Simple async in JavaScript

parallel.js

(function load(items, callback) {
    var loaded = new Array(items.length);
    var error;
    items.forEach(function(item, i) {
        getWidget(item, function(err, obj) {
            error = error || err;
            loaded[i] = err || obj;
            if (loaded.filter(function(n) { return n; }).length === loaded.length) {
                callback(error, loaded);
            }
        });
    });
})(['foo', 'bar', 'baz'], function(err, results) {
    console.log(err);
    // [Error: bar is invalid]
    console.log(results);
    // [ { type: 'widget', id: 'foo' },
    //  [Error: bar is invalid],
    //  { type: 'widget', id: 'baz' } ]
});

function getWidget(arg, callback) {
    // Force error to demonstrate error handling.
    if (arg === 'bar') return callback(new Error('bar is invalid'));
    return callback(null, {type: 'widget', id: arg});
}

serial.js

(function load(results, callback) {
    if (!results.widget) return getWidget('foo', function(err, widget) {
        if (err) return callback(err);
        results.widget = widget;
        load(results, callback);
    });
    if (!results.gadget) return getGadget('bar', function(err, gadget) {
        if (err) return callback(err);
        results.gadget = gadget;
        load(results, callback);
    });
    callback(null, results);
})({}, function(err, results) {
    console.log(results);
    // { widget: { type: 'widget', id: 'foo' },
    //   gadget: { type: 'gadget', id: 'bar' } }
});

function getWidget(arg, callback) {
    return callback(null, {type: 'widget', id: arg});
}

function getGadget(arg, callback) {
    return callback(null, {type: 'gadget', id: arg});
}