Following JavaScript code will append an external JavaScript file to the document body. This can be used as guide to load a JavaScript file on the fly.
<script type="text/javascript">
var newScript = document.createElement('script');
newScript.type = 'text/javascript';
newScript.async = true;
newScript.src = 'https://somedomain.com/JsFileName.js';
document.getElementsByTagName("body")[0].appendChild(newScript);
</script>
cheers,
Samitha