Implementations of parallel and serial "map" function for Q lib

gistfile1.js

function mapParallel(array, callback) {
    return array.reduce(function(acc_p, element, index, array) {
        return Q.when(callback(element, index, array), function(res) {
            return Q.when(acc_p, function(acc) {
                return acc.concat([res]);
            });
        });
    }, []);
}

function mapSerial(array, callback) {
    return array.reduce(function(acc_p, element, index, array) {
        return Q.when(acc_p, function(acc) {
            return Q.when(callback(element, index, array), function(res) {    
                return acc.concat([res]);
            });
        });
    }, []);
}

test.js

var Q = require("q");

function mapParallel(array, callback) {
    return array.reduce(function(acc_p, element, index, array) {
        return Q.when(callback(element, index, array), function(res) {
            return Q.when(acc_p, function(acc) {
                return acc.concat([res]);
            });
        });
    }, []);
}

function mapSerial(array, callback) {
    return array.reduce(function(acc_p, element, index, array) {
        return Q.when(acc_p, function(acc) {
            return Q.when(callback(element, index, array), function(res) {    
                return acc.concat([res]);
            });
        });
    }, []);
}

function reduceSerial(array, callback, initialValue) {
    return array.reduce(function(acc_p, element, index, array) {
        return Q.when(acc_p, function(acc) {
            return callback(acc, element, index, array);
        })
    }, initialValue);
}



var TEST_ARRAY = ["A", "B", "C", "D", "E", "F"];

Q.when(reduceSerial(TEST_ARRAY, function(accumulator, element, index, array) {
    console.log("REDUCE SERIAL ITEM  ", element, index, array);
    return delayedConcatenate(accumulator, element);
}, ""), function(result) {
    console.log("REDUCE SERIAL RESULT", result);
});

Q.when(mapSerial(TEST_ARRAY, function(element, index, array) {
    console.log("MAP SERIAL ITEM     ", element, index, array);
    return delayedMakeLower(element);
}), function(result) {
    console.log("MAP SERIAL RESULT   ", result);
});

Q.when(mapParallel(TEST_ARRAY, function(element, index, array) {
    console.log("MAP PARALLEL ITEM   ", element, index, array);
    return delayedMakeLower(element);
}), function(result) {
    console.log("MAP PARALLEL RESULT ", result);
});

function delayedMakeLower(value) {
    var d = Q.defer();
    setTimeout(function() { d.resolve(value.toLowerCase()); }, 250);
    return d.promise;
}

function delayedConcatenate(a, b) {
    var d = Q.defer();
    setTimeout(function() { d.resolve(a + b); }, 250);
    return d.promise;
}