Implement Promise.series

When you have the list of values to be passed to the same function sequentially, so that the next call is only made only when the promise returned by the previous call is resolved, you either have to “manually” chain promises or use Promise.reduce, which both looks a bit ugly and also not so obvious.

It could be nice to have something like this:

Promise.series = function (arr, iteratorAsync) {
    return Promise.reduce(arr, function(memo, item) {
        return iteratorAsync(item);
    }, 0);
}

[ericclemmons]

Thank God I found this issue. I was under the impression that Promise.all was iterative. (I was .push’ing promises onto an Array, expecting sequential execution).

[petkaantonov]

The reason for popularity of this suggestion is misunderstanding of promises and thinking of Promise.all as some kind of “parallel” which needs a sequential equivalent.

But Promise.all doesn’t “parallelize” promises or anything, it does nothing to the input promises - it is simply just telling you whether the promises you already are controlling from somewhere else are completed or not.

So because promises are not being controlled from the collection methods, Promise.all cannot have a .sequence equivalent - a Promise.sequence would always need a tailored “iteratorAsync” function to go with it - just like reduce so no net savings of lines of code are gained .