async load css files. Usage: require('style.css');

script.js

function asyncInject(array, fn, delay) {
    var i = 0;
    window.setTimeout(function iter() {
        if (i === array.length) {
            return;
        }
        fn.call(array, array[i], i++);
        window.setTimeout(iter, delay);
    }, 0);
}

require = function (paths, callback) {


    // Check if the path param is of the required type.
    if (!paths instanceof Array || !paths instanceof String) {
        console.log('Please check the useage');
        return;
    }

    // If paths param is a single param, convert it to an Array.
    paths = typeof paths === 'string' ? paths.split() : paths
    var js, css;

    // For every path in paths add the required to the head.
    asyncInject(paths, function (path) {
        var type = path.split('.').pop();
        switch (type) {
            case 'js':
                js = document.createElement('script');
                js.src = path;
                type = js;
                break;
            case 'css':
                css = document.createElement('link');
                css.href = path;
                css.rel = 'stylesheet';
                type = css;
                break;
            default:
                return;
        }

        if (callback)
            type.addEventListener('load', function (e) {
                callback(e);
            }, false);

        document.head.appendChild(type);
    }, 0);
}