Async calls per each array element and wait for completion

Question

I found many similar questions but none with the same purpose (or maybe i just didnt understand? Can be that as well).

I've an array of elements, a function A that performs an async operation with an element and returns a promise, and the function B i'm working on which should call A for each array element, and return a promise once all the A calls have been completed. How can i do that? Relevant code:

var img = [];
img[0] = "http://www.something.com/picture0.jpg";
img[1] = "http://www.something.com/picture1.jpg";
img[2] = "http://www.something.com/picture2.jpg";
function ask(img)
    {
    return new Promise((resolve, reject) =>
        {
        if(something)
            {resolve("test");}
        else
            {reject("test");}
        }
    }
function analyze_all(img)
    {
    for(var i in img)
        {
        ask(img[i])
        .then((res) => 
            {
            ???
            })
        .catch((err) =>
            {
            ???
            });
        }
    return ???
    }

I thought about pushing all results to a list, like that:

function analyze_all(img)
    {
    var ret = [];
    for(var i in img)
        {
        ask(img[i])
        .then((res) => 
            {
            ret.push({image: img[i], data: res});
            })
        .catch((err) =>
            {
            ret.push({image: img[i], error: err});
            });
        }
    return ???
    }

But then how do i know when will all the async calls be completed? Please help!

Solved. As addition, if someone has similar needs and wants to have all the "return" values whether they're resolves or rejects, i suggest checking the following link: https://nmaggioni.xyz/2016/10/13/Avoiding-Promise-all-fail-fast-behavior/

You can use `Promise.all` to turn a list of promises into a promise that resolves to a list. — Jonas Wilms, May 02 '18 at 18:01
I'm not understanding, the list must contain the Promise returned by the function call or the resolved value? — Barnack, May 02 '18 at 18:26

Tomasz Mularczyk · Accepted Answer · 2018-05-02T18:04:43.600

2

You are on the right track, use Promise.all:

function analyze_all(img) {
    var ret = [];
    for(var i in img) {
        ret.push( 
          ask(img[i]).then((res) => {
            // something
          })
          .catch((err) => {
            // something
          })
        )
    }
    return Promise.all(ret);
}

edited May 02 '18 at 18:04

answered May 02 '18 at 18:01

Tomasz Mularczyk

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@barnack no, it is not? – Jonas Wilms May 02 '18 at 18:28
ok i just need time to understand the concept and how to handle the results, thanks for the answer! – Barnack May 02 '18 at 18:30

Async calls per each array element and wait for completion

1 Answers1