Question

157

Promise.all with nested Promise.all

rated 0 times [ 161] [ 4] / answers: 1 / hits: 41330 / 8 Years ago, mon, july 18, 2016, 12:00:00

I've nested arrays, I'm able to retrieve promises for the 2nd level array but not sure how to implement a then once top level finishes as well.

result.forEach(function(entity){ // outer list ???

    return Promise.all(entity.urls.map(function(item){

        return requestURL(item.href);

    }));

});

for instance if results has two or more items and each item has 10 or more urls to fetch, how would we implement then of [Promise.all][1] for all the promises. Native solution please.

Basically to handle nested arrays of promises in a right way.

Data Structure:

var result = [

    {

        urls: [

            {href: link1},

            {href: link2},

            {href: link3}

        ]

    },

    {

        urls: [

            {href: link4},

            {href: link5},

            {href: link6}

        ]

    }

];

Answers

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luzv

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1 Year ago

answered 8 Years ago acaciac · Accepted Answer

Use map instead of forEach, and wrap it inside another Promise.all call.

var arr = [

  {subarr: [1,2,3]},

  {subarr: [4,5,6]},

  {subarr: [7,8,9]}

];

function processAsync(n) {

  return new Promise(function(resolve) {

    setTimeout(

      function() { resolve(n * n); },

      Math.random() * 1e3

    );

  });

}

Promise.all(arr.map(function(entity){

  return Promise.all(entity.subarr.map(function(item){

    return processAsync(item);

  }));

})).then(function(data) {

  console.log(data);

});

You can also use an immediately invoked generator. For example, to get flattened results,

var arr = [

  {subarr: [1,2,3]},

  {subarr: [4,5,6]},

  {subarr: [7,8,9]}

];

function processAsync(n) {

  return new Promise(function(resolve) {

    setTimeout(

      function() { resolve(n * n); },

      Math.random() * 1e3

    );

  });

}

Promise.all(function*() {

  for(var entity of arr)

    for(var item of entity.subarr)

      yield processAsync(item);

}()).then(function(data) {

  console.log(data);

});