asyncReduce.js

var asyncReduce = function(array, iterator, callback) {
    var index = 0;

    if (!(array instanceof Array)) {
        throw new Error('first argument is required to be an array');
    }

    if (!iterator || typeof iterator !== "function") {
        throw new Error('second argument required to be a function');
    }

    var previousValue;

    (function innerIterator() {
        previousValue = array[++index];
        if (index < array.length - 1) {
            previousValue = iterator(previousValue, array[index], index, innerIterator.bind(this));
        } else {
            return callback && callback(previousValue);
        }
    })();
}