If you need to load javascript by the case like domain name, following code solve your pain point.
Example 1) Loading script by the screen width
var head = document.getElementsByTagName('head')[0];
var js = document.createElement("script");
js.type = "text/javascript";
if (screen.width > 500)
{
js.src = "js/jquery_computer.js";
}
else
{
js.src = "js/mobile_version.js";
}
head.appendChild(js);
Example 2) Loading Google Adsense script by the domain name
<!-- Global site tag (gtag.js) - Google Analytics -->
<div id="adsense_space"></div>
<script>
var head = document.getElementsByTagName('adsense_space')[0];
var sUA = "your default User Agent ID";
if (document.domain.toLowerCase()=="enewtown.com") sUA="UA-1093021-17"; else sUA="UA-1093021-16";
var js = document.createElement("script");
js.type = "text/javascript";
js.src = "https://www.googletagmanager.com/gtag/js?id=" + sUA;
window.dataLayer = window.dataLayer || [];
function gtag(){dataLayer.push(arguments);}
gtag('js', new Date());
gtag('config', sUA);
</script>