Execute promises concurrently with a buffer pool size in Javascript

Question

I have a function with a promises which must be executed n times with different params each time. I want to chain the promises in a way that the script is always working on 3-4 promises at the time.

I made it with promise.all, this executes 3 concurrently and when all the promises resolves it goes on with the next 3.

How to make it work that when one of 3 resolves it starts immediatly with another but always working on max 3 at the time?

for( var i = 0; i < tasks.length; i++){

    if( i > 0 && i%3 == 0 ){

      await Promise.all([
       doTaskFunction(tasks[i]),
        doTaskFunction(tasks[i-1]),
        doTaskFunction(tasks[i-2]),
      ]);
    }

  }

Answer 1

You can achieve this fairly easy using es6-promise-pool:

const tasks = [
    (param) => new Promise(function(resolve, reject) {
        setTimeout(resolve, 2000, 'foo');
    }),
    () => new Promise(function(resolve, reject) {
        setTimeout(resolve, 2000, 'foo');
    }),
    () => new Promise(function(resolve, reject) {
        setTimeout(resolve, 2000, 'foo');
    }),
    () => Promise.resolve(1),
    () => Promise.resolve(2),
    () => Promise.resolve(3)
 ];
 
 let count = 1;

 const promiseProducer = () => {
    while(tasks.length) {
       console.log('processing ' + count++);
       const task = tasks.shift();
       return task(); // optionally you could pass a parameter here
    }
    
    return null;
 }
 
 const pool = new PromisePool(promiseProducer, 3); // concurrent Promises set to 3
 const poolPromise = pool.start();

 poolPromise.then(() => { console.log('done!'); })

<script src="https://cdn.jsdelivr.net/npm/[email protected]/es6-promise-pool.min.js"></script>

Answer 2

I am just leaving my naive with generators implementation here! :)

function* myIteratorFactory(arr) {
  for (let i = 0; i < arr.length; i++) {
    yield(arr[i])
  }
}


function delayPromise(text, ms) {
  return function() {
    return new Promise((resolve, reject) => {
      console.log('[%s] Promise with value %s just started', new Date().toISOString(), text)
      setTimeout(() => resolve(text), ms)
    }).then(() => console.log('[%s] Promise with value %s just ended', new Date().toISOString(), text))
  }
}

var promArr = [
  delayPromise('hi', 1500),
  delayPromise('alex', 2000),
  delayPromise('how', 1700),
  delayPromise('are', 1800),
  delayPromise('you', 1500),
]

var que = 0
var myIterator = myIteratorFactory(promArr)


function executor(r) {

  while (que < 3) {
    var next = myIterator.next()
    if (next.done) return;

    next.value()
      .then(() => {
        que--
        executor(r)
        if (que == 0) r()
      })
    que++
  }



}
executor(() => console.log('i am done for today!'))