Async recursive using promise

Question

So i am trying to transfer my code into the "Promise world", and in many places when i had to "loop" with async functionality - i simply used recursion in such a way

function doRecursion(idx,callback){
    if(idx < someArray.length){
        doAsync(function(){
            doRecursion(++idx,callback)
        });
    }else{
        callback('done!')
    }
}
doRecursion(0,function(msg){
    //...
});

Now i am trying to make the change into the Promise world, and i am quite stuck

var Promise = require('bluebird')
function doRecursion(idx){
    return new Promise(function(resolve){
        if(idx < someArray.length){
            doAsync(function(){
                //... doRecursion(++idx)
                // how do i call doRecusion here....
            });
        }else{
            resolve('done!')
        }
    });
}
doRecursion(0).then(function(msg){
    //...
});

Thanks.

resolve(doRecursion(++idx)); - if you resolve Promise 1 with Promise 2, then Promise 1 will resolve with the resolved value of Promise 2 — Jaromanda X
– Jaromanda X, Commented Feb 22, 2016 at 12:21
Please stop using the promise constructor anti-pattern. This goes for the answers too. — jib
– jib, Commented Feb 23, 2016 at 1:42

Ben Fortune · Accepted Answer · 2016-02-22 12:38:45Z

4

I'd go with the Promise.all approach.

What this does is wait until all the promises in the array have resolved. The map will apply the async method to each item in the array and return a promise.

function doAsyncP() {
    return new Promise((resolve) => {
        doAsync(function() {
            resolve();
        });
    });
}

Promise.all(
    someArray.map(doAsyncP)
).then((msg) => {
    //we're done.
});

answered Feb 22, 2016 at 12:38

Ben Fortune

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Comments

McMath · Accepted Answer · 2016-02-22 12:48:52Z

1

In your recursive function, you can do this:

...
if (idx < someArray.length) {
  doAsync(function() {
    resolve(doRecursion(idx + 1));
   });
} else {
...

In other words, while idx is less than someArray.length, your promise will resolve to another promise, this time the promise returned by calling doRecursion() with an idx incremented by one. The then callback the bottom will not be called until doRecursion resolves to some value other than a promise. In this case, it will eventually resolve with a value of 'done!'.

That said, if you are using promises, you probably don't need to use recursion at all. You might have to refactor your code a bit more, but I would suggest considering @BenFortune's answer as an alternative.

edited Feb 22, 2016 at 12:48

answered Feb 22, 2016 at 12:37

McMath

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2 Comments

Ben Fortune Over a year ago

Bit of an anti-pattern IMO if you're using promises.

McMath Over a year ago

@BenFortune Yes, I agree, but it's what the asker asked. I've never even thought about using recursion with promises before, and that is probably why. The asker should consider your accepting your answer instead.

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