Sequential waterfall over array of promisorys.

arrayFall.js

// Performs sequential waterfall over array of promisorys.
// Following two sentences:
//   var P = Promise.all(inArr.map(cbk))
//   var P = arrayfall(inArr, cbk)
// ...are pretty much the same.
// Both runs all promises sequentially and returns new promise resolving with
// an array with resolved data (if none rejected).
// But in the former all promises are run as fast as input array is scanned.
// Whereas with arrayfall() promises are executed in sequence: each after
// previous one is resolved (or rejected) even being completely asyncronous.
function arrayfall(inArr, cbk) {
    var arr = [];

    inArr.map(function(item, i) {
        arr[i] = i
            ? arr[i-1].then(function(data){
                arr[i-1] = data; // Release.
                return Promise.resolve(cbk(item, i));
            })
            : Promise.resolve(cbk(item, i))
        ;
    });

    return Promise.all(arr);
};