Map reduce with p-promises, based on https://gist.github.com/kriskowal/359801

map-reduce.js

// Map/Reduce with Promises
var p = require("p-promise");

// to simulate a long latency, promise-returning API
var delay = function (timeout) {
    var deferred = p.defer();
    setTimeout(function () {
        deferred.resolve();
    }, timeout);
    return deferred.promise;
};

var sum = [0,1,2,3,4,5,6,7,8,9]
.map(function (n) {
    return delay(1000).then(function () {
        return n;
    });
})
// at this point, we have an array of promises
// where each will eventually resolve to a number
// for the first iteration of reduce, sum will be the promise of
// the first number, i.e. 0
.reduce(function (sum, n) {
    return n.then(function (n) {
        return sum.then(function (sum) {
            return sum + n;
        });
    });
});

sum.then(function (sum) {
    console.log(sum);
});