async_loading_js_or_css.js

var loadFile = (function () {
    var links = document.getElementsByTagName('link');
    var scripts = document.getElementsByTagName('script');
    var head = document.getElementsByTagName('head')[0];

    var need = {
        css: true,
        js: true
    }

    function isNeed(url, ext) {
        need[ext] = true;
        var arr = (ext == 'js') ? scripts : links;
        for (var i = 0; i < arr.length; i++) {
            if (arr[i].href == url) need[ext] = false;
        }
        return need[ext];
    }

    return function (url, cb) {
        var ext = url.slice(url.lastIndexOf('.') + 1);
        if (!ext || !isNeed(url, ext)) return;

        var el = document.createElement(ext == 'css' ? 'link' : 'script');
        if (cb) { el.onload = cb; }
        if (ext == 'css') el.rel = "stylesheet";
        if (ext == 'css') el.href = url;
        if (ext == 'js') el.src = url;
        if (ext == 'js') el.async = true;
        head.appendChild(el);
    }
})();