Async Loader

loader.js

var loader = {
    script: function ( url ) {
        var script = document.createElement( 'script' );
        script.src = url;
        return script;
    },
    css: function ( url ) {
        var link = document.createElement( 'link' );
        link.rel = "stylesheet";
        link.type = "text/css";
        link.href = url;
        return link;
    },
    task: function ( url ) {
        var item;
        if ( /js$/.test( url ) ) item = loader.script( url );
        if ( /css$/.test( url ) ) item = loader.css( url );

        return new Promise( function ( resolve ) {
            if ( !item ) {
                console.log( url + " must be js or css" );
                return resolve();
            }
            document.head.appendChild( item );
            item.onload = resolve;
            item.onerror = resolve;
        } );
    }

}

module.exports = function () {
    var urls = [].slice.call( arguments );
    return new Promise( function ( resolve ) {
        function next() {
            if ( !urls.length ) return resolve();
            var url = urls.shift();
            loader.task( url ).then( next );
        }
        next();
    } );
};