Tiny Async Handler in JS

demo.js

var get = function (path, callback) {
    var xhr = new XMLHttpRequest();
    xhr.onload = function () {
        if (this.status === 200) {
            callback(this.responseText);
        }
    };
    xhr.open('get', path);
    xhr.send();
};

var articleSection = document.querySelector('#articles');

whenall(function (on) {
    get(window.location.pathname, on('model'));
    get('/template/articles.html', on('template'));
    setTimeout(on('timeout'), 2000);
}, function (data) {
    articleSection.innerHTML = Mustache.to_html(data.template, JSON.parse(data.model));
});

whenall.js

var whenall = function (begin, end) {
    var index = 0;
    var results = {};
    var on = function (name) {
        ++index;
        return function (data) {
            --index;
            results[name] = data;
            if (index === 0) {
                end(results);
            }
        };
    };
    begin(on);
};