Node JS Promise.all and forEach

Question

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 = [];

items.forEach(function(item){

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

    things.forEach(function(thing, index){

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

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

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

Answer 1

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 error 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 results.
• Concurrency is better than sequential execution if it's free - it's better to execute things concurrently and wait for them Promise.all than 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 = items.map(fn); // 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(data.map(fn))
).then(function(data){
    // the next `then` is executed after the promise has returned from the previous
    // `then` fulfilled, in this case it's an aggregate promise because of 
    // the `.all` 
    return Promise.all(data.map(fn));
}).then(function(data){
    // just for good measure
    return Promise.all(data.map(fn));
});

// now to get the results:

res2.then(function(data){
    console.log(data); // [16, 32, 48, 64, 80]
});