load js css

function loadJsCssFile(filename, filetype){
    let fileref;
    if (filetype == 'js'){ //if filename is a external JavaScript file
        fileref = document.createElement('script');
        fileref.setAttribute('type', 'text/javascript');
        fileref.setAttribute('src', filename);
    }
    else if (filetype == 'css') { //if filename is an external CSS file
        fileref = document.createElement('link');
        fileref.setAttribute('rel', 'stylesheet')
        fileref.setAttribute('type', 'text/css');
        fileref.setAttribute('href', filename);
    }
    
    if (fileref != 'undefined')
        document.getElementsByTagName('head')[0].appendChild(fileref);
}

function callJs(file, callback) {
    let script = document.getElementsByTagName('script')[0];
    let newjs = document.createElement('script');
    
    newjs.onload = function() {
        callback();
    };
    
    newjs.src = file;
    //script.parentNode.insertBefore(newjs. script);
    script.parentNode.appendChild(newjs);
}

loadJsCssFile('mycss.css', 'css');
loadJsCssFile('myjs.js', 'js');
callJs('myApp.js', function() {
    mainApp.init();
});

var addKeyEvent = function(e) {
    if(e.key === 'Enter') {
        if(typeof myJs != 'undefined') {
            myJs.init();
        }
    }
}

document.addEventListener("keydown", addKeyEvent);