Run a list of functions in series, returning a pjromise that resolves to an array of results.

promise-series.js

'use strict';

/*
 * run promises in sequence, return an array of results.
 * Returns a promise that resolves to an array of results, where funcs[i]() -> results[i]
*/
function asyncSeries(...funcs) {
  return aggregate([], funcs);
}

function aggregate(results, funcs) {
  if (funcs.length === 0) return Promise.all(results);
  return funcs[0]().then(result => aggregate([...results, result], funcs.slice(1)));
}

// usage (ES6-friendly this time):
asyncSeries(fn1, () => fn2(someArg), () => someObj.fn3(someOtherArg), fn4);

promise-series.reduce.js

// same as above, using reduce

function asyncSeries(...funcs) {
  return funcs.reduce(function execAsyncFuncsInSeries(res, func) {
    return res.then(results => {
      return func().then(result => results.concat([result]));
    });
  }, Promise.resolve([]));
}