 5 ok 6
(index):37 started 7
(index):37 started 8
(index):39 finished 7
(index):39 finished 8
(index):48 ok 7 ok 8

It is really easy to make the same mistake when writing a for loop for example by waiting on each item in turn

async function test() {

  const list = ['1', '2', '3'];
  const result = [];
  for (const item of list) {
     result.push(await value(item));


For most operation that involve Promise and loop you will normally need map/Promise.all at some point. This version should complete in around the minimum 5 seconds.

async function test() {

  const list = ['1', '2', '3'];
  const result = await Promise.all(list.map(value));


No comments: