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user3205931 user3205931 - 1 year ago 92
Javascript Question

Node JS Promise.all and forEach

I have an array like structure that exposes async methods. The async method calls return array structures that in turn expose more async methods. I am creating another JSON object to store values obtained from this structure and so I need to be careful about keeping track of references in callbacks.

I have coded a brute force solution, but I would like to learn a more idiomatic or clean solution.

  1. The pattern should be repeatable for n levels of nesting.

  2. I need to use promise.all or some similar technique to determine when to resolve the enclosing routine.

  3. Not every element will necessarily involve making an async call. So in a nested promise.all I can't simply make assignments to my JSON array elements based on index. Nevertheless, I do need to use something like promise.all in the nested forEach to ensure that all property assignments have been made prior to resolving the enclosing routine.

  4. I am using the bluebird promise lib but this is not a requirement

Here is some partial code -

var jsonItems = [];


var jsonItem = {}; =;
// or Promise.all(allItemGetThingCalls, function(things){

things.forEach(function(thing, index){

jsonItems[index].thingName =;
if(thing.type === 'file'){

thing.getFile().then(function(file){ //or promise.all?

jsonItems[index].filesize = file.getSize();


It's pretty straightforward with some simple rules:

  • Whenever you create a promise in a then, return it - any promise you don't return will not be waited for outside.
  • Whenever you create multiple promises, .all them - that way it waits for all the promises and no errors from any of them are silenced.
  • Whenever you nest thens, you can typically return in the middle - then chains are usually at most 1 level deep.
  • Whenever you perform IO, it should be with a promise - either it should be in a promise or it should use a promise to signal its completion.

And some tips:

  • Mapping is better done with .map than with for/push - if you're mapping values with a function, map lets you concisely express the notion of applying actions one by one and aggregating the result.
  • Concurrency is better than sequential execution if it's free - it's better to execute things concurrently and wait for them Promise.all then to execute things one after the other - each waiting before the next.

Ok, so let's get started:

var items = [1, 2, 3, 4, 5];
var fn = function asyncMultiplyBy2(v){ // sample async action
    return new Promise(resolve => setTimeout(() => resolve(v * 2), 100));
// map over forEach since it returns

var actions =; // run the function over all items.

// we now have a promises array and we want to wait for it

var results = Promise.all(actions); // pass array of promises

results.then(data => // or just .then(console.log)
    console.log(data) // [2, 4, 6, 8, 10]

// we can nest this of course, as I said, `then` chains:

var res2 = Promise.all([1, 2, 3, 4, 5].map(fn)).then(
    data => Promise.all(
    // the next `then` is executed after the promise returned from the previous
    // `then` fulfilled, in this case it's an aggregate promise because of 
    // the `.all` 
    return Promise.all(;
    // just for good measure
    return Promise.all(;

// now to get the results:

    console.log(data); // [16, 32, 48, 64, 80]