How to return accumulated returned Promise values as array to .then() following Array.prototype.reduce()?

Question

Given this pattern

someArray.reduce(function(p, item) {
  return p.then(function() {
    return someFunction(item);
  });
}, $.Deferred().resolve()).then(fun         


        

Answer 1

There are multiple possible strategies depending upon the specifics of what you're trying to do: Here's one option:

someArray.reduce(function(p, item) {
  return p.then(function(array) {
    return someFunction(item).then(function(val) {
        array.push(val);
        return array;
    });
  });
}, $.Deferred().resolve([])).then(function(array) {
  // all done here
  // accumulated results in array
}, function(err) {
  // err is the error from the rejected promise that stopped the chain of execution
});

Working demo: http://jsfiddle.net/jfriend00/d4q1aaa0/

---

FYI, the Bluebird Promise library (which is what I generally use) has .mapSeries() which is built for this pattern:

var someArray = [1,2,3,4];

Promise.mapSeries(someArray, function(item) {
    return someFunction(item);
}).then(function(results) {
    log(results);
});

Working demo: http://jsfiddle.net/jfriend00/7fm3wv7j/