Map an array, where the map function, and the result of the map, are asynchronous.

sync-and-async-map.js

// SYNCHRONOUS MAP

function smap(src, fn) {
    if (src.length == 0) {
        return [];
    } else {
        return [fn(src[0])].concat(smap(src.slice(1), fn));
    }
}

function sadd(n) {
    return n + 1;
}

console.log(smap([2, 3, 4], sadd));

// ASYNC (BUT ORDERED/SERIAL) MAP

function amap(arr, fn, callback) {

    if (arr.length == 0) {
        callback([]);
    } else {
        fn(arr[0], function(v) {
            amap(arr.slice(1), fn, function (list) {
                callback([v].concat(list));
            })
        });
    }

}

function aadd(n, callback) {
    callback(n + 1);
}

amap([2, 3, 4], aadd, console.log);