load() - a deadsimple javascript function to load js/css in a controlled fashion

load.js

/*
* more controlled way of loading javascript & css in index.html
*
* purposes:
*   - quicker/smoother rendering of 1st meaningful frame and css preloading animations
*   - save performance by adding delays to tracker-jscode (analytics etc) 
*   - conditional logic (only load certain js/css files based on url-flags, server port etc)
* 
* example:
* 
*       load([
*         {src:"/js/stylus.min.js"},
*         {src:"https://cdn.polyfill.io/v2/polyfill.js?features=es6,fetch,Promise"},
*       ],  
*         load([
*           {src:"/js/main.js"},
*           {src:"https://google.com/analytics/k2j3kj423", load: (i) => setTimeout(i,2000) }
*         ])
*       )()
*
*       require('/foo.js').then( () => alert("nice!") ) // manual way of loading js-file
*/

function load(files, cb) {
    window.require = function(url, type) {
        return new Promise(function(resolve, reject) {
            type = type == 'js' ? 'js' : 'css'
            var tag = document.createElement(type == 'js' ? 'script' : 'link');
            if (type == 'css')
                tag.rel = "stylesheet"
            tag[type == 'js' ? 'src' : 'href'] = url;
            tag.addEventListener('load', function() {
                resolve(tag);
            }, false);
            tag.addEventListener('error', function() {
                reject(tag);
                console.log('require(' + url + ') failed')
            }, false);
            document.body.appendChild(tag);
        }
        )
    }
    return function(files,cb){
        // load other scripts after first meaningful frame
        setTimeout(function() {
            Promise.all(files.map((f)=>{
                !f.load ? require(f.src, "js") : new Promise(function(src, delay, resolve, reject) {
                    f.load( function(){ require(f.src, "js") } )
                }
                .bind(this, f.src, f.delay))
            }
            )).then(()=>cb ? cb() : false)
        }, 1000)
    }.bind(this,files,cb)
}