codice:
$ cat index.html 
<html>
<head>
    <script type="text/javascript" src="start.js"></script>
</head>
<body onload="start()">




</body>
</html>

$ cat start.js 
function start() {
    var script = document.createElement("script");


    script.src = "config.js";
    script.type = "text/javascript";
    script.async = false;
    
    document.body.appendChild(script);
    console.log(xxx);
}

$ cat config.js 
var xxx = 123;res$
Uncaught ReferenceError: xxx is not defined
at start (start.js:9:17)
at onload (index.html:5:24)
start @ start.js:9
onload @ index.html:5